safekeeper: flush WAL on transaction commit

libs/postgres_ffi/src/xlog_utils.rs

@@ -14,7 +14,7 @@ use super::bindings::{
 };
 use super::wal_generator::LogicalMessageGenerator;
 use super::PG_MAJORVERSION;
-use crate::pg_constants;
+use crate::pg_constants::{self, XLOG_XACT_COMMIT, XLOG_XACT_COMMIT_PREPARED};
 use crate::PG_TLI;
 use crate::{uint32, uint64, Oid};
 use crate::{WAL_SEGMENT_SIZE, XLOG_BLCKSZ};
@@ -296,6 +296,13 @@ impl XLogRecord {
     pub fn is_xlog_switch_record(&self) -> bool {
         self.xl_info == pg_constants::XLOG_SWITCH && self.xl_rmid == pg_constants::RM_XLOG_ID
     }
+
+    // Is this record a transaction commit?
+    pub fn is_xact_commit(&self) -> bool {
+        self.xl_rmid == pg_constants::RM_XACT_ID
+            && (self.xl_info & pg_constants::XLOG_XACT_OPMASK == XLOG_XACT_COMMIT
+                || self.xl_info & pg_constants::XLOG_XACT_OPMASK == XLOG_XACT_COMMIT_PREPARED)
+    }
 }
 
 impl XLogPageHeaderData {

safekeeper/src/wal_storage.rs

@@ -12,7 +12,7 @@ use bytes::Bytes;
 use camino::{Utf8Path, Utf8PathBuf};
 use futures::future::BoxFuture;
 use postgres_ffi::v14::xlog_utils::{IsPartialXLogFileName, IsXLogFileName, XLogFromFileName};
-use postgres_ffi::{dispatch_pgversion, XLogSegNo, PG_TLI};
+use postgres_ffi::{dispatch_pgversion, XLogRecord, XLogSegNo, PG_TLI, XLOG_SIZE_OF_XLOG_RECORD};
 use remote_storage::RemotePath;
 use std::cmp::{max, min};
 use std::future::Future;
@@ -431,19 +431,31 @@ impl Storage for PhysicalStorage {
             self.decoder = WalStreamDecoder::new(startpos, pg_version);
         }
         self.decoder.feed_bytes(buf);
+        let mut xact_commit = false;
         loop {
             match self.decoder.poll_decode()? {
                 None => break, // no full record yet
-                Some((lsn, _rec)) => {
+                Some((lsn, rec)) => {
                     self.write_record_lsn = lsn;
                     if lsn <= self.flush_lsn {
                         debug_assert!(lsn > self.flush_lsn, "flush LSN regressed");
                         self.flush_record_lsn = lsn;
                     }
+                    // TODO: the decoder already has the record header, make it return it.
+                    let header = XLogRecord::from_slice(&rec[0..XLOG_SIZE_OF_XLOG_RECORD])
+                        .expect("invalid record header");
+                    xact_commit = xact_commit || header.is_xact_commit();
                 }
             }
         }
 
+        // If a transaction committed, flush the WAL. This will emit an AppendResponse to the
+        // compute. Otherwise, with pipelined ingestion, the txn may have to wait until the next
+        // periodic flush in 1 second, causing commit latency.
+        if xact_commit {
+            self.flush_wal().await?;
+        }
+
         Ok(())
     }