Add lookup table for replication slot conflict reasons

This commit switches the handling of the conflict cause strings for
replication slots to use a table rather than being explicitly listed,
using a C99-designated initializer syntax for the array elements.  This
makes the whole more readable while easing future maintenance with less
areas to update when adding a new conflict reason.

This is similar to 74a7306, but the scale of the change is smaller
as there are less conflict causes than LWLock builtin tranche names.

Author: Bharath Rupireddy
Reviewed-by: Jelte Fennema-Nio
Discussion: https://postgr.es/m/CALj2ACUxSLA91QGFrJsWNKs58KXb1C03mbuwKmzqqmoAKLwJaw@mail.gmail.com

Author: michaelpq

src/backend/replication/slot.c

@@ -77,6 +77,22 @@ typedef struct ReplicationSlotOnDisk
 	ReplicationSlotPersistentData slotdata;
 } ReplicationSlotOnDisk;
 
+/*
+ * Lookup table for slot invalidation causes.
+ */
+const char *const SlotInvalidationCauses[] = {
+	[RS_INVAL_NONE] = "none",
+	[RS_INVAL_WAL_REMOVED] = "wal_removed",
+	[RS_INVAL_HORIZON] = "rows_removed",
+	[RS_INVAL_WAL_LEVEL] = "wal_level_insufficient",
+};
+
+/* Maximum number of invalidation causes */
+#define	RS_INVAL_MAX_CAUSES RS_INVAL_WAL_LEVEL
+
+StaticAssertDecl(lengthof(SlotInvalidationCauses) == (RS_INVAL_MAX_CAUSES + 1),
+				 "array length mismatch");
+
 /* size of version independent data */
 #define ReplicationSlotOnDiskConstantSize \
 	offsetof(ReplicationSlotOnDisk, slotdata)
@@ -2290,23 +2306,26 @@ RestoreSlotFromDisk(const char *name)
 }
 
 /*
- * Maps the pg_replication_slots.conflict_reason text value to
- * ReplicationSlotInvalidationCause enum value
+ * Maps a conflict reason for a replication slot to
+ * ReplicationSlotInvalidationCause.
  */
 ReplicationSlotInvalidationCause
-GetSlotInvalidationCause(char *conflict_reason)
+GetSlotInvalidationCause(const char *conflict_reason)
 {
+	ReplicationSlotInvalidationCause cause;
+	bool		found PG_USED_FOR_ASSERTS_ONLY = false;
+
 	Assert(conflict_reason);
 
-	if (strcmp(conflict_reason, SLOT_INVAL_WAL_REMOVED_TEXT) == 0)
-		return RS_INVAL_WAL_REMOVED;
-	else if (strcmp(conflict_reason, SLOT_INVAL_HORIZON_TEXT) == 0)
-		return RS_INVAL_HORIZON;
-	else if (strcmp(conflict_reason, SLOT_INVAL_WAL_LEVEL_TEXT) == 0)
-		return RS_INVAL_WAL_LEVEL;
-	else
-		Assert(0);
+	for (cause = RS_INVAL_NONE; cause <= RS_INVAL_MAX_CAUSES; cause++)
+	{
+		if (strcmp(SlotInvalidationCauses[cause], conflict_reason) == 0)
+		{
+			found = true;
+			break;
+		}
+	}
 
-	/* Keep compiler quiet */
-	return RS_INVAL_NONE;
+	Assert(found);
+	return cause;
 }

src/backend/replication/slotfuncs.c

@@ -413,24 +413,12 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 			nulls[i++] = true;
 		else
 		{
-			switch (slot_contents.data.invalidated)
-			{
-				case RS_INVAL_NONE:
-					nulls[i++] = true;
-					break;
-
-				case RS_INVAL_WAL_REMOVED:
-					values[i++] = CStringGetTextDatum(SLOT_INVAL_WAL_REMOVED_TEXT);
-					break;
-
-				case RS_INVAL_HORIZON:
-					values[i++] = CStringGetTextDatum(SLOT_INVAL_HORIZON_TEXT);
-					break;
-
-				case RS_INVAL_WAL_LEVEL:
-					values[i++] = CStringGetTextDatum(SLOT_INVAL_WAL_LEVEL_TEXT);
-					break;
-			}
+			ReplicationSlotInvalidationCause cause = slot_contents.data.invalidated;
+
+			if (cause == RS_INVAL_NONE)
+				nulls[i++] = true;
+			else
+				values[i++] = CStringGetTextDatum(SlotInvalidationCauses[cause]);
 		}
 
 		values[i++] = BoolGetDatum(slot_contents.data.failover);

src/include/replication/slot.h

@@ -40,6 +40,9 @@ typedef enum ReplicationSlotPersistency
 /*
  * Slots can be invalidated, e.g. due to max_slot_wal_keep_size. If so, the
  * 'invalidated' field is set to a value other than _NONE.
+ *
+ * When adding a new invalidation cause here, remember to update
+ * SlotInvalidationCauses and RS_INVAL_MAX_CAUSES.
  */
 typedef enum ReplicationSlotInvalidationCause
 {
@@ -52,13 +55,7 @@ typedef enum ReplicationSlotInvalidationCause
 	RS_INVAL_WAL_LEVEL,
 } ReplicationSlotInvalidationCause;
 
-/*
- * The possible values for 'conflict_reason' returned in
- * pg_get_replication_slots.
- */
-#define SLOT_INVAL_WAL_REMOVED_TEXT "wal_removed"
-#define SLOT_INVAL_HORIZON_TEXT     "rows_removed"
-#define SLOT_INVAL_WAL_LEVEL_TEXT   "wal_level_insufficient"
+extern PGDLLIMPORT const char *const SlotInvalidationCauses[];
 
 /*
  * On-Disk data of a replication slot, preserved across restarts.
@@ -275,6 +272,6 @@ extern void CheckPointReplicationSlots(bool is_shutdown);
 extern void CheckSlotRequirements(void);
 extern void CheckSlotPermissions(void);
 extern ReplicationSlotInvalidationCause
-			GetSlotInvalidationCause(char *conflict_reason);
+			GetSlotInvalidationCause(const char *conflict_reason);
 
 #endif							/* SLOT_H */