During Hot Standby, fix drop database when sessions idle.

Previously we only cancelled sessions that were in-transaction.

Simple fix is to just cancel all sessions without waiting. Doing
it this way avoids complicating common code paths, which would
not be worth the trouble to cover this rare case.

Problem report and fix by Andres Freund, edited somewhat by me

src/backend/commands/dbcommands.c

@@ -13,7 +13,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.230 2010/01/02 16:57:37 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/dbcommands.c,v 1.231 2010/01/10 15:44:28 sriggs Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1945,22 +1945,27 @@ dbase_redo(XLogRecPtr lsn, XLogRecord *record)
 
 		if (InHotStandby)
 		{
-			VirtualTransactionId *database_users;
-
 			/*
-			 * Find all users connected to this database and ask them
-			 * politely to immediately kill their sessions before processing
-			 * the drop database record, after the usual grace period.
-			 * We don't wait for commit because drop database is
-			 * non-transactional.
+			 * We don't do ResolveRecoveryConflictWithVirutalXIDs() here since
+			 * that only waits for transactions and completely idle sessions
+			 * would block us. This is rare enough that we do this as simply
+			 * as possible: no wait, just force them off immediately. 
+			 *
+			 * No locking is required here because we already acquired
+			 * AccessExclusiveLock. Anybody trying to connect while we do this
+			 * will block during InitPostgres() and then disconnect when they
+			 * see the database has been removed.
 			 */
-		    database_users = GetConflictingVirtualXIDs(InvalidTransactionId,
-													   xlrec->db_id,
-													   false);
+			while (CountDBBackends(xlrec->db_id) > 0)
+			{
+				CancelDBBackends(xlrec->db_id);
 
-			ResolveRecoveryConflictWithVirtualXIDs(database_users,
-												   "drop database",
-												   CONFLICT_MODE_FATAL);
+				/*
+				 * Wait awhile for them to die so that we avoid flooding an
+				 * unresponsive backend when system is heavily loaded.
+				 */
+				pg_usleep(10000);
+			}
 		}
 
 		/* Drop pages for this database that are in the shared buffer cache */

src/backend/storage/ipc/procarray.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.54 2010/01/02 16:57:51 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.55 2010/01/10 15:44:28 sriggs Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1826,6 +1826,32 @@ CountDBBackends(Oid databaseid)
 	return count;
 }
 
+/*
+ * CancelDBBackends --- cancel backends that are using specified database
+ */
+void
+CancelDBBackends(Oid databaseid)
+{
+	ProcArrayStruct *arrayP = procArray;
+	int			index;
+
+	/* tell all backends to die */
+	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
+
+	for (index = 0; index < arrayP->numProcs; index++)
+	{
+		volatile PGPROC *proc = arrayP->procs[index];
+
+		if (proc->databaseId == databaseid)
+		{
+			proc->recoveryConflictMode = CONFLICT_MODE_FATAL;
+			kill(proc->pid, SIGINT);
+		}
+	}
+
+	LWLockRelease(ProcArrayLock);
+}
+
 /*
  * CountUserBackends --- count backends that are used by specified user
  */

src/include/storage/procarray.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/procarray.h,v 1.28 2010/01/02 16:58:08 momjian Exp $
+ * $PostgreSQL: pgsql/src/include/storage/procarray.h,v 1.29 2010/01/10 15:44:28 sriggs Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -63,6 +63,7 @@ extern pid_t CancelVirtualTransaction(VirtualTransactionId vxid,
 
 extern int	CountActiveBackends(void);
 extern int	CountDBBackends(Oid databaseid);
+extern void	CancelDBBackends(Oid databaseid);
 extern int	CountUserBackends(Oid roleid);
 extern bool CountOtherDBBackends(Oid databaseId,
 					 int *nbackends, int *nprepared);