Introduce logical decoding.

This feature, building on previous commits, allows the write-ahead log
stream to be decoded into a series of logical changes; that is,
inserts, updates, and deletes and the transactions which contain them.
It is capable of handling decoding even across changes to the schema
of the effected tables.  The output format is controlled by a
so-called "output plugin"; an example is included.  To make use of
this in a real replication system, the output plugin will need to be
modified to produce output in the format appropriate to that system,
and to perform filtering.

Currently, information can be extracted from the logical decoding
system only via SQL; future commits will add the ability to stream
changes via walsender.

Andres Freund, with review and other contributions from many other
people, including Álvaro Herrera, Abhijit Menon-Sen, Peter Gheogegan,
Kevin Grittner, Robert Haas, Heikki Linnakangas, Fujii Masao, Abhijit
Menon-Sen, Michael Paquier, Simon Riggs, Craig Ringer, and Steve
Singer.

1 files changed, 29 insertions, 1 deletions

diff --git a/src/backend/storage/ipc/standby.c b/src/backend/storage/ipc/standby.c
index fb5f18edfc7..aa8bea5538b 100644
--- a/src/backend/storage/ipc/standby.c
+++ b/src/backend/storage/ipc/standby.c
@@ -800,7 +800,9 @@ standby_redo(XLogRecPtr lsn, XLogRecord *record)
 
 /*
  * Log details of the current snapshot to WAL. This allows the snapshot state
- * to be reconstructed on the standby.
+ * to be reconstructed on the standby and for logical decoding.
+ *
+ * This is used for Hot Standby as follows:
  *
  * We can move directly to STANDBY_SNAPSHOT_READY at startup if we
  * start from a shutdown checkpoint because we know nothing was running
@@ -854,6 +856,12 @@ standby_redo(XLogRecPtr lsn, XLogRecord *record)
  * Zero xids should no longer be possible, but we may be replaying WAL
  * from a time when they were possible.
  *
+ * For logical decoding only the running xacts information is needed;
+ * there's no need to look at the locking information, but it's logged anyway,
+ * as there's no independent knob to just enable logical decoding. For
+ * details of how this is used, check snapbuild.c's introductory comment.
+ *
+ *
  * Returns the RecPtr of the last inserted record.
  */
 XLogRecPtr
@@ -879,8 +887,28 @@ LogStandbySnapshot(void)
 	 * record we write, because standby will open up when it sees this.
 	 */
 	running = GetRunningTransactionData();
+
+	/*
+	 * GetRunningTransactionData() acquired ProcArrayLock, we must release
+	 * it. For Hot Standby this can be done before inserting the WAL record
+	 * because ProcArrayApplyRecoveryInfo() rechecks the commit status using
+	 * the clog. For logical decoding, though, the lock can't be released
+	 * early becuase the clog might be "in the future" from the POV of the
+	 * historic snapshot. This would allow for situations where we're waiting
+	 * for the end of a transaction listed in the xl_running_xacts record
+	 * which, according to the WAL, have commit before the xl_running_xacts
+	 * record. Fortunately this routine isn't executed frequently, and it's
+	 * only a shared lock.
+	 */
+	if (wal_level < WAL_LEVEL_LOGICAL)
+		LWLockRelease(ProcArrayLock);
+
 	recptr = LogCurrentRunningXacts(running);
 
+	/* Release lock if we kept it longer ... */
+	if (wal_level >= WAL_LEVEL_LOGICAL)
+		LWLockRelease(ProcArrayLock);
+
 	/* GetRunningTransactionData() acquired XidGenLock, we must release it */
 	LWLockRelease(XidGenLock);