Disallow starting server with insufficient wal_level for existing slot.

Previously it was possible to create a slot, change wal_level, and
restart, even if the new wal_level was insufficient for the
slot. That's a problem for both logical and physical slots, because
the necessary WAL records are not generated.

This removes a few tests in newer versions that, somewhat
inexplicably, whether restarting with a too low wal_level worked (a
buggy behaviour!).

Reported-By: Joshua D. Drake
Author: Andres Freund
Discussion: https://postgr.es/m/20181029191304.lbsmhshkyymhw22w@alap3.anarazel.de
Backpatch: 9.4-, where replication slots where introduced

Author: anarazel

src/backend/replication/logical/logical.c

@@ -77,6 +77,11 @@ CheckLogicalDecodingRequirements(void)
 {
 	CheckSlotRequirements();
 
+	/*
+	 * NB: Adding a new requirement likely means that RestoreSlotFromDisk()
+	 * needs the same check.
+	 */
+
 	if (wal_level < WAL_LEVEL_LOGICAL)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),

src/backend/replication/slot.c

@@ -783,6 +783,11 @@ ReplicationSlotsCountDBSlots(Oid dboid, int *nslots, int *nactive)
 void
 CheckSlotRequirements(void)
 {
+	/*
+	 * NB: Adding a new requirement likely means that RestoreSlotFromDisk()
+	 * needs the same check.
+	 */
+
 	if (max_replication_slots == 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
@@ -1286,6 +1291,31 @@ RestoreSlotFromDisk(const char *name)
 		return;
 	}
 
+	/*
+	 * Verify that requirements for the specific slot type are met. That's
+	 * important because if these aren't met we're not guaranteed to retain
+	 * all the necessary resources for the slot.
+	 *
+	 * NB: We have to do so *after* the above checks for ephemeral slots,
+	 * because otherwise a slot that shouldn't exist anymore could prevent
+	 * restarts.
+	 *
+	 * NB: Changing the requirements here also requires adapting
+	 * CheckSlotRequirements() and CheckLogicalDecodingRequirements().
+	 */
+	if (cp.slotdata.database != InvalidOid && wal_level < WAL_LEVEL_LOGICAL)
+		ereport(FATAL,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("logical replication slots \"%s\" exists, but wal_level < logical",
+						NameStr(cp.slotdata.name)),
+				 errhint("Change wal_level to be replica or higher.")));
+	else if (wal_level < WAL_LEVEL_REPLICA)
+		ereport(FATAL,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("physical replication slots \"%s\" exists, but wal_level < replica",
+						NameStr(cp.slotdata.name)),
+				 errhint("Change wal_level to be replica or higher.")));
+
 	/* nothing can be active yet, don't lock anything */
 	for (i = 0; i < max_replication_slots; i++)
 	{