mz_adapter/

client.rs

// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.

use std::borrow::Cow;
use std::collections::BTreeMap;
use std::fmt::{Debug, Display, Formatter};
use std::future::Future;
use std::pin::{self, Pin};
use std::sync::Arc;
use std::time::{Duration, Instant};

use anyhow::bail;
use chrono::{DateTime, Utc};
use derivative::Derivative;
use futures::{Stream, StreamExt};
use itertools::Itertools;
use mz_adapter_types::connection::{ConnectionId, ConnectionIdType};
use mz_auth::password::Password;
use mz_auth::{Authenticated, AuthenticatorKind};
use mz_build_info::BuildInfo;
use mz_compute_types::ComputeInstanceId;
use mz_ore::channel::OneshotReceiverExt;
use mz_ore::collections::CollectionExt;
use mz_ore::id_gen::{IdAllocator, IdAllocatorInnerBitSet, MAX_ORG_ID, org_id_conn_bits};
use mz_ore::instrument;
use mz_ore::now::{EpochMillis, NowFn, to_datetime};
use mz_ore::str::StrExt;
use mz_ore::task::AbortOnDropHandle;
use mz_ore::thread::JoinOnDropHandle;
use mz_ore::tracing::OpenTelemetryContext;
use mz_repr::user::InternalUserMetadata;
use mz_repr::{CatalogItemId, ColumnIndex, SqlScalarType};
use mz_sql::ast::{Raw, Statement};
use mz_sql::catalog::{EnvironmentId, SessionCatalog};
use mz_sql::session::hint::ApplicationNameHint;
use mz_sql::session::metadata::SessionMetadata;
use mz_sql::session::user::SUPPORT_USER;
use mz_sql::session::vars::{
    CLUSTER, ENABLE_FRONTEND_PEEK_SEQUENCING, OwnedVarInput, SystemVars, Var,
};
use mz_sql_parser::parser::{ParserStatementError, StatementParseResult};
use prometheus::Histogram;
use serde_json::json;
use tokio::sync::{mpsc, oneshot};
use tracing::{debug, error};
use uuid::Uuid;

use crate::catalog::Catalog;
use crate::command::{
    CatalogDump, CatalogSnapshot, Command, CopyFromStdinWriter, ExecuteResponse, Response,
    SASLChallengeResponse, SASLVerifyProofResponse, SuperuserAttribute,
};
use crate::config::{ScopedParameters, ScopedParametersScope, SystemParameterFrontend};
use crate::coord::{Coordinator, ExecuteContextGuard};
use crate::error::AdapterError;
use crate::metrics::{self, Metrics};
use crate::session::{
    EndTransactionAction, PreparedStatement, Session, SessionConfig, StateRevision, TransactionId,
};
use crate::statement_logging::{StatementEndedExecutionReason, StatementExecutionStrategy};
use crate::telemetry::{self, EventDetails, SegmentClientExt, StatementFailureType};
use crate::webhook::AppendWebhookResponse;
use crate::{AdapterNotice, AppendWebhookError, PeekClient, PeekResponseUnary, StartupResponse};

/// A handle to a running coordinator.
///
/// The coordinator runs on its own thread. Dropping the handle will wait for
/// the coordinator's thread to exit, which will only occur after all
/// outstanding [`Client`]s for the coordinator have dropped.
pub struct Handle {
    pub(crate) session_id: Uuid,
    pub(crate) start_instant: Instant,
    pub(crate) _thread: JoinOnDropHandle<()>,
}

impl Handle {
    /// Returns the session ID associated with this coordinator.
    ///
    /// The session ID is generated on coordinator boot. It lasts for the
    /// lifetime of the coordinator. Restarting the coordinator will result
    /// in a new session ID.
    pub fn session_id(&self) -> Uuid {
        self.session_id
    }

    /// Returns the instant at which the coordinator booted.
    pub fn start_instant(&self) -> Instant {
        self.start_instant
    }
}

/// A coordinator client.
///
/// A coordinator client is a simple handle to a communication channel with the
/// coordinator. It can be cheaply cloned.
///
/// Clients keep the coordinator alive. The coordinator will not exit until all
/// outstanding clients have dropped.
#[derive(Debug, Clone)]
pub struct Client {
    build_info: &'static BuildInfo,
    inner_cmd_tx: mpsc::UnboundedSender<(OpenTelemetryContext, Command)>,
    id_alloc: IdAllocator<IdAllocatorInnerBitSet>,
    now: NowFn,
    metrics: Metrics,
    environment_id: EnvironmentId,
    segment_client: Option<mz_segment::Client>,
}

impl Client {
    pub(crate) fn new(
        build_info: &'static BuildInfo,
        cmd_tx: mpsc::UnboundedSender<(OpenTelemetryContext, Command)>,
        metrics: Metrics,
        now: NowFn,
        environment_id: EnvironmentId,
        segment_client: Option<mz_segment::Client>,
    ) -> Client {
        // Connection ids are 32 bits and have 3 parts.
        // 1. MSB bit is always 0 because these are interpreted as an i32, and it is possible some
        //    driver will not handle a negative id since postgres has never produced one because it
        //    uses process ids.
        // 2. Next 12 bits are the lower 12 bits of the org id. This allows balancerd to route
        //    incoming cancel messages to a subset of the environments.
        // 3. Last 19 bits are random.
        let env_lower = org_id_conn_bits(&environment_id.organization_id());
        Client {
            build_info,
            inner_cmd_tx: cmd_tx,
            id_alloc: IdAllocator::new(1, MAX_ORG_ID, env_lower),
            now,
            metrics,
            environment_id,
            segment_client,
        }
    }

    /// Allocates a client for an incoming connection.
    pub fn new_conn_id(&self) -> Result<ConnectionId, AdapterError> {
        self.id_alloc.alloc().ok_or(AdapterError::IdExhaustionError)
    }

    /// Creates a new session associated with this client for the given user.
    ///
    /// It is the caller's responsibility to have authenticated the user.
    /// We pass in an Authenticated marker as a guardrail to ensure the
    /// user has authenticated with an authenticator before creating a session.
    pub fn new_session(&self, config: SessionConfig, _authenticated: Authenticated) -> Session {
        // We use the system clock to determine when a session connected to Materialize. This is not
        // intended to be 100% accurate and correct, so we don't burden the timestamp oracle with
        // generating a more correct timestamp.
        Session::new(self.build_info, config, self.metrics().session_metrics())
    }

    /// Used by [mz_auth::AuthenticatorKind::Password]
    /// to verify the provided user's password against the
    /// stored credentials in the catalog.
    pub async fn authenticate(
        &self,
        user: &String,
        password: &Password,
    ) -> Result<Authenticated, AdapterError> {
        let (tx, rx) = oneshot::channel();
        self.send(Command::AuthenticatePassword {
            role_name: user.to_string(),
            password: Some(password.clone()),
            tx,
        });
        rx.await.expect("sender dropped")?;
        Ok(Authenticated)
    }

    /// Used by [mz_auth::AuthenticatorKind::Sasl] for SASL-SCRAM authentication.
    /// This is used prior to [Client::verify_sasl_proof].
    pub async fn generate_sasl_challenge(
        &self,
        user: &String,
        client_nonce: &String,
    ) -> Result<SASLChallengeResponse, AdapterError> {
        let (tx, rx) = oneshot::channel();
        self.send(Command::AuthenticateGetSASLChallenge {
            role_name: user.to_string(),
            nonce: client_nonce.to_string(),
            tx,
        });
        let response = rx.await.expect("sender dropped")?;
        Ok(response)
    }

    /// Used by [mz_auth::AuthenticatorKind::Sasl] for SASL-SCRAM authentication.
    /// This is used after [Client::generate_sasl_challenge].
    pub async fn verify_sasl_proof(
        &self,
        user: &String,
        proof: &String,
        nonce: &String,
        mock_hash: &String,
    ) -> Result<(SASLVerifyProofResponse, Authenticated), AdapterError> {
        let (tx, rx) = oneshot::channel();
        self.send(Command::AuthenticateVerifySASLProof {
            role_name: user.to_string(),
            proof: proof.to_string(),
            auth_message: nonce.to_string(),
            mock_hash: mock_hash.to_string(),
            tx,
        });
        let response = rx.await.expect("sender dropped")?;
        Ok((response, Authenticated))
    }

    /// Checks if a role exists and has the `LOGIN` attribute.
    pub async fn role_can_login(&self, role_name: &str) -> Result<(), AdapterError> {
        let (tx, rx) = oneshot::channel();
        self.send(Command::CheckRoleCanLogin {
            role_name: role_name.to_string(),
            tx,
        });
        rx.await.expect("sender dropped")
    }

    /// Upgrades this client to a session client.
    ///
    /// A session is a connection that has successfully negotiated parameters,
    /// like the user. Most coordinator operations are available only after
    /// upgrading a connection to a session.
    ///
    /// Returns a new client that is bound to the session and a response
    /// containing various details about the startup.
    #[mz_ore::instrument(level = "debug")]
    pub async fn startup(&self, session: Session) -> Result<SessionClient, AdapterError> {
        let user = session.user().clone();
        let conn_id = session.conn_id().clone();
        let secret_key = session.secret_key();
        let uuid = session.uuid();
        let client_ip = session.client_ip();
        let application_name = session.application_name().into();
        let notice_tx = session.retain_notice_transmitter();

        let (tx, rx) = oneshot::channel();

        // ~~SPOOKY ZONE~~
        //
        // This guard prevents a race where the startup command finishes, but the Future returned
        // by this function is concurrently dropped, so we never create a `SessionClient` and thus
        // never cleanup the initialized Session.
        let rx = rx.with_guard(|_| {
            self.send(Command::Terminate {
                conn_id: conn_id.clone(),
                tx: None,
            });
        });

        self.send(Command::Startup {
            tx,
            user,
            conn_id: conn_id.clone(),
            secret_key,
            uuid,
            client_ip: client_ip.copied(),
            application_name,
            notice_tx,
        });

        // When startup fails, no need to call terminate (handle_startup does this). Delay creating
        // the client until after startup to sidestep the panic in its `Drop` implementation.
        let response = rx.await.expect("sender dropped")?;

        // Create the client as soon as startup succeeds (before any await points) so its `Drop` can
        // handle termination.
        // Build the PeekClient with controller handles returned from startup.
        let StartupResponse {
            role_id,
            write_notify,
            session_defaults,
            catalog,
            storage_collections,
            transient_id_gen,
            optimizer_metrics,
            persist_client,
            statement_logging_frontend,
            superuser_attribute,
        } = response;

        let peek_client = PeekClient::new(
            self.clone(),
            &catalog,
            storage_collections,
            transient_id_gen,
            optimizer_metrics,
            persist_client,
            statement_logging_frontend,
        );

        let mut client = SessionClient {
            inner: Some(self.clone()),
            session: Some(session),
            timeouts: Timeout::new(),
            environment_id: self.environment_id.clone(),
            segment_client: self.segment_client.clone(),
            peek_client,
            enable_frontend_peek_sequencing: false, // initialized below, once we have a ConnCatalog
        };

        let session = client.session();

        // Apply the superuser attribute to the session's user if
        // it exists.
        if let SuperuserAttribute(Some(superuser)) = superuser_attribute {
            session.apply_internal_user_metadata(InternalUserMetadata { superuser });
        }

        session.initialize_role_metadata(role_id);
        let vars_mut = session.vars_mut();
        for (name, val) in session_defaults {
            if let Err(err) = vars_mut.set_default(&name, val.borrow()) {
                // Note: erroring here is unexpected, but we don't want to panic if somehow our
                // assumptions are wrong.
                tracing::error!("failed to set peristed default, {err:?}");
            }
        }
        session
            .vars_mut()
            .end_transaction(EndTransactionAction::Commit);

        // Stash the future that notifies us of builtin table writes completing, we'll block on
        // this future before allowing queries from this session against relevant relations.
        //
        // Note: We stash the future as opposed to waiting on it here to prevent blocking session
        // creation on builtin table updates. This improves the latency for session creation and
        // reduces scheduling load on any dataflows that read from these builtin relations, since
        // it allows updates to be batched.
        session.set_builtin_table_updates(write_notify);

        let catalog = catalog.for_session(session);

        let cluster_active = session.vars().cluster().to_string();
        if session.vars().welcome_message() {
            let cluster_info = if catalog.resolve_cluster(Some(&cluster_active)).is_err() {
                format!("{cluster_active} (does not exist)")
            } else {
                cluster_active.to_string()
            };

            // Emit a welcome message, optimized for readability by humans using
            // interactive tools. If you change the message, make sure that it
            // formats nicely in both `psql` and the console's SQL shell.
            session.add_notice(AdapterNotice::Welcome(format!(
                "connected to Materialize v{}
  Environment ID: {}
  Region: {}
  User: {}
  Cluster: {}
  Database: {}
  {}
  Session UUID: {}

Issue a SQL query to get started. Need help?
  View documentation: https://materialize.com/s/docs
  Join our Slack community: https://materialize.com/s/chat
    ",
                session.vars().build_info().semver_version(),
                self.environment_id,
                self.environment_id.region(),
                session.vars().user().name,
                cluster_info,
                session.vars().database(),
                match session.vars().search_path() {
                    [schema] => format!("Schema: {}", schema),
                    schemas => format!(
                        "Search path: {}",
                        schemas.iter().map(|id| id.to_string()).join(", ")
                    ),
                },
                session.uuid(),
            )));
        }

        if session.vars().current_object_missing_warnings() {
            if catalog.active_database().is_none() {
                let db = session.vars().database().into();
                session.add_notice(AdapterNotice::UnknownSessionDatabase(db));
            }
        }

        // Users stub their toe on their default cluster not existing, so we provide a notice to
        // help guide them on what do to.
        let cluster_var = session
            .vars()
            .inspect(CLUSTER.name())
            .expect("cluster should exist");
        if session.vars().current_object_missing_warnings()
            && catalog.resolve_cluster(Some(&cluster_active)).is_err()
        {
            let cluster_notice = 'notice: {
                if cluster_var.inspect_session_value().is_some() {
                    break 'notice Some(AdapterNotice::DefaultClusterDoesNotExist {
                        name: cluster_active,
                        kind: "session",
                        suggested_action: "Pick an extant cluster with SET CLUSTER = name. Run SHOW CLUSTERS to see available clusters.".into(),
                    });
                }

                let role_default = catalog.get_role(catalog.active_role_id());
                let role_cluster = match role_default.vars().get(CLUSTER.name()) {
                    Some(OwnedVarInput::Flat(name)) => Some(name),
                    None => None,
                    // This is unexpected!
                    Some(v @ OwnedVarInput::SqlSet(_)) => {
                        tracing::warn!(?v, "SqlSet found for cluster Role Default");
                        break 'notice None;
                    }
                };

                let alter_role = "with `ALTER ROLE <role> SET cluster TO <cluster>;`";
                match role_cluster {
                    // If there is no default, suggest a Role default.
                    None => Some(AdapterNotice::DefaultClusterDoesNotExist {
                        name: cluster_active,
                        kind: "system",
                        suggested_action: format!(
                            "Set a default cluster for the current role {alter_role}."
                        ),
                    }),
                    // If the default does not exist, suggest to change it.
                    Some(_) => Some(AdapterNotice::DefaultClusterDoesNotExist {
                        name: cluster_active,
                        kind: "role",
                        suggested_action: format!(
                            "Change the default cluster for the current role {alter_role}."
                        ),
                    }),
                }
            };

            if let Some(notice) = cluster_notice {
                session.add_notice(notice);
            }
        }

        client.enable_frontend_peek_sequencing = ENABLE_FRONTEND_PEEK_SEQUENCING
            .require(catalog.system_vars())
            .is_ok();

        Ok(client)
    }

    /// Cancels the query currently running on the specified connection.
    pub fn cancel_request(&self, conn_id: ConnectionIdType, secret_key: u32) {
        self.send(Command::CancelRequest {
            conn_id,
            secret_key,
        });
    }

    /// Executes a single SQL statement that returns rows as the
    /// `mz_support` user.
    pub async fn support_execute_one(
        &self,
        sql: &str,
    ) -> Result<Pin<Box<dyn Stream<Item = PeekResponseUnary> + Send>>, anyhow::Error> {
        // Connect to the coordinator.
        let conn_id = self.new_conn_id()?;
        let session = self.new_session(
            SessionConfig {
                conn_id,
                uuid: Uuid::new_v4(),
                user: SUPPORT_USER.name.clone(),
                client_ip: None,
                external_metadata_rx: None,
                helm_chart_version: None,
                authenticator_kind: AuthenticatorKind::None,
                groups: None,
            },
            Authenticated,
        );
        let mut session_client = self.startup(session).await?;

        // Parse the SQL statement.
        let stmts = mz_sql::parse::parse(sql)?;
        if stmts.len() != 1 {
            bail!("must supply exactly one query");
        }
        let StatementParseResult { ast: stmt, sql } = stmts.into_element();

        const EMPTY_PORTAL: &str = "";
        session_client.start_transaction(Some(1))?;
        session_client
            .declare(EMPTY_PORTAL.into(), stmt, sql.to_string())
            .await?;

        let execute_result = session_client
            .execute(EMPTY_PORTAL.into(), futures::future::pending(), None)
            .await?;
        match execute_result {
            (ExecuteResponse::SendingRowsStreaming { mut rows, .. }, _) => {
                // We have to only drop the session client _after_ we read the
                // result. Otherwise the peek will get cancelled right when we
                // drop the session client. So we wrap it up in an extra stream
                // like this, which owns the client and can return it.
                let owning_response_stream = async_stream::stream! {
                    while let Some(rows) = rows.next().await {
                        yield rows;
                    }
                    drop(session_client);
                };
                Ok(Box::pin(owning_response_stream))
            }
            r => bail!("unsupported response type: {r:?}"),
        }
    }

    /// Returns the metrics associated with the adapter layer.
    pub fn metrics(&self) -> &Metrics {
        &self.metrics
    }

    /// The current time according to the [`Client`].
    pub fn now(&self) -> DateTime<Utc> {
        to_datetime((self.now)())
    }

    /// Get a metadata and a channel that can be used to append to a webhook source.
    pub async fn get_webhook_appender(
        &self,
        database: String,
        schema: String,
        name: String,
    ) -> Result<AppendWebhookResponse, AppendWebhookError> {
        let (tx, rx) = oneshot::channel();

        // Send our request.
        self.send(Command::GetWebhook {
            database,
            schema,
            name,
            tx,
        });

        // Using our one shot channel to get the result, returning an error if the sender dropped.
        let response = rx
            .await
            .map_err(|_| anyhow::anyhow!("failed to receive webhook response"))?;

        response
    }

    /// Gets the current value of all system variables.
    pub async fn get_system_vars(&self) -> SystemVars {
        let (tx, rx) = oneshot::channel();
        self.send(Command::GetSystemVars { tx });
        rx.await.expect("coordinator unexpectedly gone")
    }

    /// Returns a snapshot of the catalog.
    ///
    /// Does a Coordinator round-trip. Session-bound callers should
    /// prefer [`SessionClient::catalog_snapshot`], which serves from the
    /// session's snapshot cache.
    pub async fn catalog_snapshot_expensive(&self) -> Arc<Catalog> {
        let (tx, rx) = oneshot::channel();
        self.send(Command::CatalogSnapshot { tx });
        let CatalogSnapshot { catalog } = rx.await.expect("coordinator unexpectedly gone");
        catalog
    }

    /// Reconciles the coordinator's scoped feature-flag working copy towards
    /// `overrides`. Used by the system-parameter sync loop from continuous
    /// LaunchDarkly evaluation.
    ///
    /// `prune_scope` bounds which objects' rows the reconcile may remove (the
    /// objects `overrides` was evaluated for). The sync loop passes the live
    /// objects from its snapshot; `None` is a full replace, used by the
    /// disabled-feature clear path. See
    /// [`crate::catalog::Op::UpdateScopedSystemParameters`].
    pub async fn update_scoped_system_parameters(
        &self,
        overrides: ScopedParameters,
        prune_scope: Option<ScopedParametersScope>,
    ) {
        let (tx, rx) = oneshot::channel();
        self.send(Command::UpdateScopedSystemParameters {
            overrides,
            prune_scope,
            tx,
        });
        let _ = rx.await;
    }

    /// Installs (or replaces) the shared system-parameter frontend on the
    /// coordinator, letting the create-cluster / create-replica paths resolve a
    /// new object's scoped overrides synchronously. Sent by the sync loop each
    /// time it (re)initializes the frontend. Fire-and-forget.
    pub fn install_scoped_system_parameter_frontend(&self, frontend: Arc<SystemParameterFrontend>) {
        self.send(Command::InstallScopedSystemParameterFrontend { frontend });
    }

    #[instrument(level = "debug")]
    pub(crate) fn send(&self, cmd: Command) {
        self.inner_cmd_tx
            .send((OpenTelemetryContext::obtain(), cmd))
            .expect("coordinator unexpectedly gone");
    }
}

/// A coordinator client that is bound to a connection.
///
/// See also [`Client`].
pub struct SessionClient {
    // Invariant: inner may only be `None` after the session has been terminated.
    // Once the session is terminated, no communication to the Coordinator
    // should be attempted.
    inner: Option<Client>,
    // Invariant: session may only be `None` during a method call. Every public
    // method must ensure that `Session` is `Some` before it returns.
    session: Option<Session>,
    timeouts: Timeout,
    segment_client: Option<mz_segment::Client>,
    environment_id: EnvironmentId,
    /// Client for frontend peek sequencing; populated at connection startup.
    peek_client: PeekClient,
    /// Whether frontend peek sequencing is enabled; initialized at connection startup.
    // TODO(peek-seq): Currently, this is initialized only at session startup. We'll be able to
    // check the actual feature flag value at every peek (without a Coordinator call) once we'll
    // always have a catalog snapshot at hand.
    pub enable_frontend_peek_sequencing: bool,
}

impl SessionClient {
    /// Parses a SQL expression, reporting failures as a telemetry event if
    /// possible.
    pub fn parse<'a>(
        &self,
        sql: &'a str,
    ) -> Result<Result<Vec<StatementParseResult<'a>>, ParserStatementError>, String> {
        match mz_sql::parse::parse_with_limit(sql) {
            Ok(Err(e)) => {
                self.track_statement_parse_failure(&e);
                Ok(Err(e))
            }
            r => r,
        }
    }

    fn track_statement_parse_failure(&self, parse_error: &ParserStatementError) {
        let session = self.session.as_ref().expect("session invariant violated");
        let Some(user_id) = session.user().external_metadata.as_ref().map(|m| m.user_id) else {
            return;
        };
        let Some(segment_client) = &self.segment_client else {
            return;
        };
        let Some(statement_kind) = parse_error.statement else {
            return;
        };
        let Some((action, object_type)) = telemetry::analyze_audited_statement(statement_kind)
        else {
            return;
        };
        let event_type = StatementFailureType::ParseFailure;
        let event_name = format!(
            "{} {} {}",
            object_type.as_title_case(),
            action.as_title_case(),
            event_type.as_title_case(),
        );
        segment_client.environment_track(
            &self.environment_id,
            event_name,
            json!({
                "statement_kind": statement_kind,
                "error": &parse_error.error,
            }),
            EventDetails {
                user_id: Some(user_id),
                application_name: Some(session.application_name()),
                ..Default::default()
            },
        );
    }

    // Verify and return the named prepared statement. We need to verify each use
    // to make sure the prepared statement is still safe to use.
    pub async fn get_prepared_statement(
        &mut self,
        name: &str,
    ) -> Result<&PreparedStatement, AdapterError> {
        let catalog = self.catalog_snapshot("get_prepared_statement").await;
        Coordinator::verify_prepared_statement(&catalog, self.session(), name)?;
        Ok(self
            .session()
            .get_prepared_statement_unverified(name)
            .expect("must exist"))
    }

    /// Saves the parsed statement as a prepared statement.
    ///
    /// The prepared statement is saved in the connection's [`crate::session::Session`]
    /// under the specified name.
    pub async fn prepare(
        &mut self,
        name: String,
        stmt: Option<Statement<Raw>>,
        sql: String,
        param_types: Vec<Option<SqlScalarType>>,
    ) -> Result<(), AdapterError> {
        let catalog = self.catalog_snapshot("prepare").await;

        // Note: This failpoint is used to simulate a request outliving the external connection
        // that made it.
        let mut async_pause = false;
        (|| {
            fail::fail_point!("async_prepare", |val| {
                async_pause = val.map_or(false, |val| val.parse().unwrap_or(false))
            });
        })();
        if async_pause {
            tokio::time::sleep(Duration::from_secs(1)).await;
        };

        let desc = Coordinator::describe(&catalog, self.session(), stmt.clone(), param_types)?;
        let now = self.now();
        let state_revision = StateRevision {
            catalog_revision: catalog.transient_revision(),
            session_state_revision: self.session().state_revision(),
        };
        self.session()
            .set_prepared_statement(name, stmt, sql, desc, state_revision, now);
        Ok(())
    }

    /// Binds a statement to a portal.
    #[mz_ore::instrument(level = "debug")]
    pub async fn declare(
        &mut self,
        name: String,
        stmt: Statement<Raw>,
        sql: String,
    ) -> Result<(), AdapterError> {
        let catalog = self.catalog_snapshot("declare").await;
        let param_types = vec![];
        let desc =
            Coordinator::describe(&catalog, self.session(), Some(stmt.clone()), param_types)?;
        let params = vec![];
        let result_formats = vec![mz_pgwire_common::Format::Text; desc.arity()];
        let now = self.now();
        let logging = self.session().mint_logging(sql, Some(&stmt), now);
        let state_revision = StateRevision {
            catalog_revision: catalog.transient_revision(),
            session_state_revision: self.session().state_revision(),
        };
        self.session().set_portal(
            name,
            desc,
            Some(stmt),
            logging,
            params,
            result_formats,
            state_revision,
        )?;
        Ok(())
    }

    /// Executes a previously-bound portal.
    ///
    /// Note: the provided `cancel_future` must be cancel-safe as it's polled in a `select!` loop.
    ///
    /// `outer_ctx_extra` is Some when we are executing as part of an outer statement, e.g., a FETCH
    /// triggering the execution of the underlying query.
    #[mz_ore::instrument(level = "debug")]
    pub async fn execute(
        &mut self,
        portal_name: String,
        cancel_future: impl Future<Output = std::io::Error> + Send,
        outer_ctx_extra: Option<ExecuteContextGuard>,
    ) -> Result<(ExecuteResponse, Instant), AdapterError> {
        let execute_started = Instant::now();

        let mut outer_ctx_extra = outer_ctx_extra;

        // Unroll SQL `EXECUTE <prepared> (...)` so the inner statement
        // flows through `try_frontend_peek` below, rather than being
        // re-dispatched via `Command::Execute` from the coordinator's
        // `Plan::Execute` handler. Without this, a prepared statement
        // would route differently from the same statement issued
        // directly.
        //
        // On a successful unroll, `unroll_sql_execute` also begins
        // EXECUTE-level statement logging on the outer portal, so that
        // `mz_statement_execution_history` records `EXECUTE foo (...)`
        // rather than the inner SQL, and installs the resulting
        // `ExecuteContextGuard` into `outer_ctx_extra`.
        let portal_name = self
            .unroll_sql_execute(portal_name, &mut outer_ctx_extra)
            .await?;

        // Attempt peek sequencing in the session task.
        // If unsupported, fall back to the Coordinator path.
        // TODO(peek-seq): wire up cancel_future
        let peek_result = self
            .try_frontend_peek(&portal_name, &mut outer_ctx_extra)
            .await?;
        if let Some(resp) = peek_result {
            debug!("frontend peek succeeded");
            // Frontend peek handled the execution and retired outer_ctx_extra if it existed.
            // No additional work needed here.
            return Ok((resp, execute_started));
        } else {
            debug!("frontend peek did not happen, falling back to `Command::Execute`");
            // If we bailed out, outer_ctx_extra is still present (if it was originally).
            // `Command::Execute` will handle it.
            // (This is not true if we bailed out _after_ the frontend peek sequencing has already
            // begun its own statement logging. That case would be a bug.)
        }

        let response = self
            .send_with_cancel(
                |tx, session| Command::Execute {
                    portal_name,
                    session,
                    tx,
                    outer_ctx_extra,
                },
                cancel_future,
            )
            .await?;
        Ok((response, execute_started))
    }

    /// If the named portal binds a SQL `EXECUTE <prepared>`, resolve the
    /// prepared statement, install a fresh portal for the inner statement
    /// (carrying the EXECUTE's actual parameter values), and return that
    /// portal's name so the caller can run `try_frontend_peek` against it.
    ///
    /// Only ever unrolls one level: the parser rejects
    /// `PREPARE foo AS EXECUTE bar` (matching Postgres), so the inner
    /// statement is guaranteed not to be another `EXECUTE`. A failsafe below
    /// surfaces an internal error if that invariant is ever violated.
    ///
    /// When the portal does not bind an `EXECUTE` — the common case —
    /// returns the original portal name, costing only a portal lookup.
    async fn unroll_sql_execute(
        &mut self,
        portal_name: String,
        outer_ctx_extra: &mut Option<ExecuteContextGuard>,
    ) -> Result<String, AdapterError> {
        let (stmt, params, outer_logging, outer_lifecycle_timestamps) = {
            let session = self.session.as_ref().expect("SessionClient invariant");
            let portal = match session.get_portal_unverified(&portal_name) {
                Some(p) => p,
                // No portal: let `try_frontend_peek` surface the
                // standard "missing portal" error.
                None => return Ok(portal_name),
            };
            match &portal.stmt {
                Some(stmt) => (
                    Arc::clone(stmt),
                    portal.parameters.clone(),
                    Arc::clone(&portal.logging),
                    portal.lifecycle_timestamps.clone(),
                ),
                None => return Ok(portal_name),
            }
        };

        // Only EXECUTE statements need unrolling. Bail out before taking a
        // catalog snapshot in the (overwhelmingly common) non-EXECUTE case.
        if !matches!(&*stmt, Statement::Execute(_)) {
            return Ok(portal_name);
        }

        let catalog = self.catalog_snapshot("unroll_sql_execute").await;

        // Validate the outer EXECUTE portal against the (possibly newer)
        // catalog: ensures the recorded portal description still matches
        // what describing the EXECUTE would produce now.
        {
            let session = self.session.as_mut().expect("SessionClient invariant");
            Coordinator::verify_portal(&catalog, session, &portal_name)?;
        }

        // Bump query_total for the outer EXECUTE itself. The inner
        // statement gets its own increment inside `try_frontend_peek_inner`
        // (or, on bailout, in the coordinator's `handle_execute`).
        {
            let session = self.session.as_ref().expect("SessionClient invariant");
            session
                .metrics()
                .query_total(&[
                    metrics::session_type_label_value(session.user()),
                    metrics::statement_type_label_value(&stmt),
                ])
                .inc();
        }

        // Begin EXECUTE-level statement logging up front, so that planning
        // errors below produce an `Errored` end-event in
        // `mz_statement_execution_history` rather than no entry at all.
        //
        // We pass the *outer* portal's `logging` and pgwire-bound `params`
        // so the recorded entry shows the user-visible `EXECUTE foo (...)`,
        // not the inner SQL. The id (if any) moves into `outer_ctx_extra`
        // below for `try_frontend_peek` to retire; on planning error we
        // explicitly emit an `Errored` end-event below.
        let began_outer_logging = outer_ctx_extra.is_none();
        let logging_id: Option<crate::statement_logging::StatementLoggingId> =
            if began_outer_logging {
                let session = self.session.as_mut().expect("SessionClient invariant");
                let result = self
                    .peek_client
                    .statement_logging_frontend
                    .begin_statement_execution(
                        session,
                        &params,
                        &outer_logging,
                        catalog.system_config(),
                        outer_lifecycle_timestamps,
                    );
                if let Some((id, began_execution, mseh_update, prepared_statement)) = result {
                    self.peek_client.log_began_execution(
                        began_execution,
                        mseh_update,
                        prepared_statement,
                    );
                    Some(id)
                } else {
                    None
                }
            } else {
                None
            };

        let new_portal_name = match self.install_inner_portal_for_execute(&catalog, &stmt, &params)
        {
            Ok(name) => name,
            Err(err) => {
                if let Some(id) = logging_id {
                    self.peek_client.log_ended_execution(
                        id,
                        StatementEndedExecutionReason::Errored {
                            error: err.to_string(),
                        },
                    );
                }
                return Err(err);
            }
        };

        // Hand off to `outer_ctx_extra` whenever we entered the begin path
        // for the outer EXECUTE — even if `begin_statement_execution`
        // returned `None` (sampling decided not to sample, or logging is
        // disabled for the user). This mirrors the original coord path,
        // which always installs a guard via
        // `ExecuteContextGuard::new(maybe_uuid, ...)`. Without this, the
        // inner portal would be treated as a fresh statement by
        // `try_frontend_peek` (or the fallback `Command::Execute` path)
        // and re-account its bytes against
        // `mz_statement_logging_unsampled_bytes`, double-counting the
        // inner SQL.
        if began_outer_logging {
            // Soft invariant: `try_frontend_peek` takes ownership of
            // `outer_ctx_extra` immediately, so this guard's `Drop` is
            // unreachable on the normal flow and the dummy channel is
            // never used. If a panic does fire `Drop` between here and
            // that takeover, the `Aborted` end-event is silently lost
            // — an acceptable trade given the panic implies the
            // connection is going down anyway.
            let (dummy_tx, _dummy_rx) = mpsc::unbounded_channel();
            *outer_ctx_extra = Some(ExecuteContextGuard::new(logging_id, dummy_tx));
        }

        Ok(new_portal_name)
    }

    /// Helper for [`Self::unroll_sql_execute`]: plans the outer
    /// `Statement::Execute`, verifies the referenced prepared statement, and
    /// installs a fresh portal carrying the inner statement plus the
    /// EXECUTE's bound parameter values. Returns the new portal's name.
    ///
    /// Split out so [`Self::unroll_sql_execute`] can wrap the fallible work
    /// in a single error-handling site that emits an `Errored` end-event
    /// for the EXECUTE-level statement-logging entry.
    fn install_inner_portal_for_execute(
        &mut self,
        catalog: &Arc<Catalog>,
        stmt: &Arc<Statement<Raw>>,
        params: &mz_sql::plan::Params,
    ) -> Result<String, AdapterError> {
        use mz_sql::plan::Plan;

        let execute_plan = {
            let session = self.session.as_mut().expect("SessionClient invariant");
            let conn_catalog = catalog.for_session(session);
            let (resolved_stmt, resolved_ids) =
                mz_sql::names::resolve(&conn_catalog, (**stmt).clone())?;
            let pcx = session.pcx();
            let (plan, _sql_impl_ids) = mz_sql::plan::plan(
                Some(pcx),
                &conn_catalog,
                resolved_stmt,
                params,
                &resolved_ids,
            )?;
            match plan {
                Plan::Execute(plan) => plan,
                other => {
                    // Planning a `Statement::Execute` must yield
                    // `Plan::Execute`. If it doesn't, the planner
                    // contract is broken.
                    return Err(AdapterError::Internal(format!(
                        "planning Statement::Execute yielded unexpected plan: {:?}",
                        mz_sql::plan::PlanKind::from(&other),
                    )));
                }
            }
        };

        // Verify and install the inner portal. Mirrors
        // `Coordinator::sequence_execute`. The new portal carries the inner
        // prepared statement's `logging`, but `try_frontend_peek` will see
        // `outer_ctx_extra=Some(...)` and inherit the EXECUTE-level logging
        // instead of starting fresh from this portal.
        let session = self.session.as_mut().expect("SessionClient invariant");
        Coordinator::verify_prepared_statement(catalog, session, &execute_plan.name)?;
        let ps = session
            .get_prepared_statement_unverified(&execute_plan.name)
            .expect("verified above");
        let inner_stmt = ps.stmt().cloned();
        let inner_desc = ps.desc().clone();
        let state_revision = ps.state_revision;
        let inner_logging = Arc::clone(ps.logging());

        // Failsafe: `PREPARE foo AS EXECUTE bar` is rejected by the parser,
        // so the resolved inner statement must not be another `EXECUTE`. If
        // that ever changes, we'd silently skip frontend sequencing for the
        // deeper EXECUTEs — surface it as an internal error instead.
        if let Some(inner) = inner_stmt.as_ref() {
            if matches!(inner, Statement::Execute(_)) {
                return Err(AdapterError::Internal(format!(
                    "nested EXECUTE: prepared statement {} resolves to another EXECUTE; \
                     parser should reject `PREPARE ... AS EXECUTE ...`",
                    execute_plan.name.quoted(),
                )));
            }
        }

        session.create_new_portal(
            inner_stmt,
            inner_logging,
            inner_desc,
            execute_plan.params,
            Vec::new(),
            state_revision,
        )
    }

    fn now(&self) -> EpochMillis {
        (self.inner().now)()
    }

    fn now_datetime(&self) -> DateTime<Utc> {
        to_datetime(self.now())
    }

    /// Starts a transaction based on implicit:
    /// - `None`: InTransaction
    /// - `Some(1)`: Started
    /// - `Some(n > 1)`: InTransactionImplicit
    /// - `Some(0)`: no change
    pub fn start_transaction(&mut self, implicit: Option<usize>) -> Result<(), AdapterError> {
        let now = self.now_datetime();
        let session = self.session.as_mut().expect("session invariant violated");
        let result = match implicit {
            None => session.start_transaction(now, None, None),
            Some(stmts) => {
                session.start_transaction_implicit(now, stmts);
                Ok(())
            }
        };
        result
    }

    /// Ends a transaction. Even if an error is returned, guarantees that the transaction in the
    /// session and Coordinator has cleared its state.
    #[instrument(level = "debug")]
    pub async fn end_transaction(
        &mut self,
        action: EndTransactionAction,
    ) -> Result<ExecuteResponse, AdapterError> {
        let res = self
            .send(|tx, session| Command::Commit {
                action,
                session,
                tx,
            })
            .await;
        // Commit isn't guaranteed to set the session's state to anything specific, so clear it
        // here. It's safe to ignore the returned `TransactionStatus` because that doesn't contain
        // any data that the Coordinator must act on for correctness.
        let _ = self.session().clear_transaction();
        res
    }

    /// Fails a transaction.
    pub fn fail_transaction(&mut self) {
        let session = self.session.take().expect("session invariant violated");
        let session = session.fail_transaction();
        self.session = Some(session);
    }

    /// Fetches the catalog, served from the session-side snapshot cache when
    /// the catalog is unchanged since the cached snapshot was taken. See
    /// [`PeekClient::catalog_snapshot`].
    #[instrument(level = "debug")]
    pub async fn catalog_snapshot(&mut self, context: &str) -> Arc<Catalog> {
        self.peek_client.catalog_snapshot(context).await
    }

    /// Dumps the catalog to a JSON string.
    ///
    /// No authorization is performed, so access to this function must be limited to internal
    /// servers or superusers.
    pub async fn dump_catalog(&mut self) -> Result<CatalogDump, AdapterError> {
        let catalog = self.catalog_snapshot("dump_catalog").await;
        catalog.dump().map_err(AdapterError::from)
    }

    /// Checks the catalog for internal consistency, returning a JSON object describing the
    /// inconsistencies, if there are any.
    ///
    /// No authorization is performed, so access to this function must be limited to internal
    /// servers or superusers.
    pub async fn check_catalog(&mut self) -> Result<(), serde_json::Value> {
        let catalog = self.catalog_snapshot("check_catalog").await;
        catalog.check_consistency()
    }

    /// Checks the coordinator for internal consistency, returning a JSON object describing the
    /// inconsistencies, if there are any. This is a superset of checks that check_catalog performs,
    ///
    /// No authorization is performed, so access to this function must be limited to internal
    /// servers or superusers.
    pub async fn check_coordinator(&self) -> Result<(), serde_json::Value> {
        self.send_without_session(|tx| Command::CheckConsistency { tx })
            .await
            .map_err(|inconsistencies| {
                serde_json::to_value(inconsistencies).unwrap_or_else(|_| {
                    serde_json::Value::String("failed to serialize inconsistencies".to_string())
                })
            })
    }

    pub async fn dump_coordinator_state(&self) -> Result<serde_json::Value, anyhow::Error> {
        self.send_without_session(|tx| Command::Dump { tx }).await
    }

    /// Tells the coordinator a statement has finished execution, in the cases
    /// where we have no other reason to communicate with the coordinator.
    pub fn retire_execute(
        &self,
        guard: ExecuteContextGuard,
        reason: StatementEndedExecutionReason,
    ) {
        if !guard.is_trivial() {
            let data = guard.defuse();
            let cmd = Command::RetireExecute { data, reason };
            self.inner().send(cmd);
        }
    }

    /// Sets up a streaming COPY FROM STDIN operation.
    ///
    /// Sends a command to the coordinator to create a background batch
    /// builder task. Returns a [`CopyFromStdinWriter`] that pgwire uses
    /// to stream decoded rows.
    pub async fn start_copy_from_stdin(
        &mut self,
        target_id: CatalogItemId,
        target_name: String,
        columns: Vec<ColumnIndex>,
        row_desc: mz_repr::RelationDesc,
        params: mz_pgcopy::CopyFormatParams<'static>,
    ) -> Result<CopyFromStdinWriter, AdapterError> {
        self.send(|tx, session| Command::StartCopyFromStdin {
            target_id,
            target_name,
            columns,
            row_desc,
            params,
            session,
            tx,
        })
        .await
    }

    /// Commits staged COPY FROM STDIN batches to a table.
    ///
    /// Adds the pre-built persist batches to the session's transaction
    /// operations. The actual commit happens when the transaction ends.
    pub fn stage_copy_from_stdin_batches(
        &mut self,
        target_id: CatalogItemId,
        batches: Vec<mz_persist_client::batch::ProtoBatch>,
    ) -> Result<(), AdapterError> {
        use crate::session::{TransactionOps, WriteOp};
        use mz_storage_client::client::TableData;

        self.session()
            .add_transaction_ops(TransactionOps::Writes(vec![WriteOp {
                id: target_id,
                rows: TableData::Batches(batches.into()),
            }]))?;
        Ok(())
    }

    /// Gets the current value of all system variables.
    pub async fn get_system_vars(&self) -> SystemVars {
        self.inner().get_system_vars().await
    }

    /// Updates the specified system variables to the specified values.
    pub async fn set_system_vars(
        &mut self,
        vars: BTreeMap<String, String>,
    ) -> Result<(), AdapterError> {
        let conn_id = self.session().conn_id().clone();
        self.send_without_session(|tx| Command::SetSystemVars { vars, conn_id, tx })
            .await
    }

    /// Injects audit events into the catalog via the coordinator.
    ///
    /// No authorization is performed, so access to this function must be limited to internal
    /// servers or superusers.
    pub async fn inject_audit_events(
        &mut self,
        events: Vec<crate::catalog::InjectedAuditEvent>,
    ) -> Result<(), AdapterError> {
        let conn_id = self.session().conn_id().clone();
        self.send_without_session(|tx| Command::InjectAuditEvents {
            events,
            conn_id,
            tx,
        })
        .await
    }

    /// Terminates the client session.
    pub async fn terminate(&mut self) {
        let conn_id = self.session().conn_id().clone();
        let res = self
            .send_without_session(|tx| Command::Terminate {
                conn_id,
                tx: Some(tx),
            })
            .await;
        if let Err(e) = res {
            // Nothing we can do to handle a failed terminate so we just log and ignore it.
            error!("Unable to terminate session: {e:?}");
        }
        // Prevent any communication with Coordinator after session is terminated.
        self.inner = None;
    }

    /// Returns a mutable reference to the session bound to this client.
    pub fn session(&mut self) -> &mut Session {
        self.session.as_mut().expect("session invariant violated")
    }

    /// Returns a reference to the inner client.
    pub fn inner(&self) -> &Client {
        self.inner.as_ref().expect("inner invariant violated")
    }

    async fn send_without_session<T, F>(&self, f: F) -> T
    where
        F: FnOnce(oneshot::Sender<T>) -> Command,
    {
        let (tx, rx) = oneshot::channel();
        self.inner().send(f(tx));
        rx.await.expect("sender dropped")
    }

    #[instrument(level = "debug")]
    async fn send<T, F>(&mut self, f: F) -> Result<T, AdapterError>
    where
        F: FnOnce(oneshot::Sender<Response<T>>, Session) -> Command,
    {
        self.send_with_cancel(f, futures::future::pending()).await
    }

    /// Send a [`Command`] to the Coordinator, with the ability to cancel the command.
    ///
    /// Note: the provided `cancel_future` must be cancel-safe as it's polled in a `select!` loop.
    #[instrument(level = "debug")]
    async fn send_with_cancel<T, F>(
        &mut self,
        f: F,
        cancel_future: impl Future<Output = std::io::Error> + Send,
    ) -> Result<T, AdapterError>
    where
        F: FnOnce(oneshot::Sender<Response<T>>, Session) -> Command,
    {
        let session = self.session.take().expect("session invariant violated");
        let mut typ = None;
        let application_name = session.application_name();
        let name_hint = ApplicationNameHint::from_str(application_name);
        let conn_id = session.conn_id().clone();
        let (tx, rx) = oneshot::channel();

        // Destructure self so we can hold a mutable reference to the inner client and session at
        // the same time.
        let Self {
            inner: inner_client,
            session: client_session,
            ..
        } = self;

        // TODO(parkmycar): Leaking this invariant here doesn't feel great, but calling
        // `self.client()` doesn't work because then Rust takes a borrow on the entirity of self.
        let inner_client = inner_client.as_ref().expect("inner invariant violated");

        // ~~SPOOKY ZONE~~
        //
        // This guard prevents a race where a `Session` is returned on `rx` but never placed
        // back in `self` because the Future returned by this function is concurrently dropped
        // with the Coordinator sending a response.
        let mut guarded_rx = rx.with_guard(|response: Response<_>| {
            *client_session = Some(response.session);
        });

        inner_client.send({
            let cmd = f(tx, session);
            // Measure the success and error rate of certain commands:
            // - declare reports success of SQL statement planning
            // - execute reports success of dataflow execution
            match cmd {
                Command::Execute { .. } => typ = Some("execute"),
                Command::GetWebhook { .. } => typ = Some("webhook"),
                Command::StartCopyFromStdin { .. }
                | Command::Startup { .. }
                | Command::AuthenticatePassword { .. }
                | Command::AuthenticateGetSASLChallenge { .. }
                | Command::AuthenticateVerifySASLProof { .. }
                | Command::CheckRoleCanLogin { .. }
                | Command::CatalogSnapshot { .. }
                | Command::Commit { .. }
                | Command::CancelRequest { .. }
                | Command::PrivilegedCancelRequest { .. }
                | Command::GetSystemVars { .. }
                | Command::SetSystemVars { .. }
                | Command::UpdateScopedSystemParameters { .. }
                | Command::InstallScopedSystemParameterFrontend { .. }
                | Command::Terminate { .. }
                | Command::RetireExecute { .. }
                | Command::CheckConsistency { .. }
                | Command::Dump { .. }
                | Command::GetComputeInstanceClient { .. }
                | Command::GetOracle { .. }
                | Command::DetermineRealTimeRecentTimestamp { .. }
                | Command::GetTransactionReadHoldsBundle { .. }
                | Command::StoreTransactionReadHolds { .. }
                | Command::ExecuteSlowPathPeek { .. }
                | Command::ExecuteSubscribe { .. }
                | Command::CopyToPreflight { .. }
                | Command::ExecuteCopyTo { .. }
                | Command::ExecuteSideEffectingFunc { .. }
                | Command::LookupConnection { .. }
                | Command::RegisterFrontendPeek { .. }
                | Command::UnregisterFrontendPeek { .. }
                | Command::ExplainTimestamp { .. }
                | Command::FrontendStatementLogging(..)
                | Command::InjectAuditEvents { .. } => {}
            };
            cmd
        });

        let mut cancel_future = pin::pin!(cancel_future);
        let mut cancelled = false;
        loop {
            tokio::select! {
                res = &mut guarded_rx => {
                    // We received a result, so drop our guard to drop our borrows.
                    drop(guarded_rx);

                    let res = res.expect("sender dropped");
                    let status = res.result.is_ok().then_some("success").unwrap_or("error");
                    if let Err(err) = res.result.as_ref() {
                        if name_hint.should_trace_errors() {
                            tracing::warn!(?err, ?name_hint, "adapter response error");
                        }
                    }

                    if let Some(typ) = typ {
                        inner_client
                            .metrics
                            .commands
                            .with_label_values(&[typ, status, name_hint.as_str()])
                            .inc();
                    }
                    *client_session = Some(res.session);
                    return res.result;
                },
                _err = &mut cancel_future, if !cancelled => {
                    cancelled = true;
                    inner_client.send(Command::PrivilegedCancelRequest {
                        conn_id: conn_id.clone(),
                    });
                }
            };
        }
    }

    pub fn add_idle_in_transaction_session_timeout(&mut self) {
        let session = self.session();
        let timeout_dur = session.vars().idle_in_transaction_session_timeout();
        if !timeout_dur.is_zero() {
            let timeout_dur = timeout_dur.clone();
            if let Some(txn) = session.transaction().inner() {
                let txn_id = txn.id.clone();
                let timeout = TimeoutType::IdleInTransactionSession(txn_id);
                self.timeouts.add_timeout(timeout, timeout_dur);
            }
        }
    }

    pub fn remove_idle_in_transaction_session_timeout(&mut self) {
        let session = self.session();
        if let Some(txn) = session.transaction().inner() {
            let txn_id = txn.id.clone();
            self.timeouts
                .remove_timeout(&TimeoutType::IdleInTransactionSession(txn_id));
        }
    }

    /// # Cancel safety
    ///
    /// This method is cancel safe. If `recv` is used as the event in a
    /// `tokio::select!` statement and some other branch
    /// completes first, it is guaranteed that no messages were received on this
    /// channel.
    pub async fn recv_timeout(&mut self) -> Option<TimeoutType> {
        self.timeouts.recv().await
    }

    /// Attempt to sequence a peek from the session task.
    ///
    /// Returns `Ok(Some(response))` if we handled the peek, or `Ok(None)` to fall back to the
    /// Coordinator's sequencing. If it returns an error, it should be returned to the user.
    ///
    /// `outer_ctx_extra` is Some when we are executing as part of an outer statement, e.g., a FETCH
    /// triggering the execution of the underlying query.
    pub(crate) async fn try_frontend_peek(
        &mut self,
        portal_name: &str,
        outer_ctx_extra: &mut Option<ExecuteContextGuard>,
    ) -> Result<Option<ExecuteResponse>, AdapterError> {
        if self.enable_frontend_peek_sequencing {
            let session = self.session.as_mut().expect("SessionClient invariant");
            self.peek_client
                .try_frontend_peek(portal_name, session, outer_ctx_extra)
                .await
        } else {
            Ok(None)
        }
    }
}

impl Drop for SessionClient {
    fn drop(&mut self) {
        // We may not have a session if this client was dropped while awaiting
        // a response. In this case, it is the coordinator's responsibility to
        // terminate the session.
        if let Some(session) = self.session.take() {
            // We may not have a connection to the Coordinator if the session was
            // prematurely terminated, for example due to a timeout.
            if let Some(inner) = &self.inner {
                inner.send(Command::Terminate {
                    conn_id: session.conn_id().clone(),
                    tx: None,
                })
            }
        }
    }
}

#[derive(Hash, PartialEq, Eq, PartialOrd, Ord, Clone, Debug)]
pub enum TimeoutType {
    IdleInTransactionSession(TransactionId),
}

impl Display for TimeoutType {
    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
        match self {
            TimeoutType::IdleInTransactionSession(txn_id) => {
                writeln!(f, "Idle in transaction session for transaction '{txn_id}'")
            }
        }
    }
}

impl From<TimeoutType> for AdapterError {
    fn from(timeout: TimeoutType) -> Self {
        match timeout {
            TimeoutType::IdleInTransactionSession(_) => {
                AdapterError::IdleInTransactionSessionTimeout
            }
        }
    }
}

struct Timeout {
    tx: mpsc::UnboundedSender<TimeoutType>,
    rx: mpsc::UnboundedReceiver<TimeoutType>,
    active_timeouts: BTreeMap<TimeoutType, AbortOnDropHandle<()>>,
}

impl Timeout {
    fn new() -> Self {
        let (tx, rx) = mpsc::unbounded_channel();
        Timeout {
            tx,
            rx,
            active_timeouts: BTreeMap::new(),
        }
    }

    /// # Cancel safety
    ///
    /// This method is cancel safe. If `recv` is used as the event in a
    /// `tokio::select!` statement and some other branch
    /// completes first, it is guaranteed that no messages were received on this
    /// channel.
    ///
    /// <https://docs.rs/tokio/latest/tokio/sync/mpsc/struct.UnboundedReceiver.html#cancel-safety>
    async fn recv(&mut self) -> Option<TimeoutType> {
        self.rx.recv().await
    }

    fn add_timeout(&mut self, timeout: TimeoutType, duration: Duration) {
        let tx = self.tx.clone();
        let timeout_key = timeout.clone();
        let handle = mz_ore::task::spawn(|| format!("{timeout_key}"), async move {
            tokio::time::sleep(duration).await;
            let _ = tx.send(timeout);
        })
        .abort_on_drop();
        self.active_timeouts.insert(timeout_key, handle);
    }

    fn remove_timeout(&mut self, timeout: &TimeoutType) {
        self.active_timeouts.remove(timeout);

        // Remove the timeout from the rx queue if it exists.
        let mut timeouts = Vec::new();
        while let Ok(pending_timeout) = self.rx.try_recv() {
            if timeout != &pending_timeout {
                timeouts.push(pending_timeout);
            }
        }
        for pending_timeout in timeouts {
            self.tx.send(pending_timeout).expect("rx is in this struct");
        }
    }
}

/// A wrapper around a Stream of PeekResponseUnary that records when it sees the
/// first row data in the given histogram. It also keeps track of whether we have already observed
/// the end of the underlying stream.
#[derive(Derivative)]
#[derivative(Debug)]
pub struct RecordFirstRowStream {
    /// The underlying stream of rows.
    #[derivative(Debug = "ignore")]
    pub rows: Box<dyn Stream<Item = PeekResponseUnary> + Unpin + Send + Sync>,
    /// The Instant when execution started.
    pub execute_started: Instant,
    /// The histogram where the time since `execute_started` will be recorded when we see the first
    /// row.
    pub time_to_first_row_seconds: Histogram,
    /// Whether we've seen any rows.
    pub saw_rows: bool,
    /// The Instant when we saw the first row.
    pub recorded_first_row_instant: Option<Instant>,
    /// Whether we have already observed the end of the underlying stream.
    pub no_more_rows: bool,
    /// Whether the first-to-last-byte metric has already been recorded for this stream.
    pub metric_recorded: bool,
}

impl RecordFirstRowStream {
    /// Create a new [`RecordFirstRowStream`]
    pub fn new(
        rows: Box<dyn Stream<Item = PeekResponseUnary> + Unpin + Send + Sync>,
        execute_started: Instant,
        client: &SessionClient,
        instance_id: Option<ComputeInstanceId>,
        strategy: Option<StatementExecutionStrategy>,
    ) -> Self {
        let histogram = Self::histogram(client, instance_id, strategy);
        Self {
            rows,
            execute_started,
            time_to_first_row_seconds: histogram,
            saw_rows: false,
            recorded_first_row_instant: None,
            no_more_rows: false,
            metric_recorded: false,
        }
    }

    fn histogram(
        client: &SessionClient,
        instance_id: Option<ComputeInstanceId>,
        strategy: Option<StatementExecutionStrategy>,
    ) -> Histogram {
        let session = client.session.as_ref().expect("session invariant");
        let isolation_level = *session.vars().transaction_isolation();
        let name_hint = ApplicationNameHint::from_str(session.application_name());
        let instance = match instance_id {
            Some(i) => Cow::Owned(i.to_string()),
            None => Cow::Borrowed("none"),
        };
        let strategy = match strategy {
            Some(s) => s.name(),
            None => "none",
        };

        client
            .inner()
            .metrics()
            .time_to_first_row_seconds
            .with_label_values(&[
                instance.as_ref(),
                isolation_level.as_variant_str(),
                strategy,
                name_hint.as_str(),
            ])
    }

    /// If you want to match [`RecordFirstRowStream`]'s logic but don't need
    /// a UnboundedReceiver, you can tell it when to record an observation.
    pub fn record(
        execute_started: Instant,
        client: &SessionClient,
        instance_id: Option<ComputeInstanceId>,
        strategy: Option<StatementExecutionStrategy>,
    ) {
        Self::histogram(client, instance_id, strategy)
            .observe(execute_started.elapsed().as_secs_f64());
    }

    pub async fn recv(&mut self) -> Option<PeekResponseUnary> {
        let msg = self.rows.next().await;
        if !self.saw_rows && matches!(msg, Some(PeekResponseUnary::Rows(_))) {
            self.saw_rows = true;
            self.time_to_first_row_seconds
                .observe(self.execute_started.elapsed().as_secs_f64());
            self.recorded_first_row_instant = Some(Instant::now());
        }
        if msg.is_none() {
            self.no_more_rows = true;
        }
        msg
    }
}