Adjust walsender usage of xlogreader, simplify APIs

* Have both physical and logical walsender share a 'xlogreader' state
  struct for tracking state.  This replaces the existing globals sendSeg
  and sendCxt.

* Change WALRead not to receive XLogReaderState->seg and ->segcxt as
  separate arguments anymore; just use the ones from 'state'.  This is
  made possible by the above change.

* have the XLogReader segment_open contract require the callbacks to
  install the file descriptor in the state struct themselves instead of
  returning it.  xlogreader was already ignoring any possible failed
  return from the callbacks, relying solely on them never returning.

  (This point is not altogether excellent, as it means the callbacks
  have to know more of XLogReaderState; but to really improve on that
  we would have to pass back error info from the callbacks to
  xlogreader.  And the complexity would not be saved but instead just
  transferred to the callers of WALRead, which would have to learn how
  to throw errors from the open_segment callback in addition of, as
  currently, from pg_pread.)

* segment_open no longer receives the 'segcxt' as a separate argument,
  since it's part of the XLogReaderState argument.

Per comments from Kyotaro Horiguchi.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200511203336.GA9913@alvherre.pgsql

src/backend/access/transam/xlogreader.c

@@ -1044,14 +1044,12 @@ err:
 
 /*
  * Helper function to ease writing of XLogRoutine->page_read callbacks.
- * If this function is used, caller must supply an open_segment callback in
+ * If this function is used, caller must supply a segment_open callback in
  * 'state', as that is used here.
  *
  * Read 'count' bytes into 'buf', starting at location 'startptr', from WAL
  * fetched from timeline 'tli'.
  *
- * 'seg/segcxt' identify the last segment used.
- *
  * Returns true if succeeded, false if an error occurs, in which case
  * 'errinfo' receives error details.
  *
@@ -1061,7 +1059,6 @@ err:
 bool
 WALRead(XLogReaderState *state,
 		char *buf, XLogRecPtr startptr, Size count, TimeLineID tli,
-		WALOpenSegment *seg, WALSegmentContext *segcxt,
 		WALReadError *errinfo)
 {
 	char	   *p;
@@ -1078,34 +1075,36 @@ WALRead(XLogReaderState *state,
 		int			segbytes;
 		int			readbytes;
 
-		startoff = XLogSegmentOffset(recptr, segcxt->ws_segsize);
+		startoff = XLogSegmentOffset(recptr, state->segcxt.ws_segsize);
 
 		/*
 		 * If the data we want is not in a segment we have open, close what we
 		 * have (if anything) and open the next one, using the caller's
 		 * provided openSegment callback.
 		 */
-		if (seg->ws_file < 0 ||
-			!XLByteInSeg(recptr, seg->ws_segno, segcxt->ws_segsize) ||
-			tli != seg->ws_tli)
+		if (state->seg.ws_file < 0 ||
+			!XLByteInSeg(recptr, state->seg.ws_segno, state->segcxt.ws_segsize) ||
+			tli != state->seg.ws_tli)
 		{
 			XLogSegNo	nextSegNo;
 
-			if (seg->ws_file >= 0)
+			if (state->seg.ws_file >= 0)
 				state->routine.segment_close(state);
 
-			XLByteToSeg(recptr, nextSegNo, segcxt->ws_segsize);
-			seg->ws_file = state->routine.segment_open(state, nextSegNo,
-													   segcxt, &tli);
+			XLByteToSeg(recptr, nextSegNo, state->segcxt.ws_segsize);
+			state->routine.segment_open(state, nextSegNo, &tli);
+
+			/* This shouldn't happen -- indicates a bug in segment_open */
+			Assert(state->seg.ws_file >= 0);
 
 			/* Update the current segment info. */
-			seg->ws_tli = tli;
-			seg->ws_segno = nextSegNo;
+			state->seg.ws_tli = tli;
+			state->seg.ws_segno = nextSegNo;
 		}
 
 		/* How many bytes are within this segment? */
-		if (nbytes > (segcxt->ws_segsize - startoff))
-			segbytes = segcxt->ws_segsize - startoff;
+		if (nbytes > (state->segcxt.ws_segsize - startoff))
+			segbytes = state->segcxt.ws_segsize - startoff;
 		else
 			segbytes = nbytes;
 
@@ -1115,7 +1114,7 @@ WALRead(XLogReaderState *state,
 
 		/* Reset errno first; eases reporting non-errno-affecting errors */
 		errno = 0;
-		readbytes = pg_pread(seg->ws_file, p, segbytes, (off_t) startoff);
+		readbytes = pg_pread(state->seg.ws_file, p, segbytes, (off_t) startoff);
 
 #ifndef FRONTEND
 		pgstat_report_wait_end();
@@ -1127,7 +1126,7 @@ WALRead(XLogReaderState *state,
 			errinfo->wre_req = segbytes;
 			errinfo->wre_read = readbytes;
 			errinfo->wre_off = startoff;
-			errinfo->wre_seg = *seg;
+			errinfo->wre_seg = state->seg;
 			return false;
 		}
 

src/backend/access/transam/xlogutils.c

@@ -784,18 +784,17 @@ XLogReadDetermineTimeline(XLogReaderState *state, XLogRecPtr wantPage, uint32 wa
 }
 
 /* XLogReaderRoutine->segment_open callback for local pg_wal files */
-int
+void
 wal_segment_open(XLogReaderState *state, XLogSegNo nextSegNo,
-				 WALSegmentContext *segcxt, TimeLineID *tli_p)
+				 TimeLineID *tli_p)
 {
 	TimeLineID	tli = *tli_p;
 	char		path[MAXPGPATH];
-	int			fd;
 
-	XLogFilePath(path, tli, nextSegNo, segcxt->ws_segsize);
-	fd = BasicOpenFile(path, O_RDONLY | PG_BINARY);
-	if (fd >= 0)
-		return fd;
+	XLogFilePath(path, tli, nextSegNo, state->segcxt.ws_segsize);
+	state->seg.ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
+	if (state->seg.ws_file >= 0)
+		return;
 
 	if (errno == ENOENT)
 		ereport(ERROR,
@@ -807,8 +806,6 @@ wal_segment_open(XLogReaderState *state, XLogSegNo nextSegNo,
 				(errcode_for_file_access(),
 				 errmsg("could not open file \"%s\": %m",
 						path)));
-
-	return -1;					/* keep compiler quiet */
 }
 
 /* stock XLogReaderRoutine->segment_close callback */
@@ -947,7 +944,6 @@ read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
 	 * zero-padded up to the page boundary if it's incomplete.
 	 */
 	if (!WALRead(state, cur_page, targetPagePtr, XLOG_BLCKSZ, tli,
-				 &state->seg, &state->segcxt,
 				 &errinfo))
 		WALReadRaiseError(&errinfo);
 

src/backend/replication/walsender.c

@@ -129,8 +129,14 @@ bool		log_replication_commands = false;
  */
 bool		wake_wal_senders = false;
 
-static WALOpenSegment *sendSeg = NULL;
-static WALSegmentContext *sendCxt = NULL;
+/*
+ * Physical walsender does not use xlogreader to read WAL, but it does use a
+ * fake one to keep state.  Logical walsender uses a proper xlogreader.  Both
+ * keep the 'xlogreader' pointer to the right one, for the sake of common
+ * routines.
+ */
+static XLogReaderState fake_xlogreader;
+static XLogReaderState *xlogreader;
 
 /*
  * These variables keep track of the state of the timeline we're currently
@@ -248,8 +254,8 @@ static void LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time);
 static TimeOffset LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now);
 static bool TransactionIdInRecentPast(TransactionId xid, uint32 epoch);
 
-static int	WalSndSegmentOpen(XLogReaderState *state, XLogSegNo nextSegNo,
-							  WALSegmentContext *segcxt, TimeLineID *tli_p);
+static void WalSndSegmentOpen(XLogReaderState *state, XLogSegNo nextSegNo,
+							  TimeLineID *tli_p);
 static void UpdateSpillStats(LogicalDecodingContext *ctx);
 
 
@@ -280,12 +286,19 @@ InitWalSender(void)
 	/* Initialize empty timestamp buffer for lag tracking. */
 	lag_tracker = MemoryContextAllocZero(TopMemoryContext, sizeof(LagTracker));
 
-	/* Make sure we can remember the current read position in XLOG. */
-	sendSeg = (WALOpenSegment *)
-		MemoryContextAlloc(TopMemoryContext, sizeof(WALOpenSegment));
-	sendCxt = (WALSegmentContext *)
-		MemoryContextAlloc(TopMemoryContext, sizeof(WALSegmentContext));
-	WALOpenSegmentInit(sendSeg, sendCxt, wal_segment_size, NULL);
+	/*
+	 * Prepare physical walsender's fake xlogreader struct.  Logical walsender
+	 * does this later.
+	 */
+	if (!am_db_walsender)
+	{
+		xlogreader = &fake_xlogreader;
+		xlogreader->routine =
+			*XL_ROUTINE(.segment_open = WalSndSegmentOpen,
+						.segment_close = wal_segment_close);
+		WALOpenSegmentInit(&xlogreader->seg, &xlogreader->segcxt,
+						   wal_segment_size, NULL);
+	}
 }
 
 /*
@@ -302,11 +315,8 @@ WalSndErrorCleanup(void)
 	ConditionVariableCancelSleep();
 	pgstat_report_wait_end();
 
-	if (sendSeg->ws_file >= 0)
-	{
-		close(sendSeg->ws_file);
-		sendSeg->ws_file = -1;
-	}
+	if (xlogreader->seg.ws_file >= 0)
+		wal_segment_close(xlogreader);
 
 	if (MyReplicationSlot != NULL)
 		ReplicationSlotRelease();
@@ -837,11 +847,9 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 				 cur_page,
 				 targetPagePtr,
 				 XLOG_BLCKSZ,
-				 sendSeg->ws_tli,	/* Pass the current TLI because only
+				 state->seg.ws_tli, /* Pass the current TLI because only
 									 * WalSndSegmentOpen controls whether new
 									 * TLI is needed. */
-				 sendSeg,
-				 sendCxt,
 				 &errinfo))
 		WALReadRaiseError(&errinfo);
 
@@ -852,8 +860,8 @@ logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int req
 	 * read() succeeds in that case, but the data we tried to read might
 	 * already have been overwritten with new WAL records.
 	 */
-	XLByteToSeg(targetPagePtr, segno, sendCxt->ws_segsize);
-	CheckXLogRemoved(segno, sendSeg->ws_tli);
+	XLByteToSeg(targetPagePtr, segno, state->segcxt.ws_segsize);
+	CheckXLogRemoved(segno, state->seg.ws_tli);
 
 	return count;
 }
@@ -1176,6 +1184,7 @@ StartLogicalReplication(StartReplicationCmd *cmd)
 										 .segment_close = wal_segment_close),
 							  WalSndPrepareWrite, WalSndWriteData,
 							  WalSndUpdateProgress);
+	xlogreader = logical_decoding_ctx->reader;
 
 	WalSndSetState(WALSNDSTATE_CATCHUP);
 
@@ -2447,13 +2456,11 @@ WalSndKill(int code, Datum arg)
 }
 
 /* XLogReaderRoutine->segment_open callback */
-static int
-WalSndSegmentOpen(XLogReaderState *state,
-				  XLogSegNo nextSegNo, WALSegmentContext *segcxt,
+static void
+WalSndSegmentOpen(XLogReaderState *state, XLogSegNo nextSegNo,
 				  TimeLineID *tli_p)
 {
 	char		path[MAXPGPATH];
-	int			fd;
 
 	/*-------
 	 * When reading from a historic timeline, and there is a timeline switch
@@ -2484,15 +2491,15 @@ WalSndSegmentOpen(XLogReaderState *state,
 	{
 		XLogSegNo	endSegNo;
 
-		XLByteToSeg(sendTimeLineValidUpto, endSegNo, segcxt->ws_segsize);
-		if (sendSeg->ws_segno == endSegNo)
+		XLByteToSeg(sendTimeLineValidUpto, endSegNo, state->segcxt.ws_segsize);
+		if (state->seg.ws_segno == endSegNo)
 			*tli_p = sendTimeLineNextTLI;
 	}
 
-	XLogFilePath(path, *tli_p, nextSegNo, segcxt->ws_segsize);
-	fd = BasicOpenFile(path, O_RDONLY | PG_BINARY);
-	if (fd >= 0)
-		return fd;
+	XLogFilePath(path, *tli_p, nextSegNo, state->segcxt.ws_segsize);
+	state->seg.ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
+	if (state->seg.ws_file >= 0)
+		return;
 
 	/*
 	 * If the file is not found, assume it's because the standby asked for a
@@ -2515,7 +2522,6 @@ WalSndSegmentOpen(XLogReaderState *state,
 				(errcode_for_file_access(),
 				 errmsg("could not open file \"%s\": %m",
 						path)));
-	return -1;					/* keep compiler quiet */
 }
 
 /*
@@ -2537,12 +2543,6 @@ XLogSendPhysical(void)
 	Size		nbytes;
 	XLogSegNo	segno;
 	WALReadError errinfo;
-	static XLogReaderState fake_xlogreader =
-	{
-		/* Fake xlogreader state for WALRead */
-		.routine.segment_open = WalSndSegmentOpen,
-		.routine.segment_close = wal_segment_close
-	};
 
 	/* If requested switch the WAL sender to the stopping state. */
 	if (got_STOPPING)
@@ -2685,9 +2685,8 @@ XLogSendPhysical(void)
 	if (sendTimeLineIsHistoric && sendTimeLineValidUpto <= sentPtr)
 	{
 		/* close the current file. */
-		if (sendSeg->ws_file >= 0)
-			close(sendSeg->ws_file);
-		sendSeg->ws_file = -1;
+		if (xlogreader->seg.ws_file >= 0)
+			wal_segment_close(xlogreader);
 
 		/* Send CopyDone */
 		pq_putmessage_noblock('c', NULL, 0);
@@ -2760,21 +2759,19 @@ XLogSendPhysical(void)
 	enlargeStringInfo(&output_message, nbytes);
 
 retry:
-	if (!WALRead(&fake_xlogreader,
+	if (!WALRead(xlogreader,
 				 &output_message.data[output_message.len],
 				 startptr,
 				 nbytes,
-				 sendSeg->ws_tli,	/* Pass the current TLI because only
-									 * WalSndSegmentOpen controls whether new
-									 * TLI is needed. */
-				 sendSeg,
-				 sendCxt,
+				 xlogreader->seg.ws_tli,	/* Pass the current TLI because
+											 * only WalSndSegmentOpen controls
+											 * whether new TLI is needed. */
 				 &errinfo))
 		WALReadRaiseError(&errinfo);
 
 	/* See logical_read_xlog_page(). */
-	XLByteToSeg(startptr, segno, sendCxt->ws_segsize);
-	CheckXLogRemoved(segno, sendSeg->ws_tli);
+	XLByteToSeg(startptr, segno, xlogreader->segcxt.ws_segsize);
+	CheckXLogRemoved(segno, xlogreader->seg.ws_tli);
 
 	/*
 	 * During recovery, the currently-open WAL file might be replaced with the
@@ -2792,10 +2789,9 @@ retry:
 		walsnd->needreload = false;
 		SpinLockRelease(&walsnd->mutex);
 
-		if (reload && sendSeg->ws_file >= 0)
+		if (reload && xlogreader->seg.ws_file >= 0)
 		{
-			close(sendSeg->ws_file);
-			sendSeg->ws_file = -1;
+			wal_segment_close(xlogreader);
 
 			goto retry;
 		}

src/bin/pg_waldump/pg_waldump.c

@@ -280,17 +280,15 @@ identify_target_directory(char *directory, char *fname)
 }
 
 /* pg_waldump's XLogReaderRoutine->segment_open callback */
-static int
-WALDumpOpenSegment(XLogReaderState *state,
-				   XLogSegNo nextSegNo, WALSegmentContext *segcxt,
+static void
+WALDumpOpenSegment(XLogReaderState *state, XLogSegNo nextSegNo,
 				   TimeLineID *tli_p)
 {
 	TimeLineID	tli = *tli_p;
 	char		fname[MAXPGPATH];
-	int			fd;
 	int			tries;
 
-	XLogFileName(fname, tli, nextSegNo, segcxt->ws_segsize);
+	XLogFileName(fname, tli, nextSegNo, state->segcxt.ws_segsize);
 
 	/*
 	 * In follow mode there is a short period of time after the server has
@@ -300,9 +298,9 @@ WALDumpOpenSegment(XLogReaderState *state,
 	 */
 	for (tries = 0; tries < 10; tries++)
 	{
-		fd = open_file_in_directory(segcxt->ws_dir, fname);
-		if (fd >= 0)
-			return fd;
+		state->seg.ws_file = open_file_in_directory(state->segcxt.ws_dir, fname);
+		if (state->seg.ws_file >= 0)
+			return;
 		if (errno == ENOENT)
 		{
 			int			save_errno = errno;
@@ -318,7 +316,6 @@ WALDumpOpenSegment(XLogReaderState *state,
 	}
 
 	fatal_error("could not find file \"%s\": %m", fname);
-	return -1;					/* keep compiler quiet */
 }
 
 /*
@@ -356,7 +353,6 @@ WALDumpReadPage(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
 	}
 
 	if (!WALRead(state, readBuff, targetPagePtr, count, private->timeline,
-				 &state->seg, &state->segcxt,
 				 &errinfo))
 	{
 		WALOpenSegment *seg = &errinfo.wre_seg;

src/include/access/xlogreader.h

@@ -63,10 +63,9 @@ typedef int (*XLogPageReadCB) (XLogReaderState *xlogreader,
 							   int reqLen,
 							   XLogRecPtr targetRecPtr,
 							   char *readBuf);
-typedef int (*WALSegmentOpenCB) (XLogReaderState *xlogreader,
-								 XLogSegNo nextSegNo,
-								 WALSegmentContext *segcxt,
-								 TimeLineID *tli_p);
+typedef void (*WALSegmentOpenCB) (XLogReaderState *xlogreader,
+								  XLogSegNo nextSegNo,
+								  TimeLineID *tli_p);
 typedef void (*WALSegmentCloseCB) (XLogReaderState *xlogreader);
 
 typedef struct XLogReaderRoutine
@@ -94,21 +93,16 @@ typedef struct XLogReaderRoutine
 	XLogPageReadCB page_read;
 
 	/*
-	 * Callback to open the specified WAL segment for reading.  The file
-	 * descriptor of the opened segment shall be returned.  In case of
+	 * Callback to open the specified WAL segment for reading.  ->seg.ws_file
+	 * shall be set to the file descriptor of the opened segment.  In case of
 	 * failure, an error shall be raised by the callback and it shall not
 	 * return.
 	 *
 	 * "nextSegNo" is the number of the segment to be opened.
 	 *
-	 * "segcxt" is additional information about the segment.
-	 *
 	 * "tli_p" is an input/output argument. WALRead() uses it to pass the
 	 * timeline in which the new segment should be found, but the callback can
 	 * use it to return the TLI that it actually opened.
-	 *
-	 * BasicOpenFile() is the preferred way to open the segment file in
-	 * backend code, whereas open(2) should be used in frontend.
 	 */
 	WALSegmentOpenCB segment_open;
 
@@ -301,9 +295,7 @@ typedef struct WALReadError
 
 extern bool WALRead(XLogReaderState *state,
 					char *buf, XLogRecPtr startptr, Size count,
-					TimeLineID tli, WALOpenSegment *seg,
-					WALSegmentContext *segcxt,
-					WALReadError *errinfo);
+					TimeLineID tli, WALReadError *errinfo);
 
 /* Functions for decoding an XLogRecord */
 

src/include/access/xlogutils.h

@@ -50,9 +50,8 @@ extern void FreeFakeRelcacheEntry(Relation fakerel);
 extern int	read_local_xlog_page(XLogReaderState *state,
 								 XLogRecPtr targetPagePtr, int reqLen,
 								 XLogRecPtr targetRecPtr, char *cur_page);
-extern int	wal_segment_open(XLogReaderState *state,
+extern void wal_segment_open(XLogReaderState *state,
 							 XLogSegNo nextSegNo,
-							 WALSegmentContext *segcxt,
 							 TimeLineID *tli_p);
 extern void wal_segment_close(XLogReaderState *state);