XLogReader general code cleanup

Some minor tweaks and comment additions, for cleanliness sake and to
avoid having the upcoming timeline-following patch be polluted with
unrelated cleanup.

Extracted from a larger patch by Craig Ringer, reviewed by Andres
Freund, with some additions by myself.

src/backend/access/transam/xlogreader.c

@@ -10,9 +10,11 @@
  *
  * NOTES
  *		See xlogreader.h for more notes on this facility.
+ *
+ *		This file is compiled as both front-end and backend code, so it
+ *		may not use ereport, server-defined static variables, etc.
  *-------------------------------------------------------------------------
  */
-
 #include "postgres.h"
 
 #include "access/transam.h"
@@ -192,7 +194,7 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 {
 	XLogRecord *record;
 	XLogRecPtr	targetPagePtr;
-	bool		randAccess = false;
+	bool		randAccess;
 	uint32		len,
 				total_len;
 	uint32		targetRecOff;
@@ -200,6 +202,13 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 	bool		gotheader;
 	int			readOff;
 
+	/*
+	 * randAccess indicates whether to verify the previous-record pointer of
+	 * the record we're reading.  We only do this if we're reading
+	 * sequentially, which is what we initially assume.
+	 */
+	randAccess = false;
+
 	/* reset error state */
 	*errormsg = NULL;
 	state->errormsg_buf[0] = '\0';
@@ -208,6 +217,7 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 
 	if (RecPtr == InvalidXLogRecPtr)
 	{
+		/* No explicit start point; read the record after the one we just read */
 		RecPtr = state->EndRecPtr;
 
 		if (state->ReadRecPtr == InvalidXLogRecPtr)
@@ -223,11 +233,13 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 	else
 	{
 		/*
+		 * Caller supplied a position to start at.
+		 *
 		 * In this case, the passed-in record pointer should already be
 		 * pointing to a valid record starting position.
 		 */
 		Assert(XRecOffIsValid(RecPtr));
-		randAccess = true;		/* allow readPageTLI to go backwards too */
+		randAccess = true;
 	}
 
 	state->currRecPtr = RecPtr;
@@ -309,8 +321,10 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 		/* XXX: more validation should be done here */
 		if (total_len < SizeOfXLogRecord)
 		{
-			report_invalid_record(state, "invalid record length at %X/%X",
-								  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+			report_invalid_record(state,
+						"invalid record length at %X/%X: wanted %lu, got %u",
+								  (uint32) (RecPtr >> 32), (uint32) RecPtr,
+								  SizeOfXLogRecord, total_len);
 			goto err;
 		}
 		gotheader = false;
@@ -463,12 +477,10 @@ XLogReadRecord(XLogReaderState *state, XLogRecPtr RecPtr, char **errormsg)
 err:
 
 	/*
-	 * Invalidate the xlog page we've cached. We might read from a different
-	 * source after failure.
+	 * Invalidate the read state. We might read from a different source after
+	 * failure.
 	 */
-	state->readSegNo = 0;
-	state->readOff = 0;
-	state->readLen = 0;
+	XLogReaderInvalReadState(state);
 
 	if (state->errormsg_buf[0] != '\0')
 		*errormsg = state->errormsg_buf;
@@ -572,7 +584,7 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 	if (!ValidXLogPageHeader(state, pageptr, hdr))
 		goto err;
 
-	/* update cache information */
+	/* update read state information */
 	state->readSegNo = targetSegNo;
 	state->readOff = targetPageOff;
 	state->readLen = readLen;
@@ -580,10 +592,19 @@ ReadPageInternal(XLogReaderState *state, XLogRecPtr pageptr, int reqLen)
 	return readLen;
 
 err:
+	XLogReaderInvalReadState(state);
+	return -1;
+}
+
+/*
+ * Invalidate the xlogreader's read state to force a re-read.
+ */
+void
+XLogReaderInvalReadState(XLogReaderState *state)
+{
 	state->readSegNo = 0;
 	state->readOff = 0;
 	state->readLen = 0;
-	return -1;
 }
 
 /*
@@ -600,8 +621,9 @@ ValidXLogRecordHeader(XLogReaderState *state, XLogRecPtr RecPtr,
 	if (record->xl_tot_len < SizeOfXLogRecord)
 	{
 		report_invalid_record(state,
-							  "invalid record length at %X/%X",
-							  (uint32) (RecPtr >> 32), (uint32) RecPtr);
+						"invalid record length at %X/%X: wanted %lu, got %u",
+							  (uint32) (RecPtr >> 32), (uint32) RecPtr,
+							  SizeOfXLogRecord, record->xl_tot_len);
 		return false;
 	}
 	if (record->xl_rmid > RM_MAX_ID)
@@ -907,11 +929,9 @@ XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr)
 err:
 out:
 	/* Reset state to what we had before finding the record */
-	state->readSegNo = 0;
-	state->readOff = 0;
-	state->readLen = 0;
 	state->ReadRecPtr = saved_state.ReadRecPtr;
 	state->EndRecPtr = saved_state.EndRecPtr;
+	XLogReaderInvalReadState(state);
 
 	return found;
 }

src/backend/access/transam/xlogutils.c

@@ -19,12 +19,11 @@
 
 #include <unistd.h>
 
-#include "miscadmin.h"
-
 #include "access/xlog.h"
 #include "access/xlog_internal.h"
 #include "access/xlogutils.h"
 #include "catalog/catalog.h"
+#include "miscadmin.h"
 #include "storage/smgr.h"
 #include "utils/guc.h"
 #include "utils/hsearch.h"
@@ -638,8 +637,17 @@ XLogTruncateRelation(RelFileNode rnode, ForkNumber forkNum,
 }
 
 /*
- * TODO: This is duplicate code with pg_xlogdump, similar to walsender.c, but
- * we currently don't have the infrastructure (elog!) to share it.
+ * Read 'count' bytes from WAL into 'buf', starting at location 'startptr'
+ * in timeline 'tli'.
+ *
+ * Will open, and keep open, one WAL segment stored in the static file
+ * descriptor 'sendFile'. This means if XLogRead is used once, there will
+ * always be one descriptor left open until the process ends, but never
+ * more than one.
+ *
+ * XXX This is very similar to pg_xlogdump's XLogDumpXLogRead and to XLogRead
+ * in walsender.c but for small differences (such as lack of elog() in
+ * frontend).  Probably these should be merged at some point.
  */
 static void
 XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
@@ -648,6 +656,7 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 	XLogRecPtr	recptr;
 	Size		nbytes;
 
+	/* state maintained across calls */
 	static int	sendFile = -1;
 	static XLogSegNo sendSegNo = 0;
 	static uint32 sendOff = 0;
@@ -664,11 +673,11 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
 
 		startoff = recptr % XLogSegSize;
 
+		/* Do we need to switch to a different xlog segment? */
 		if (sendFile < 0 || !XLByteInSeg(recptr, sendSegNo))
 		{
 			char		path[MAXPGPATH];
 
-			/* Switch to another logfile segment */
 			if (sendFile >= 0)
 				close(sendFile);
 
@@ -745,7 +754,7 @@ XLogRead(char *buf, TimeLineID tli, XLogRecPtr startptr, Size count)
  * Public because it would likely be very helpful for someone writing another
  * output method outside walsender, e.g. in a bgworker.
  *
- * TODO: The walsender has it's own version of this, but it relies on the
+ * TODO: The walsender has its own version of this, but it relies on the
  * walsender's latch being set whenever WAL is flushed. No such infrastructure
  * exists for normal backends, so we have to do a check/sleep/repeat style of
  * loop for now.

src/backend/replication/logical/logicalfuncs.c

@@ -241,6 +241,10 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 
 	PG_TRY();
 	{
+		/*
+		 * Passing InvalidXLogRecPtr here causes replay to start at the slot's
+		 * confirmed_flush.
+		 */
 		ctx = CreateDecodingContext(InvalidXLogRecPtr,
 									options,
 									logical_read_local_xlog_page,
@@ -263,6 +267,14 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 
 		ctx->output_writer_private = p;
 
+		/*
+		 * We start reading xlog from the restart lsn, even though in
+		 * CreateDecodingContext we set the snapshot builder up using the
+		 * slot's confirmed_flush. This means we might read xlog we don't
+		 * actually decode rows from, but the snapshot builder might need it
+		 * to get to a consistent point. The point we start returning data to
+		 * *users* at is the candidate restart lsn from the decoding context.
+		 */
 		startptr = MyReplicationSlot->data.restart_lsn;
 
 		CurrentResourceOwner = ResourceOwnerCreate(CurrentResourceOwner, "logical decoding");
@@ -280,6 +292,10 @@ pg_logical_slot_get_changes_guts(FunctionCallInfo fcinfo, bool confirm, bool bin
 			if (errm)
 				elog(ERROR, "%s", errm);
 
+			/*
+			 * Now that we've set up the xlog reader state, subsequent calls
+			 * pass InvalidXLogRecPtr to say "continue from last record"
+			 */
 			startptr = InvalidXLogRecPtr;
 
 			/*

src/include/access/xlogreader.h

@@ -139,16 +139,22 @@ struct XLogReaderState
 	 * ----------------------------------------
 	 */
 
-	/* Buffer for currently read page (XLOG_BLCKSZ bytes) */
+	/*
+	 * Buffer for currently read page (XLOG_BLCKSZ bytes, valid up to at least
+	 * readLen bytes)
+	 */
 	char	   *readBuf;
+	uint32		readLen;
 
-	/* last read segment, segment offset, read length, TLI */
+	/* last read segment, segment offset, TLI for data currently in readBuf */
 	XLogSegNo	readSegNo;
 	uint32		readOff;
-	uint32		readLen;
 	TimeLineID	readPageTLI;
 
-	/* beginning of last page read, and its TLI  */
+	/*
+	 * beginning of prior page read, and its TLI.  Doesn't necessarily
+	 * correspond to what's in readBuf; used for timeline sanity checks.
+	 */
 	XLogRecPtr	latestPagePtr;
 	TimeLineID	latestPageTLI;
 
@@ -174,6 +180,9 @@ extern void XLogReaderFree(XLogReaderState *state);
 extern struct XLogRecord *XLogReadRecord(XLogReaderState *state,
 			   XLogRecPtr recptr, char **errormsg);
 
+/* Invalidate read state */
+extern void XLogReaderInvalReadState(XLogReaderState *state);
+
 #ifdef FRONTEND
 extern XLogRecPtr XLogFindNextRecord(XLogReaderState *state, XLogRecPtr RecPtr);
 #endif   /* FRONTEND */

src/include/access/xlogutils.h

@@ -47,7 +47,9 @@ extern Buffer XLogReadBufferExtended(RelFileNode rnode, ForkNumber forknum,
 extern Relation CreateFakeRelcacheEntry(RelFileNode rnode);
 extern void FreeFakeRelcacheEntry(Relation fakerel);
 
-extern int read_local_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr,
-	int reqLen, XLogRecPtr targetRecPtr, char *cur_page, TimeLineID *pageTLI);
+extern int read_local_xlog_page(XLogReaderState *state,
+					 XLogRecPtr targetPagePtr, int reqLen,
+					 XLogRecPtr targetRecPtr, char *cur_page,
+					 TimeLineID *pageTLI);
 
 #endif