From 5c21ad07cc06f00f96e9401209a7da839b326d3a Mon Sep 17 00:00:00 2001
From: Andres Freund <andres@anarazel.de>
Date: Wed, 5 Apr 2017 20:56:35 -0700
Subject: [PATCH] Code review for recent slot.c changes.

---
 src/backend/replication/slot.c | 48 ++++++++++++++++------------------
 1 file changed, 22 insertions(+), 26 deletions(-)

diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index 6c5ec7a00e..6ac0ce69c6 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -410,7 +410,7 @@ ReplicationSlotRelease(void)
  * Cleanup all temporary slots created in current session.
  */
 void
-ReplicationSlotCleanup()
+ReplicationSlotCleanup(void)
 {
 	int			i;
 
@@ -802,12 +802,12 @@ ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
  * pg_database oid for the database to prevent creation of new slots on the db
  * or replay from existing slots.
  *
- * This routine isn't as efficient as it could be - but we don't drop databases
- * often, especially databases with lots of slots.
- *
  * Another session that concurrently acquires an existing slot on the target DB
  * (most likely to drop it) may cause this function to ERROR. If that happens
  * it may have dropped some but not all slots.
+ *
+ * This routine isn't as efficient as it could be - but we don't drop
+ * databases often, especially databases with lots of slots.
  */
 void
 ReplicationSlotsDropDBSlots(Oid dboid)
@@ -822,7 +822,7 @@ restart:
 	for (i = 0; i < max_replication_slots; i++)
 	{
 		ReplicationSlot *s;
-		NameData slotname;
+		char *slotname;
 		int active_pid;
 
 		s = &ReplicationSlotCtl->replication_slots[i];
@@ -839,10 +839,10 @@ restart:
 		if (s->data.database != dboid)
 			continue;
 
-		/* Claim the slot, as if ReplicationSlotAcquire()ing. */
+		/* acquire slot, so ReplicationSlotDropAcquired can be reused  */
 		SpinLockAcquire(&s->mutex);
-		strncpy(NameStr(slotname), NameStr(s->data.name), NAMEDATALEN);
-		NameStr(slotname)[NAMEDATALEN-1] = '\0';
+		/* can't change while ReplicationSlotControlLock is held */
+		slotname = NameStr(s->data.name);
 		active_pid = s->active_pid;
 		if (active_pid == 0)
 		{
@@ -852,36 +852,32 @@ restart:
 		SpinLockRelease(&s->mutex);
 
 		/*
-		 * We might fail here if the slot was active. Even though we hold an
-		 * exclusive lock on the database object a logical slot for that DB can
-		 * still be active if it's being dropped by a backend connected to
-		 * another DB or is otherwise acquired.
+		 * Even though we hold an exclusive lock on the database object a
+		 * logical slot for that DB can still be active, e.g. if it's
+		 * concurrently being dropped by a backend connected to another DB.
 		 *
-		 * It's an unlikely race that'll only arise from concurrent user action,
-		 * so we'll just bail out.
+		 * That's fairly unlikely in practice, so we'll just bail out.
 		 */
 		if (active_pid)
-			elog(ERROR, "replication slot %s is in use by pid %d",
-						NameStr(slotname), active_pid);
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_IN_USE),
+					 errmsg("replication slot \"%s\" is active for PID %d",
+							slotname, active_pid)));
 
 		/*
-		 * To avoid largely duplicating ReplicationSlotDropAcquired() or
-		 * complicating it with already_locked flags for ProcArrayLock,
-		 * ReplicationSlotControlLock and ReplicationSlotAllocationLock, we
-		 * just release our ReplicationSlotControlLock to drop the slot.
+		 * To avoid duplicating ReplicationSlotDropAcquired() and to avoid
+		 * holding ReplicationSlotControlLock over filesystem operations,
+		 * release ReplicationSlotControlLock and use
+		 * ReplicationSlotDropAcquired.
 		 *
-		 * For safety we'll restart our scan from the beginning each
-		 * time we release the lock.
+		 * As that means the set of slots could change, restart scan from the
+		 * beginning each time we release the lock.
 		 */
 		LWLockRelease(ReplicationSlotControlLock);
 		ReplicationSlotDropAcquired();
 		goto restart;
 	}
 	LWLockRelease(ReplicationSlotControlLock);
-
-	/* recompute limits once after all slots are dropped */
-	ReplicationSlotsComputeRequiredXmin(false);
-	ReplicationSlotsComputeRequiredLSN();
 }
 
 
-- 
2.40.0