Fix parallel operations that prevent oldest xmin from advancing.

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

src/backend/access/heap/vacuumlazy.c

@@ -4159,6 +4159,12 @@ parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
 	LVRelState	vacrel;
 	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(MyProc->statusFlags == PROC_IN_VACUUM);
+
 	lvshared = (LVShared *) shm_toc_lookup(toc, PARALLEL_VACUUM_KEY_SHARED,
 										   false);
 	elevel = lvshared->elevel;

src/backend/access/nbtree/nbtsort.c

@@ -1808,6 +1808,13 @@ _bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
 		ResetUsage();
 #endif							/* BTREE_BUILD_STATS */
 
+	/*
+	 * The only possible status flag that can be set to the parallel worker is
+	 * PROC_IN_SAFE_IC.
+	 */
+	Assert((MyProc->statusFlags == 0) ||
+		   (MyProc->statusFlags == PROC_IN_SAFE_IC));
+
 	/* Set debug_query_string for individual workers first */
 	sharedquery = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, true);
 	debug_query_string = sharedquery;

src/backend/storage/ipc/procarray.c

@@ -2638,6 +2638,10 @@ ProcArrayInstallImportedXmin(TransactionId xmin,
  * PGPROC of the transaction from which we imported the snapshot, rather than
  * an XID.
  *
+ * Note that this function also copies statusFlags from the source `proc` in
+ * order to avoid the case where MyProc's xmin needs to be skipped for
+ * computing xid horizon.
+ *
  * Returns true if successful, false if source xact is no longer running.
  */
 bool
@@ -2649,8 +2653,10 @@ 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 statusFlags from source proc.
+	 */
+	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
 	/*
 	 * Be certain that the referenced PGPROC has an advertised xmin which is
@@ -2663,7 +2669,14 @@ ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
 		TransactionIdIsNormal(xid) &&
 		TransactionIdPrecedesOrEquals(xid, xmin))
 	{
+		/* Install xmin */
 		MyProc->xmin = TransactionXmin = xmin;
+
+		/* Flags being copied must be valid copy-able flags. */
+		Assert((proc->statusFlags & (~PROC_COPYABLE_FLAGS)) == 0);
+		MyProc->statusFlags = proc->statusFlags;
+		ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
+
 		result = true;
 	}
 

src/include/storage/proc.h

@@ -65,6 +65,13 @@ struct XidCache
 #define		PROC_VACUUM_STATE_MASK \
 	(PROC_IN_VACUUM | PROC_IN_SAFE_IC | PROC_VACUUM_FOR_WRAPAROUND)
 
+/*
+ * Flags that are valid to copy from another proc, the parallel leader
+ * process in practice.  Currently, flags that are set during parallel
+ * vacuum and parallel index creation are allowed.
+ */
+#define		PROC_COPYABLE_FLAGS (PROC_IN_VACUUM | PROC_IN_SAFE_IC)
+
 /*
  * We allow a small number of "weak" relation locks (AccessShareLock,
  * RowShareLock, RowExclusiveLock) to be recorded in the PGPROC structure