Generalize parallel slot result handling.

Instead of having a hard-coded behavior that we ignore missing
tables and report all other errors, let the caller decide what
to do by setting a callback.

Mark Dilger, reviewed and somewhat revised by me. The larger patch
series of which this is a part has also had review from Peter
Geoghegan, Andres Freund, Álvaro Herrera, Michael Paquier, and Amul
Sul, but I don't know whether any of them have reviewed this bit
specifically.

Discussion: http://postgr.es/m/12ED3DA8-25F0-4B68-937D-D907CFBF08E7@enterprisedb.com
Discussion: http://postgr.es/m/5F743835-3399-419C-8324-2D424237E999@enterprisedb.com
Discussion: http://postgr.es/m/70655DF3-33CE-4527-9A4D-DDEB582B6BA0@enterprisedb.com

src/bin/scripts/reindexdb.c

@@ -466,6 +466,7 @@ reindex_one_database(const ConnParams *cparams, ReindexType type,
 			goto finish;
 		}
 
+		ParallelSlotSetHandler(free_slot, TableCommandResultHandler, NULL);
 		run_reindex_command(free_slot->connection, process_type, objname,
 							echo, verbose, concurrently, true);
 

src/bin/scripts/vacuumdb.c

@@ -713,6 +713,7 @@ vacuum_one_database(const ConnParams *cparams,
 		 * Execute the vacuum.  All errors are handled in processQueryResult
 		 * through ParallelSlotsGetIdle.
 		 */
+		ParallelSlotSetHandler(free_slot, TableCommandResultHandler, NULL);
 		run_vacuum_command(free_slot->connection, sql.data,
 						   echo, tabname);
 

src/fe_utils/parallel_slot.c

@@ -30,7 +30,7 @@
 
 static void init_slot(ParallelSlot *slot, PGconn *conn);
 static int	select_loop(int maxFd, fd_set *workerset);
-static bool processQueryResult(PGconn *conn, PGresult *result);
+static bool processQueryResult(ParallelSlot *slot, PGresult *result);
 
 static void
 init_slot(ParallelSlot *slot, PGconn *conn)
@@ -38,34 +38,24 @@ init_slot(ParallelSlot *slot, PGconn *conn)
 	slot->connection = conn;
 	/* Initially assume connection is idle */
 	slot->isFree = true;
+	ParallelSlotClearHandler(slot);
 }
 
 /*
- * Process (and delete) a query result.  Returns true if there's no error,
- * false otherwise -- but errors about trying to work on a missing relation
- * are reported and subsequently ignored.
+ * Process (and delete) a query result.  Returns true if there's no problem,
+ * false otherwise. It's up to the handler to decide what cosntitutes a
+ * problem.
  */
 static bool
-processQueryResult(PGconn *conn, PGresult *result)
+processQueryResult(ParallelSlot *slot, PGresult *result)
 {
-	/*
-	 * If it's an error, report it.  Errors about a missing table are harmless
-	 * so we continue processing; but die for other errors.
-	 */
-	if (PQresultStatus(result) != PGRES_COMMAND_OK)
-	{
-		char	   *sqlState = PQresultErrorField(result, PG_DIAG_SQLSTATE);
+	Assert(slot->handler != NULL);
 
-		pg_log_error("processing of database \"%s\" failed: %s",
-					 PQdb(conn), PQerrorMessage(conn));
-
-		if (sqlState && strcmp(sqlState, ERRCODE_UNDEFINED_TABLE) != 0)
-		{
-			PQclear(result);
+	/* On failure, the handler should return NULL after freeing the result */
+	if (!slot->handler(result, slot->connection, slot->handler_context))
 		return false;
-		}
-	}
 
+	/* Ok, we have to free it ourself */
 	PQclear(result);
 	return true;
 }
@@ -76,15 +66,15 @@ processQueryResult(PGconn *conn, PGresult *result)
  * Note that this will block if the connection is busy.
  */
 static bool
-consumeQueryResult(PGconn *conn)
+consumeQueryResult(ParallelSlot *slot)
 {
 	bool		ok = true;
 	PGresult   *result;
 
-	SetCancelConn(conn);
-	while ((result = PQgetResult(conn)) != NULL)
+	SetCancelConn(slot->connection);
+	while ((result = PQgetResult(slot->connection)) != NULL)
 	{
-		if (!processQueryResult(conn, result))
+		if (!processQueryResult(slot, result))
 			ok = false;
 	}
 	ResetCancelConn();
@@ -227,14 +217,15 @@ ParallelSlotsGetIdle(ParallelSlot *slots, int numslots)
 
 				if (result != NULL)
 				{
-					/* Check and discard the command result */
-					if (!processQueryResult(slots[i].connection, result))
+					/* Handle and discard the command result */
+					if (!processQueryResult(slots + i, result))
 						return NULL;
 				}
 				else
 				{
 					/* This connection has become idle */
 					slots[i].isFree = true;
+					ParallelSlotClearHandler(slots + i);
 					if (firstFree < 0)
 						firstFree = i;
 					break;
@@ -329,9 +320,53 @@ ParallelSlotsWaitCompletion(ParallelSlot *slots, int numslots)
 
 	for (i = 0; i < numslots; i++)
 	{
-		if (!consumeQueryResult((slots + i)->connection))
+		if (!consumeQueryResult(slots + i))
+			return false;
+	}
+
+	return true;
+}
+
+/*
+ * TableCommandResultHandler
+ *
+ * ParallelSlotResultHandler for results of commands (not queries) against
+ * tables.
+ *
+ * Requires that the result status is either PGRES_COMMAND_OK or an error about
+ * a missing table.  This is useful for utilities that compile a list of tables
+ * to process and then run commands (vacuum, reindex, or whatever) against
+ * those tables, as there is a race condition between the time the list is
+ * compiled and the time the command attempts to open the table.
+ *
+ * For missing tables, logs an error but allows processing to continue.
+ *
+ * For all other errors, logs an error and terminates further processing.
+ *
+ * res: PGresult from the query executed on the slot's connection
+ * conn: connection belonging to the slot
+ * context: unused
+ */
+bool
+TableCommandResultHandler(PGresult *res, PGconn *conn, void *context)
+{
+	/*
+	 * If it's an error, report it.  Errors about a missing table are harmless
+	 * so we continue processing; but die for other errors.
+	 */
+	if (PQresultStatus(res) != PGRES_COMMAND_OK)
+	{
+		char	   *sqlState = PQresultErrorField(res, PG_DIAG_SQLSTATE);
+
+		pg_log_error("processing of database \"%s\" failed: %s",
+					 PQdb(conn), PQerrorMessage(conn));
+
+		if (sqlState && strcmp(sqlState, ERRCODE_UNDEFINED_TABLE) != 0)
+		{
+			PQclear(res);
 			return false;
 		}
+	}
 
 	return true;
 }

src/include/fe_utils/parallel_slot.h

@@ -15,12 +15,39 @@
 #include "fe_utils/connect_utils.h"
 #include "libpq-fe.h"
 
+typedef bool (*ParallelSlotResultHandler) (PGresult *res, PGconn *conn,
+										   void *context);
+
 typedef struct ParallelSlot
 {
 	PGconn	   *connection;		/* One connection */
 	bool		isFree;			/* Is it known to be idle? */
+
+	/*
+	 * Prior to issuing a command or query on 'connection', a handler callback
+	 * function may optionally be registered to be invoked to process the
+	 * results, and context information may optionally be registered for use
+	 * by the handler.  If unset, these fields should be NULL.
+	 */
+	ParallelSlotResultHandler handler;
+	void	   *handler_context;
 } ParallelSlot;
 
+static inline void
+ParallelSlotSetHandler(ParallelSlot *slot, ParallelSlotResultHandler handler,
+					   void *context)
+{
+	slot->handler = handler;
+	slot->handler_context = context;
+}
+
+static inline void
+ParallelSlotClearHandler(ParallelSlot *slot)
+{
+	slot->handler = NULL;
+	slot->handler_context = NULL;
+}
+
 extern ParallelSlot *ParallelSlotsGetIdle(ParallelSlot *slots, int numslots);
 
 extern ParallelSlot *ParallelSlotsSetup(const ConnParams *cparams,
@@ -31,5 +58,7 @@ extern void ParallelSlotsTerminate(ParallelSlot *slots, int numslots);
 
 extern bool ParallelSlotsWaitCompletion(ParallelSlot *slots, int numslots);
 
+extern bool TableCommandResultHandler(PGresult *res, PGconn *conn,
+									  void *context);
 
 #endif							/* PARALLEL_SLOT_H */