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//! LISTEN/NOTIFY support for PostgreSQL connections.
//!
//! PostgreSQL sends `NotificationResponse` messages asynchronously when
//! a channel the connection is LISTENing on receives a NOTIFY.
//!
//! This module provides:
//! - `Notification` struct — channel name + payload
//! - `listen()` / `unlisten()` — subscribe/unsubscribe to channels
//! - `poll_notifications()` — drain buffered notifications (non-blocking)
//! - `recv_notification()` — block-wait for the next notification
use super::{
PgConnection, PgError, PgResult, io::MAX_MESSAGE_SIZE, is_ignorable_session_message,
unexpected_backend_message,
};
use crate::protocol::PgEncoder;
/// A notification received from PostgreSQL LISTEN/NOTIFY.
#[derive(Debug, Clone)]
pub struct Notification {
/// The PID of the notifying backend process
pub process_id: i32,
/// The channel name
pub channel: String,
/// The payload (may be empty)
pub payload: String,
}
#[inline]
fn return_with_desync<T>(conn: &mut PgConnection, err: PgError) -> PgResult<T> {
if matches!(
err,
PgError::Protocol(_) | PgError::Connection(_) | PgError::Timeout(_)
) {
conn.mark_io_desynced();
}
Err(err)
}
impl PgConnection {
/// Subscribe to a notification channel.
///
/// ```ignore
/// conn.listen("price_calendar_changed").await?;
/// ```
pub async fn listen(&mut self, channel: &str) -> PgResult<()> {
// Channel names are identifiers, quote them to prevent injection
let sql = format!("LISTEN \"{}\"", channel.replace('"', "\"\""));
self.execute_simple(&sql).await
}
/// Unsubscribe from a notification channel.
pub async fn unlisten(&mut self, channel: &str) -> PgResult<()> {
let sql = format!("UNLISTEN \"{}\"", channel.replace('"', "\"\""));
self.execute_simple(&sql).await
}
/// Unsubscribe from all notification channels.
pub async fn unlisten_all(&mut self) -> PgResult<()> {
self.execute_simple("UNLISTEN *").await
}
/// Drain all buffered notifications without blocking.
///
/// Notifications arrive asynchronously from PostgreSQL and are buffered
/// whenever `recv()` encounters a `NotificationResponse`. This method
/// returns all currently buffered notifications.
pub fn poll_notifications(&mut self) -> Vec<Notification> {
self.notifications.drain(..).collect()
}
/// Wait for the next notification, blocking until one arrives.
///
/// Unlike `recv()`, this does NOT use the 30-second Slowloris timeout
/// guard. LISTEN connections idle for long periods — that's normal,
/// not a DoS attack.
///
/// Useful for a dedicated LISTEN connection in a background task.
pub async fn recv_notification(&mut self) -> PgResult<Notification> {
use crate::protocol::BackendMessage;
// Return buffered notification immediately if available
if let Some(n) = self.notifications.pop_front() {
return Ok(n);
}
// Send empty query to flush any pending notifications from server
let bytes = PgEncoder::try_encode_query_string("")?;
self.write_all_with_timeout(&bytes, "stream write").await?;
// Read messages — use recv() for the initial empty query response
// (which completes quickly), then switch to no-timeout reads
let mut got_ready = false;
loop {
// Try to decode from the existing buffer first
if self.buffer.len() >= 5 {
let msg_len = u32::from_be_bytes([
self.buffer[1],
self.buffer[2],
self.buffer[3],
self.buffer[4],
]) as usize;
if msg_len < 4 {
return return_with_desync(
self,
PgError::Protocol(format!(
"Invalid message length: {} (minimum 4)",
msg_len
)),
);
}
if msg_len > MAX_MESSAGE_SIZE {
return return_with_desync(
self,
PgError::Protocol(format!(
"Message too large: {} bytes (max {})",
msg_len, MAX_MESSAGE_SIZE
)),
);
}
if self.buffer.len() > msg_len {
let msg_bytes = self.buffer.split_to(msg_len + 1);
let (msg, _) = match BackendMessage::decode(&msg_bytes) {
Ok(decoded) => decoded,
Err(err) => return return_with_desync(self, PgError::Protocol(err)),
};
match msg {
BackendMessage::NotificationResponse {
process_id,
channel,
payload,
} => {
return Ok(Notification {
process_id,
channel,
payload,
});
}
BackendMessage::EmptyQueryResponse => continue,
BackendMessage::NoticeResponse(_) => continue,
BackendMessage::ParameterStatus { .. } => continue,
BackendMessage::CommandComplete(_) => continue,
BackendMessage::ReadyForQuery(_) => {
got_ready = true;
// Check buffer for notifications that arrived with this batch
if let Some(n) = self.notifications.pop_front() {
return Ok(n);
}
continue;
}
BackendMessage::ErrorResponse(err) => {
return Err(PgError::QueryServer(err.into()));
}
msg if is_ignorable_session_message(&msg) => continue,
other => {
return return_with_desync(
self,
unexpected_backend_message("listen/notify wait", &other),
);
}
}
}
}
// Read from socket — use tokio read (no timeout!) if we've
// already gotten ReadyForQuery (now we're just waiting for NOTIFY)
if self.buffer.capacity() - self.buffer.len() < 65536 {
self.buffer.reserve(131072);
}
if got_ready {
// LISTEN connections can stay idle for hours (empty buffer),
// but a partially buffered backend frame should still timeout
// to fail-closed on slowloris-style partial writes.
let n = if self.buffer.is_empty() {
self.read_without_timeout().await?
} else {
self.read_with_timeout().await?
};
if n == 0 {
return return_with_desync(
self,
PgError::Connection("Connection closed".to_string()),
);
}
} else {
// Initial flush — use the normal timeout to avoid hanging
// if the server is unresponsive during the empty query
let n = self.read_with_timeout().await?;
if n == 0 {
return return_with_desync(
self,
PgError::Connection("Connection closed".to_string()),
);
}
}
}
}
}
#[cfg(test)]
mod tests {
use super::return_with_desync;
use crate::driver::{PgConnection, PgError};
#[cfg(unix)]
fn test_conn() -> PgConnection {
use crate::driver::connection::StatementCache;
use crate::driver::stream::PgStream;
use bytes::BytesMut;
use std::collections::{HashMap, VecDeque};
use std::num::NonZeroUsize;
use tokio::net::UnixStream;
let (unix_stream, _peer) = UnixStream::pair().expect("unix stream pair");
PgConnection {
stream: PgStream::Unix(unix_stream),
buffer: BytesMut::with_capacity(1024),
write_buf: BytesMut::with_capacity(1024),
sql_buf: BytesMut::with_capacity(256),
params_buf: Vec::new(),
prepared_statements: HashMap::new(),
stmt_cache: StatementCache::new(NonZeroUsize::new(2).expect("non-zero")),
column_info_cache: HashMap::new(),
process_id: 0,
cancel_key_bytes: Vec::new(),
requested_protocol_minor: PgConnection::default_protocol_minor(),
negotiated_protocol_minor: PgConnection::default_protocol_minor(),
notifications: VecDeque::new(),
replication_stream_active: false,
replication_mode_enabled: false,
last_replication_wal_end: None,
io_desynced: false,
pending_statement_closes: Vec::new(),
draining_statement_closes: false,
}
}
#[cfg(unix)]
#[tokio::test]
async fn notification_return_with_desync_marks_protocol_error() {
let mut conn = test_conn();
let err =
return_with_desync::<()>(&mut conn, PgError::Protocol("bad notify frame".to_string()))
.expect_err("protocol error must be returned");
assert!(err.to_string().contains("bad notify frame"));
assert!(conn.is_io_desynced());
}
}