Fix parallel operations that prevent oldest xmin from advancing.

Amit Kapila · Amit Kapila · commit 33b6dd83e26f · 2021-11-19T09:24:00.000+05:30
While determining xid horizons, we skip over backends that are running Vacuum. We also ignore Create Index Concurrently, or Reindex Concurrently for the purposes of computing Xmin for Vacuum. But we were not setting the flags corresponding to these operations when they are performed in parallel which was preventing Xid horizon from advancing. The optimization related to skipping Create Index Concurrently, or Reindex Concurrently operations was implemented in PG-14 but the fix is the same for the Parallel Vacuum as well so back-patched till PG-13. Author: Masahiko Sawada Reviewed-by: Amit Kapila Backpatch-through: 13 Discussion: https://postgr.es/m/CAD21AoCLQqgM1sXh9BrDFq0uzd3RBFKi=Vfo6cjjKODm0Onr5w@mail.gmail.com
diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c
@@ -3482,6 +3482,12 @@ parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
 	LVRelStats	vacrelstats;
 	ErrorContextCallback errcallback;
 
+	/*
+	 * A parallel vacuum worker must have only PROC_IN_VACUUM flag since we
+	 * don't support parallel vacuum for autovacuum as of now.
+	 */
+	Assert(MyPgXact->vacuumFlags == PROC_IN_VACUUM);
+
 	lvshared = (LVShared *) shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_SHARED,
 										   false);
 	elevel = lvshared->elevel;
diff --git a/src/backend/storage/ipc/procarray.c b/src/backend/storage/ipc/procarray.c
@@ -1864,6 +1864,10 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
  * PGPROC of the transaction from which we imported the snapshot, rather than
  * an XID.
  *
+ * Note that this function also copies vacuumFlags from the source `proc` in
+ * order to avoid the case where MyPgXact's xmin needs to be skipped for
+ * computing xid horizon.
+ *
  * Returns true if successful, false if source xact is no longer running.
  */
 bool
@@ -1876,8 +1880,10 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 	Assert(TransactionIdIsNormal(xmin));
 	Assert(proc != NULL);
 
-	/* Get lock so source xact can't end while we're doing this */
-	LWLockAcquire(ProcArrayLock, LW_SHARED);
+	/*
+	 * Get an exclusive lock so that we can copy vacuumFlags from source proc.
+	 */
+	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
 	pgxact = &allPgXact[proc->pgprocno];
 
@@ -1892,7 +1898,13 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 		TransactionIdIsNormal(xid) &&
 		TransactionIdPrecedesOrEquals(xid, xmin))
 	{
+		/* Install xmin */
 		MyPgXact->xmin = TransactionXmin = xmin;
+
+		/* Flags being copied must be valid copy-able flags. */
+		Assert((pgxact->vacuumFlags & (~PROC_COPYABLE_FLAGS)) == 0);
+		MyPgXact->vacuumFlags = pgxact->vacuumFlags;
+
 		result = true;
 	}
 
diff --git a/src/include/storage/proc.h b/src/include/storage/proc.h
@@ -62,6 +62,13 @@ struct XidCache
 #define		PROC_VACUUM_STATE_MASK \
 	(PROC_IN_VACUUM | PROC_IN_ANALYZE | PROC_VACUUM_FOR_WRAPAROUND)
 
+/*
+ * Flags that are valid to copy from another proc, the parallel leader
+ * process in practice.  Currently, a flag that is set during parallel
+ * vacuum is allowed.
+ */
+#define		PROC_COPYABLE_FLAGS (PROC_IN_VACUUM)
+
 /*
  * We allow a small number of "weak" relation locks (AccessShareLock,
  * RowShareLock, RowExclusiveLock) to be recorded in the PGPROC structure
