neon/proxy/src/cancellation.rs

use anyhow::Context;
use dashmap::DashMap;
use pq_proto::CancelKeyData;
use std::{net::SocketAddr, sync::Arc};
use tokio::net::TcpStream;
use tokio_postgres::{CancelToken, NoTls};
use tracing::info;

/// Enables serving `CancelRequest`s.
#[derive(Default)]
pub struct CancelMap(DashMap<CancelKeyData, Option<CancelClosure>>);

impl CancelMap {
    /// Cancel a running query for the corresponding connection.
    pub async fn cancel_session(&self, key: CancelKeyData) -> anyhow::Result<()> {
        // NB: we should immediately release the lock after cloning the token.
        let cancel_closure = self
            .0
            .get(&key)
            .and_then(|x| x.clone())
            .with_context(|| format!("query cancellation key not found: {key}"))?;

        info!("cancelling query per user's request using key {key}");
        cancel_closure.try_cancel_query().await
    }

    /// Run async action within an ephemeral session identified by [`CancelKeyData`].
    pub fn get_session(self: Arc<Self>) -> Session {
        // HACK: We'd rather get the real backend_pid but tokio_postgres doesn't
        // expose it and we don't want to do another roundtrip to query
        // for it. The client will be able to notice that this is not the
        // actual backend_pid, but backend_pid is not used for anything
        // so it doesn't matter.
        let key = loop {
            let key = rand::random();

            // Random key collisions are unlikely to happen here, but they're still possible,
            // which is why we have to take care not to rewrite an existing key.
            match self.0.entry(key) {
                dashmap::mapref::entry::Entry::Occupied(_) => continue,
                dashmap::mapref::entry::Entry::Vacant(e) => {
                    e.insert(None);
                }
            }
            break key;
        };

        info!("registered new query cancellation key {key}");
        Session {
            key,
            cancel_map: self,
        }
    }

    #[cfg(test)]
    fn contains(&self, session: &Session) -> bool {
        self.0.contains_key(&session.key)
    }

    #[cfg(test)]
    fn is_empty(&self) -> bool {
        self.0.is_empty()
    }
}

/// This should've been a [`std::future::Future`], but
/// it's impossible to name a type of an unboxed future
/// (we'd need something like `#![feature(type_alias_impl_trait)]`).
#[derive(Clone)]
pub struct CancelClosure {
    socket_addr: SocketAddr,
    cancel_token: CancelToken,
}

impl CancelClosure {
    pub fn new(socket_addr: SocketAddr, cancel_token: CancelToken) -> Self {
        Self {
            socket_addr,
            cancel_token,
        }
    }

    /// Cancels the query running on user's compute node.
    pub async fn try_cancel_query(self) -> anyhow::Result<()> {
        let socket = TcpStream::connect(self.socket_addr).await?;
        self.cancel_token.cancel_query_raw(socket, NoTls).await?;

        Ok(())
    }
}

/// Helper for registering query cancellation tokens.
pub struct Session {
    /// The user-facing key identifying this session.
    key: CancelKeyData,
    /// The [`CancelMap`] this session belongs to.
    cancel_map: Arc<CancelMap>,
}

impl Session {
    /// Store the cancel token for the given session.
    /// This enables query cancellation in `crate::proxy::prepare_client_connection`.
    pub fn enable_query_cancellation(&self, cancel_closure: CancelClosure) -> CancelKeyData {
        info!("enabling query cancellation for this session");
        self.cancel_map.0.insert(self.key, Some(cancel_closure));

        self.key
    }
}

impl Drop for Session {
    fn drop(&mut self) {
        self.cancel_map.0.remove(&self.key);
        info!("dropped query cancellation key {}", &self.key);
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[tokio::test]
    async fn check_session_drop() -> anyhow::Result<()> {
        let cancel_map: Arc<CancelMap> = Default::default();

        let session = cancel_map.clone().get_session();
        assert!(cancel_map.contains(&session));
        drop(session);
        // Check that the session has been dropped.
        assert!(cancel_map.is_empty());

        Ok(())
    }
}