Use callbacks in SlruScanDirectory for the actual action

Previously, the code assumed that the only possible action to take was
to delete files behind a certain cutoff point.  The async notify code
was already a crock: it used a different "pagePrecedes" function for
truncation than for regular operation.  By allowing it to pass a
callback to SlruScanDirectory it can do cleanly exactly what it needs to
do.

The clog.c code also had its own use for SlruScanDirectory, which is
made a bit simpler with this.

src/backend/access/transam/clog.c

@@ -606,7 +606,7 @@ TruncateCLOG(TransactionId oldestXact)
 	cutoffPage = TransactionIdToPage(oldestXact);
 
 	/* Check to see if there's any files that could be removed */
-	if (!SlruScanDirectory(ClogCtl, cutoffPage, false))
+	if (!SlruScanDirectory(ClogCtl, SlruScanDirCbReportPresence, &cutoffPage))
 		return;					/* nothing to remove */
 
 	/* Write XLOG record and flush XLOG to disk */

src/backend/access/transam/slru.c

@@ -132,6 +132,8 @@ static bool SlruPhysicalWritePage(SlruCtl ctl, int pageno, int slotno,
 static void SlruReportIOError(SlruCtl ctl, int pageno, TransactionId xid);
 static int	SlruSelectLRUPage(SlruCtl ctl, int pageno);
 
+static bool SlruScanDirCbDeleteCutoff(SlruCtl ctl, char *filename,
+						  int segpage, void *data);
 
 /*
  * Initialization of shared memory
@@ -1137,33 +1139,84 @@ restart:;
 	LWLockRelease(shared->ControlLock);
 
 	/* Now we can remove the old segment(s) */
-	(void) SlruScanDirectory(ctl, cutoffPage, true);
+	(void) SlruScanDirectory(ctl, SlruScanDirCbDeleteCutoff, &cutoffPage);
 }
 
 /*
- * SimpleLruTruncate subroutine: scan directory for removable segments.
- * Actually remove them iff doDeletions is true.  Return TRUE iff any
- * removable segments were found.  Note: no locking is needed.
- *
- * This can be called directly from clog.c, for reasons explained there.
+ * SlruScanDirectory callback
+ * 		This callback reports true if there's any segment prior to the one
+ * 		containing the page passed as "data".
  */
 bool
-SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions)
+SlruScanDirCbReportPresence(SlruCtl ctl, char *filename, int segpage, void *data)
+{
+	int		cutoffPage = *(int *) data;
+
+	cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT;
+
+	if (ctl->PagePrecedes(segpage, cutoffPage))
+		return true;	/* found one; don't iterate any more */
+
+	return false;	/* keep going */
+}
+
+/*
+ * SlruScanDirectory callback.
+ *		This callback deletes segments prior to the one passed in as "data".
+ */
+static bool
+SlruScanDirCbDeleteCutoff(SlruCtl ctl, char *filename, int segpage, void *data)
+{
+	char	path[MAXPGPATH];
+	int		cutoffPage = *(int *) data;
+
+	if (ctl->PagePrecedes(segpage, cutoffPage))
+	{
+		snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, filename);
+		ereport(DEBUG2,
+				(errmsg("removing file \"%s\"", path)));
+		unlink(path);
+	}
+
+	return false;	/* keep going */
+}
+
+/*
+ * SlruScanDirectory callback.
+ *		This callback deletes all segments.
+ */
+bool
+SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int segpage, void *data)
+{
+	char	path[MAXPGPATH];
+
+	snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, filename);
+	ereport(DEBUG2,
+			(errmsg("removing file \"%s\"", path)));
+	unlink(path);
+
+	return false;	/* keep going */
+}
+
+/*
+ * Scan the SimpleLRU directory and apply a callback to each file found in it.
+ *
+ * If the callback returns true, the scan is stopped.  The last return value
+ * from the callback is returned.
+ *
+ * Note that the ordering in which the directory is scanned is not guaranteed.
+ *
+ * Note that no locking is applied.
+ */
+bool
+SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
 {
-	bool		found = false;
 	DIR		   *cldir;
 	struct dirent *clde;
 	int			segno;
 	int			segpage;
-	char		path[MAXPGPATH];
-
-	/*
-	 * The cutoff point is the start of the segment containing cutoffPage.
-	 * (This is redundant when called from SimpleLruTruncate, but not when
-	 * called directly from clog.c.)
-	 */
-	cutoffPage -= cutoffPage % SLRU_PAGES_PER_SEGMENT;
-
+	bool		retval;
+	
 	cldir = AllocateDir(ctl->Dir);
 	while ((clde = ReadDir(cldir, ctl->Dir)) != NULL)
 	{
@@ -1172,20 +1225,15 @@ SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions)
 		{
 			segno = (int) strtol(clde->d_name, NULL, 16);
 			segpage = segno * SLRU_PAGES_PER_SEGMENT;
-			if (ctl->PagePrecedes(segpage, cutoffPage))
-			{
-				found = true;
-				if (doDeletions)
-				{
-					snprintf(path, MAXPGPATH, "%s/%s", ctl->Dir, clde->d_name);
-					ereport(DEBUG2,
-							(errmsg("removing file \"%s\"", path)));
-					unlink(path);
-				}
-			}
+
+			elog(DEBUG2, "SlruScanDirectory invoking callback on %s/%s",
+				 ctl->Dir, clde->d_name);
+			retval = callback(ctl, clde->d_name, segpage, data);
+			if (retval)
+				break;
 		}
 	}
 	FreeDir(cldir);
 
-	return found;
+	return retval;
 }

src/backend/commands/async.c

@@ -194,7 +194,7 @@ typedef struct QueuePosition
 
 /* choose logically smaller QueuePosition */
 #define QUEUE_POS_MIN(x,y) \
-	(asyncQueuePagePrecedesLogically((x).page, (y).page) ? (x) : \
+	(asyncQueuePagePrecedes((x).page, (y).page) ? (x) : \
 	 (x).page != (y).page ? (y) : \
 	 (x).offset < (y).offset ? (x) : (y))
 
@@ -360,8 +360,7 @@ static bool backendHasExecutedInitialListen = false;
 bool		Trace_notify = false;
 
 /* local function prototypes */
-static bool asyncQueuePagePrecedesPhysically(int p, int q);
-static bool asyncQueuePagePrecedesLogically(int p, int q);
+static bool asyncQueuePagePrecedes(int p, int q);
 static void queue_listen(ListenActionKind action, const char *channel);
 static void Async_UnlistenOnExit(int code, Datum arg);
 static void Exec_ListenPreCommit(void);
@@ -388,25 +387,11 @@ static void NotifyMyFrontEnd(const char *channel,
 static bool AsyncExistsPendingNotify(const char *channel, const char *payload);
 static void ClearPendingActionsAndNotifies(void);
 
-
 /*
  * We will work on the page range of 0..QUEUE_MAX_PAGE.
- *
- * asyncQueuePagePrecedesPhysically just checks numerically without any magic
- * if one page precedes another one.  This is wrong for normal operation but
- * is helpful when clearing pg_notify/ during startup.
- *
- * asyncQueuePagePrecedesLogically compares using wraparound logic, as is
- * required by slru.c.
  */
 static bool
-asyncQueuePagePrecedesPhysically(int p, int q)
-{
-	return p < q;
-}
-
-static bool
-asyncQueuePagePrecedesLogically(int p, int q)
+asyncQueuePagePrecedes(int p, int q)
 {
 	int			diff;
 
@@ -484,7 +469,7 @@ AsyncShmemInit(void)
 	/*
 	 * Set up SLRU management of the pg_notify data.
 	 */
-	AsyncCtl->PagePrecedes = asyncQueuePagePrecedesLogically;
+	AsyncCtl->PagePrecedes = asyncQueuePagePrecedes;
 	SimpleLruInit(AsyncCtl, "Async Ctl", NUM_ASYNC_BUFFERS, 0,
 				  AsyncCtlLock, "pg_notify");
 	/* Override default assumption that writes should be fsync'd */
@@ -494,15 +479,8 @@ AsyncShmemInit(void)
 	{
 		/*
 		 * During start or reboot, clean out the pg_notify directory.
-		 *
-		 * Since we want to remove every file, we temporarily use
-		 * asyncQueuePagePrecedesPhysically() and pass INT_MAX as the
-		 * comparison value; every file in the directory should therefore
-		 * appear to be less than that.
 		 */
-		AsyncCtl->PagePrecedes = asyncQueuePagePrecedesPhysically;
-		(void) SlruScanDirectory(AsyncCtl, INT_MAX, true);
-		AsyncCtl->PagePrecedes = asyncQueuePagePrecedesLogically;
+		(void) SlruScanDirectory(AsyncCtl, SlruScanDirCbDeleteAll, NULL);
 
 		/* Now initialize page zero to empty */
 		LWLockAcquire(AsyncCtlLock, LW_EXCLUSIVE);
@@ -1223,7 +1201,7 @@ asyncQueueIsFull(void)
 		nexthead = 0;			/* wrap around */
 	boundary = QUEUE_POS_PAGE(QUEUE_TAIL);
 	boundary -= boundary % SLRU_PAGES_PER_SEGMENT;
-	return asyncQueuePagePrecedesLogically(nexthead, boundary);
+	return asyncQueuePagePrecedes(nexthead, boundary);
 }
 
 /*
@@ -2074,7 +2052,7 @@ asyncQueueAdvanceTail(void)
 	 */
 	newtailpage = QUEUE_POS_PAGE(min);
 	boundary = newtailpage - (newtailpage % SLRU_PAGES_PER_SEGMENT);
-	if (asyncQueuePagePrecedesLogically(oldtailpage, boundary))
+	if (asyncQueuePagePrecedes(oldtailpage, boundary))
 	{
 		/*
 		 * SimpleLruTruncate() will ask for AsyncCtlLock but will also release

src/include/access/slru.h

@@ -145,6 +145,15 @@ extern int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno,
 extern void SimpleLruWritePage(SlruCtl ctl, int slotno);
 extern void SimpleLruFlush(SlruCtl ctl, bool checkpoint);
 extern void SimpleLruTruncate(SlruCtl ctl, int cutoffPage);
-extern bool SlruScanDirectory(SlruCtl ctl, int cutoffPage, bool doDeletions);
+
+typedef bool (*SlruScanCallback) (SlruCtl ctl, char *filename, int segpage,
+					 void *data);
+extern bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data);
+
+/* SlruScanDirectory public callbacks */
+extern bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename,
+							int segpage, void *data);
+extern bool SlruScanDirCbDeleteAll(SlruCtl ctl, char *filename, int segpage,
+					   void *data);
 
 #endif   /* SLRU_H */