Add a mechanism to let dynamically loaded modules register post-commit/

post-abort cleanup hooks.  I'm surprised that we have not needed this
already, but I need it now to fix a plpgsql problem, and the usefulness
for other dynamically loaded modules seems obvious.

src/backend/access/transam/xact.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/transam/xact.c,v 1.154 2003/09/25 06:57:57 petere Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/transam/xact.c,v 1.155 2003/09/28 23:26:20 tgl Exp $
  *
  * NOTES
  *		Transaction aborts can now occur two ways:
@@ -183,6 +183,7 @@ static void AtCommit_Memory(void);
 static void AtStart_Cache(void);
 static void AtStart_Locks(void);
 static void AtStart_Memory(void);
+static void CallEOXactCallbacks(bool isCommit);
 static void CleanupTransaction(void);
 static void CommitTransaction(void);
 static void RecordTransactionAbort(void);
@@ -217,6 +218,18 @@ int			CommitSiblings = 5; /* number of concurrent xacts needed to
 								 * sleep */
 
 
+/*
+ * List of add-on end-of-xact callbacks
+ */
+typedef struct EOXactCallbackItem
+{
+	struct EOXactCallbackItem *next;
+	EOXactCallback callback;
+	void	   *arg;
+} EOXactCallbackItem;
+
+static EOXactCallbackItem *EOXact_callbacks = NULL;
+
 static void (*_RollbackFunc) (void *) = NULL;
 static void *_RollbackData = NULL;
 
@@ -964,6 +977,7 @@ CommitTransaction(void)
 
 	AtCommit_Locks();
 
+	CallEOXactCallbacks(true);
 	AtEOXact_GUC(true);
 	AtEOXact_SPI();
 	AtEOXact_gist();
@@ -1073,6 +1087,7 @@ AbortTransaction(void)
 
 	AtAbort_Locks();
 
+	CallEOXactCallbacks(false);
 	AtEOXact_GUC(false);
 	AtEOXact_SPI();
 	AtEOXact_gist();
@@ -1430,6 +1445,62 @@ RequireTransactionChain(void *stmtNode, const char *stmtType)
 }
 
 
+/*
+ * Register or deregister callback functions for end-of-xact cleanup
+ *
+ * These functions are intended for use by dynamically loaded modules.
+ * For built-in modules we generally just hardwire the appropriate calls
+ * (mainly because it's easier to control the order that way, where needed).
+ *
+ * Note that the callback occurs post-commit or post-abort, so the callback
+ * functions can only do noncritical cleanup.
+ */
+void
+RegisterEOXactCallback(EOXactCallback callback, void *arg)
+{
+	EOXactCallbackItem *item;
+
+	item = (EOXactCallbackItem *)
+		MemoryContextAlloc(TopMemoryContext, sizeof(EOXactCallbackItem));
+	item->callback = callback;
+	item->arg = arg;
+	item->next = EOXact_callbacks;
+	EOXact_callbacks = item;
+}
+
+void
+UnregisterEOXactCallback(EOXactCallback callback, void *arg)
+{
+	EOXactCallbackItem *item;
+	EOXactCallbackItem *prev;
+
+	prev = NULL;
+	for (item = EOXact_callbacks; item; prev = item, item = item->next)
+	{
+		if (item->callback == callback && item->arg == arg)
+		{
+			if (prev)
+				prev->next = item->next;
+			else
+				EOXact_callbacks = item->next;
+			pfree(item);
+			break;
+		}
+	}
+}
+
+static void
+CallEOXactCallbacks(bool isCommit)
+{
+	EOXactCallbackItem *item;
+
+	for (item = EOXact_callbacks; item; item = item->next)
+	{
+		(*item->callback) (isCommit, item->arg);
+	}
+}
+
+
 /* ----------------------------------------------------------------
  *					   transaction block support
  * ----------------------------------------------------------------

src/include/access/xact.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: xact.h,v 1.55 2003/08/08 21:42:32 momjian Exp $
+ * $Id: xact.h,v 1.56 2003/09/28 23:26:20 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -59,9 +59,13 @@ typedef enum TBlockState
 	TBLOCK_ENDABORT
 } TBlockState;
 
-/* ----------------
+/*
+ *	end-of-transaction cleanup callbacks for dynamically loaded modules
+ */
+typedef void (*EOXactCallback) (bool isCommit, void *arg);
+
+/*
  *	transaction state structure
- * ----------------
  */
 typedef struct TransactionStateData
 {
@@ -130,6 +134,8 @@ extern void UserAbortTransactionBlock(void);
 extern void AbortOutOfAnyTransaction(void);
 extern void PreventTransactionChain(void *stmtNode, const char *stmtType);
 extern void RequireTransactionChain(void *stmtNode, const char *stmtType);
+extern void RegisterEOXactCallback(EOXactCallback callback, void *arg);
+extern void UnregisterEOXactCallback(EOXactCallback callback, void *arg);
 
 extern void RecordTransactionCommit(void);