Track invalidation_reason in pg_replication_slots.

Till now, the reason for replication slot invalidation is not tracked
directly in pg_replication_slots. A recent commit 007693f2a3 added
'conflict_reason' to show the reasons for slot conflict/invalidation, but
only for logical slots.

This commit adds a new column 'invalidation_reason' to show invalidation
reasons for both physical and logical slots. And, this commit also turns
'conflict_reason' text column to 'conflicting' boolean column (effectively
reverting commit 007693f2a3). The 'conflicting' column is true for
invalidation reasons 'rows_removed' and 'wal_level_insufficient' because
those make the slot conflict with recovery. When 'conflicting' is true,
one can now look at the new 'invalidation_reason' column for the reason
for the logical slot's conflict with recovery.

The new 'invalidation_reason' column will also be useful to track other
invalidation reasons in the future commit.

Author: Bharath Rupireddy
Reviewed-by: Bertrand Drouvot, Amit Kapila, Shveta Malik
Discussion: https://www.postgresql.org/message-id/ZfR7HuzFEswakt/a%40ip-10-97-1-34.eu-west-3.compute.internal
Discussion: https://www.postgresql.org/message-id/CALj2ACW4aUe-_uFQOjdWCEN-xXoLGhmvRFnL8SNw_TZ5nJe+aw@mail.gmail.com

4 files changed, 44 insertions, 35 deletions

diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 04227a72d10..f69b7f55801 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1023,7 +1023,8 @@ CREATE VIEW pg_replication_slots AS
             L.wal_status,
             L.safe_wal_size,
             L.two_phase,
-            L.conflict_reason,
+            L.conflicting,
+            L.invalidation_reason,
             L.failover,
             L.synced
     FROM pg_get_replication_slots() AS L
diff --git a/src/backend/replication/logical/slotsync.c b/src/backend/replication/logical/slotsync.c
index 7b180bdb5c8..30480960c55 100644
--- a/src/backend/replication/logical/slotsync.c
+++ b/src/backend/replication/logical/slotsync.c
@@ -663,7 +663,7 @@ synchronize_slots(WalReceiverConn *wrconn)
 	bool		started_tx = false;
 	const char *query = "SELECT slot_name, plugin, confirmed_flush_lsn,"
 		" restart_lsn, catalog_xmin, two_phase, failover,"
-		" database, conflict_reason"
+		" database, invalidation_reason"
 		" FROM pg_catalog.pg_replication_slots"
 		" WHERE failover and NOT temporary";
 
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 91ca397857a..cdf0c450c59 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -1525,14 +1525,14 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 	XLogRecPtr	initial_effective_xmin = InvalidXLogRecPtr;
 	XLogRecPtr	initial_catalog_effective_xmin = InvalidXLogRecPtr;
 	XLogRecPtr	initial_restart_lsn = InvalidXLogRecPtr;
-	ReplicationSlotInvalidationCause conflict_prev PG_USED_FOR_ASSERTS_ONLY = RS_INVAL_NONE;
+	ReplicationSlotInvalidationCause invalidation_cause_prev PG_USED_FOR_ASSERTS_ONLY = RS_INVAL_NONE;
 
 	for (;;)
 	{
 		XLogRecPtr	restart_lsn;
 		NameData	slotname;
 		int			active_pid = 0;
-		ReplicationSlotInvalidationCause conflict = RS_INVAL_NONE;
+		ReplicationSlotInvalidationCause invalidation_cause = RS_INVAL_NONE;
 
 		Assert(LWLockHeldByMeInMode(ReplicationSlotControlLock, LW_SHARED));
 
@@ -1554,17 +1554,14 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 
 		restart_lsn = s->data.restart_lsn;
 
-		/*
-		 * If the slot is already invalid or is a non conflicting slot, we
-		 * don't need to do anything.
-		 */
+		/* we do nothing if the slot is already invalid */
 		if (s->data.invalidated == RS_INVAL_NONE)
 		{
 			/*
 			 * The slot's mutex will be released soon, and it is possible that
 			 * those values change since the process holding the slot has been
 			 * terminated (if any), so record them here to ensure that we
-			 * would report the correct conflict cause.
+			 * would report the correct invalidation cause.
 			 */
 			if (!terminated)
 			{
@@ -1578,7 +1575,7 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 				case RS_INVAL_WAL_REMOVED:
 					if (initial_restart_lsn != InvalidXLogRecPtr &&
 						initial_restart_lsn < oldestLSN)
-						conflict = cause;
+						invalidation_cause = cause;
 					break;
 				case RS_INVAL_HORIZON:
 					if (!SlotIsLogical(s))
@@ -1589,15 +1586,15 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 					if (TransactionIdIsValid(initial_effective_xmin) &&
 						TransactionIdPrecedesOrEquals(initial_effective_xmin,
 													  snapshotConflictHorizon))
-						conflict = cause;
+						invalidation_cause = cause;
 					else if (TransactionIdIsValid(initial_catalog_effective_xmin) &&
 							 TransactionIdPrecedesOrEquals(initial_catalog_effective_xmin,
 														   snapshotConflictHorizon))
-						conflict = cause;
+						invalidation_cause = cause;
 					break;
 				case RS_INVAL_WAL_LEVEL:
 					if (SlotIsLogical(s))
-						conflict = cause;
+						invalidation_cause = cause;
 					break;
 				case RS_INVAL_NONE:
 					pg_unreachable();
@@ -1605,14 +1602,14 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 		}
 
 		/*
-		 * The conflict cause recorded previously should not change while the
-		 * process owning the slot (if any) has been terminated.
+		 * The invalidation cause recorded previously should not change while
+		 * the process owning the slot (if any) has been terminated.
 		 */
-		Assert(!(conflict_prev != RS_INVAL_NONE && terminated &&
-				 conflict_prev != conflict));
+		Assert(!(invalidation_cause_prev != RS_INVAL_NONE && terminated &&
+				 invalidation_cause_prev != invalidation_cause));
 
-		/* if there's no conflict, we're done */
-		if (conflict == RS_INVAL_NONE)
+		/* if there's no invalidation, we're done */
+		if (invalidation_cause == RS_INVAL_NONE)
 		{
 			SpinLockRelease(&s->mutex);
 			if (released_lock)
@@ -1632,13 +1629,13 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 		{
 			MyReplicationSlot = s;
 			s->active_pid = MyProcPid;
-			s->data.invalidated = conflict;
+			s->data.invalidated = invalidation_cause;
 
 			/*
 			 * XXX: We should consider not overwriting restart_lsn and instead
 			 * just rely on .invalidated.
 			 */
-			if (conflict == RS_INVAL_WAL_REMOVED)
+			if (invalidation_cause == RS_INVAL_WAL_REMOVED)
 				s->data.restart_lsn = InvalidXLogRecPtr;
 
 			/* Let caller know */
@@ -1681,7 +1678,7 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 			 */
 			if (last_signaled_pid != active_pid)
 			{
-				ReportSlotInvalidation(conflict, true, active_pid,
+				ReportSlotInvalidation(invalidation_cause, true, active_pid,
 									   slotname, restart_lsn,
 									   oldestLSN, snapshotConflictHorizon);
 
@@ -1694,7 +1691,7 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 
 				last_signaled_pid = active_pid;
 				terminated = true;
-				conflict_prev = conflict;
+				invalidation_cause_prev = invalidation_cause;
 			}
 
 			/* Wait until the slot is released. */
@@ -1727,7 +1724,7 @@ InvalidatePossiblyObsoleteSlot(ReplicationSlotInvalidationCause cause,
 			ReplicationSlotSave();
 			ReplicationSlotRelease();
 
-			ReportSlotInvalidation(conflict, false, active_pid,
+			ReportSlotInvalidation(invalidation_cause, false, active_pid,
 								   slotname, restart_lsn,
 								   oldestLSN, snapshotConflictHorizon);
 
@@ -2356,21 +2353,21 @@ RestoreSlotFromDisk(const char *name)
 }
 
 /*
- * Maps a conflict reason for a replication slot to
+ * Maps an invalidation reason for a replication slot to
  * ReplicationSlotInvalidationCause.
  */
 ReplicationSlotInvalidationCause
-GetSlotInvalidationCause(const char *conflict_reason)
+GetSlotInvalidationCause(const char *invalidation_reason)
 {
 	ReplicationSlotInvalidationCause cause;
 	ReplicationSlotInvalidationCause result = RS_INVAL_NONE;
 	bool		found PG_USED_FOR_ASSERTS_ONLY = false;
 
-	Assert(conflict_reason);
+	Assert(invalidation_reason);
 
 	for (cause = RS_INVAL_NONE; cause <= RS_INVAL_MAX_CAUSES; cause++)
 	{
-		if (strcmp(SlotInvalidationCauses[cause], conflict_reason) == 0)
+		if (strcmp(SlotInvalidationCauses[cause], invalidation_reason) == 0)
 		{
 			found = true;
 			result = cause;
diff --git a/src/backend/replication/slotfuncs.c b/src/backend/replication/slotfuncs.c
index ad79e1fccd6..4232c1e52e5 100644
--- a/src/backend/replication/slotfuncs.c
+++ b/src/backend/replication/slotfuncs.c
@@ -239,7 +239,7 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 Datum
 pg_get_replication_slots(PG_FUNCTION_ARGS)
 {
-#define PG_GET_REPLICATION_SLOTS_COLS 17
+#define PG_GET_REPLICATION_SLOTS_COLS 18
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 	XLogRecPtr	currlsn;
 	int			slotno;
@@ -263,6 +263,7 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		bool		nulls[PG_GET_REPLICATION_SLOTS_COLS];
 		WALAvailability walstate;
 		int			i;
+		ReplicationSlotInvalidationCause cause;
 
 		if (!slot->in_use)
 			continue;
@@ -409,18 +410,28 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 
 		values[i++] = BoolGetDatum(slot_contents.data.two_phase);
 
-		if (slot_contents.data.database == InvalidOid)
+		cause = slot_contents.data.invalidated;
+
+		if (SlotIsPhysical(&slot_contents))
 			nulls[i++] = true;
 		else
 		{
-			ReplicationSlotInvalidationCause cause = slot_contents.data.invalidated;
-
-			if (cause == RS_INVAL_NONE)
-				nulls[i++] = true;
+			/*
+			 * rows_removed and wal_level_insufficient are the only two
+			 * reasons for the logical slot's conflict with recovery.
+			 */
+			if (cause == RS_INVAL_HORIZON ||
+				cause == RS_INVAL_WAL_LEVEL)
+				values[i++] = BoolGetDatum(true);
 			else
-				values[i++] = CStringGetTextDatum(SlotInvalidationCauses[cause]);
+				values[i++] = BoolGetDatum(false);
 		}
 
+		if (cause == RS_INVAL_NONE)
+			nulls[i++] = true;
+		else
+			values[i++] = CStringGetTextDatum(SlotInvalidationCauses[cause]);
+
 		values[i++] = BoolGetDatum(slot_contents.data.failover);
 
 		values[i++] = BoolGetDatum(slot_contents.data.synced);