Pre-pad WAL files when streaming transaction log

Instead of filling files as they appear, pre-pad the
WAL files received when streaming xlog the same way
that the server does. Data is streamed into a .partial
file which is then renamed()d into palce when it's complete,
but it will always be 16MB.

This also means that the starting position for pg_receivexlog
is now simply right after the last complete segment, and we
never need to deal with partial segments there.

Patch by me, review by Fujii Masao

src/bin/pg_basebackup/pg_receivexlog.c

@@ -71,33 +71,10 @@ usage(void)
 static bool
 segment_callback(XLogRecPtr segendpos, uint32 timeline)
 {
-	char		fn[MAXPGPATH];
-	struct stat statbuf;
-
 	if (verbose)
 		fprintf(stderr, _("%s: finished segment at %X/%X (timeline %u)\n"),
 				progname, segendpos.xlogid, segendpos.xrecoff, timeline);
 
-	/*
-	 * Check if there is a partial file for the name we just finished, and if
-	 * there is, remove it under the assumption that we have now got all the
-	 * data we need.
-	 */
-	segendpos.xrecoff /= XLOG_SEG_SIZE;
-	PrevLogSeg(segendpos.xlogid, segendpos.xrecoff);
-	snprintf(fn, sizeof(fn), "%s/%08X%08X%08X.partial",
-			 basedir, timeline,
-			 segendpos.xlogid,
-			 segendpos.xrecoff);
-	if (stat(fn, &statbuf) == 0)
-	{
-		/* File existed, get rid of it */
-		if (verbose)
-			fprintf(stderr, _("%s: removing file \"%s\"\n"),
-					progname, fn);
-		unlink(fn);
-	}
-
 	/*
 	 * Never abort from this - we handle all aborting in continue_streaming()
 	 */
@@ -119,9 +96,8 @@ continue_streaming(void)
 /*
  * Determine starting location for streaming, based on:
  * 1. If there are existing xlog segments, start at the end of the last one
- * 2. If the last one is a partial segment, rename it and start over, since
- *	  we don't sync after every write.
- * 3. If no existing xlog exists, start from the beginning of the current
+ *    that is complete (size matches XLogSegSize)
+ * 2. If no valid xlog exists, start from the beginning of the current
  *	  WAL segment.
  */
 static XLogRecPtr
@@ -133,7 +109,6 @@ FindStreamingStart(XLogRecPtr currentpos, uint32 currenttimeline)
 	bool		b;
 	uint32		high_log = 0;
 	uint32		high_seg = 0;
-	bool		partial = false;
 
 	dir = opendir(basedir);
 	if (dir == NULL)
@@ -195,7 +170,7 @@ FindStreamingStart(XLogRecPtr currentpos, uint32 currenttimeline)
 			disconnect_and_exit(1);
 		}
 
-		if (statbuf.st_size == 16 * 1024 * 1024)
+		if (statbuf.st_size == XLOG_SEG_SIZE)
 		{
 			/* Completed segment */
 			if (log > high_log ||
@@ -208,37 +183,9 @@ FindStreamingStart(XLogRecPtr currentpos, uint32 currenttimeline)
 		}
 		else
 		{
-			/*
-			 * This is a partial file. Rename it out of the way.
-			 */
-			char		newfn[MAXPGPATH];
-
-			fprintf(stderr, _("%s: renaming partial file \"%s\" to \"%s.partial\"\n"),
-					progname, dirent->d_name, dirent->d_name);
-
-			snprintf(newfn, sizeof(newfn), "%s/%s.partial",
-					 basedir, dirent->d_name);
-
-			if (stat(newfn, &statbuf) == 0)
-			{
-				/*
-				 * XXX: perhaps we should only error out if the existing file
-				 * is larger?
-				 */
-				fprintf(stderr, _("%s: file \"%s\" already exists. Check and clean up manually.\n"),
-						progname, newfn);
-				disconnect_and_exit(1);
-			}
-			if (rename(fullpath, newfn) != 0)
-			{
-				fprintf(stderr, _("%s: could not rename \"%s\" to \"%s\": %s\n"),
-						progname, fullpath, newfn, strerror(errno));
-				disconnect_and_exit(1);
-			}
-
-			/* Don't continue looking for more, we assume this is the last */
-			partial = true;
-			break;
+			fprintf(stderr, _("%s: segment file '%s' is incorrect size %d, skipping\n"),
+					progname, dirent->d_name, (int) statbuf.st_size);
+			continue;
 		}
 	}
 
@@ -247,17 +194,11 @@ FindStreamingStart(XLogRecPtr currentpos, uint32 currenttimeline)
 	if (high_log > 0 || high_seg > 0)
 	{
 		XLogRecPtr	high_ptr;
-
-		if (!partial)
-		{
-			/*
-			 * If the segment was partial, the pointer is already at the right
-			 * location since we want to re-transmit that segment. If it was
-			 * not, we need to move it to the next segment, since we are
-			 * tracking the last one that was complete.
-			 */
-			NextLogSeg(high_log, high_seg);
-		}
+		/*
+		 * Move the starting pointer to the start of the next segment,
+		 * since the highest one we've seen was completed.
+		 */
+		NextLogSeg(high_log, high_seg);
 
 		high_ptr.xlogid = high_log;
 		high_ptr.xrecoff = high_seg * XLOG_SEG_SIZE;

src/bin/pg_basebackup/receivelog.c

@@ -27,6 +27,7 @@
 #include "receivelog.h"
 #include "streamutil.h"
 
+#include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/types.h>
 #include <unistd.h>
@@ -41,24 +42,128 @@ const XLogRecPtr InvalidXLogRecPtr = {0, 0};
  * Open a new WAL file in the specified directory. Store the name
  * (not including the full directory) in namebuf. Assumes there is
  * enough room in this buffer...
+ *
+ * The file will be padded to 16Mb with zeroes.
  */
 static int
 open_walfile(XLogRecPtr startpoint, uint32 timeline, char *basedir, char *namebuf)
 {
 	int			f;
 	char		fn[MAXPGPATH];
+	struct stat	statbuf;
+	char	   *zerobuf;
+	int			bytes;
 
 	XLogFileName(namebuf, timeline, startpoint.xlogid,
 				 startpoint.xrecoff / XLOG_SEG_SIZE);
 
-	snprintf(fn, sizeof(fn), "%s/%s", basedir, namebuf);
-	f = open(fn, O_WRONLY | O_CREAT | O_EXCL | PG_BINARY, 0666);
+	snprintf(fn, sizeof(fn), "%s/%s.partial", basedir, namebuf);
+	f = open(fn, O_WRONLY | O_CREAT | PG_BINARY, S_IRUSR | S_IWUSR);
 	if (f == -1)
+	{
 		fprintf(stderr, _("%s: Could not open WAL segment %s: %s\n"),
-				progname, namebuf, strerror(errno));
+				progname, fn, strerror(errno));
+		return -1;
+	}
+
+	/*
+	 * Verify that the file is either empty (just created), or a complete
+	 * XLogSegSize segment. Anything in between indicates a corrupt file.
+	 */
+	if (fstat(f, &statbuf) != 0)
+	{
+		fprintf(stderr, _("%s: could not stat WAL segment %s: %s\n"),
+				progname, fn, strerror(errno));
+		close(f);
+		return -1;
+	}
+	if (statbuf.st_size == XLogSegSize)
+		return f; /* File is open and ready to use */
+	if (statbuf.st_size != 0)
+	{
+		fprintf(stderr, _("%s: WAL segment %s is %d bytes, should be 0 or %d\n"),
+				progname, fn, (int) statbuf.st_size, XLogSegSize);
+		close(f);
+		return -1;
+	}
+
+	/* New, empty, file. So pad it to 16Mb with zeroes */
+	zerobuf = xmalloc0(XLOG_BLCKSZ);
+	for (bytes = 0; bytes < XLogSegSize; bytes += XLOG_BLCKSZ)
+	{
+		if (write(f, zerobuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
+		{
+			fprintf(stderr, _("%s: could not pad WAL segment %s: %s\n"),
+					progname, fn, strerror(errno));
+			close(f);
+			unlink(fn);
+			return -1;
+		}
+	}
+	free(zerobuf);
+
+	if (lseek(f, SEEK_SET, 0) != 0)
+	{
+		fprintf(stderr, _("%s: could not seek back to beginning of WAL segment %s: %s\n"),
+				progname, fn, strerror(errno));
+		close(f);
+		return -1;
+	}
 	return f;
 }
 
+static bool
+close_walfile(int walfile, char *basedir, char *walname)
+{
+	off_t		currpos = lseek(walfile, 0, SEEK_CUR);
+
+	if (currpos == -1)
+	{
+		fprintf(stderr, _("%s: could not get current position in file %s: %s\n"),
+				progname, walname, strerror(errno));
+		return false;
+	}
+
+	if (fsync(walfile) != 0)
+	{
+		fprintf(stderr, _("%s: could not fsync file %s: %s\n"),
+				progname, walname, strerror(errno));
+		return false;
+	}
+
+	if (close(walfile) != 0)
+	{
+		fprintf(stderr, _("%s: could not close file %s: %s\n"),
+				progname, walname, strerror(errno));
+		return false;
+	}
+
+	/*
+	 * Rename the .partial file only if we've completed writing the
+	 * whole segment.
+	 */
+	if (currpos == XLOG_SEG_SIZE)
+	{
+		char		oldfn[MAXPGPATH];
+		char		newfn[MAXPGPATH];
+
+		snprintf(oldfn, sizeof(oldfn), "%s/%s.partial", basedir, walname);
+		snprintf(newfn, sizeof(newfn), "%s/%s", basedir, walname);
+		if (rename(oldfn, newfn) != 0)
+		{
+			fprintf(stderr, _("%s: could not rename file %s: %s\n"),
+					progname, walname, strerror(errno));
+			return false;
+		}
+	}
+	else
+		fprintf(stderr, _("%s: not renaming %s, segment is not complete.\n"),
+				progname, walname);
+
+	return true;
+}
+
+
 /*
  * Local version of GetCurrentTimestamp(), since we are not linked with
  * backend code.
@@ -178,10 +283,8 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline, char *sysi
 		if (stream_continue && stream_continue())
 		{
 			if (walfile != -1)
-			{
-				fsync(walfile);
-				close(walfile);
-			}
+				/* Potential error message is written by close_walfile */
+				return close_walfile(walfile, basedir, current_walfile_name);
 			return true;
 		}
 
@@ -360,8 +463,10 @@ ReceiveXlogStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline, char *sysi
 			/* Did we reach the end of a WAL segment? */
 			if (blockpos.xrecoff % XLOG_SEG_SIZE == 0)
 			{
-				fsync(walfile);
-				close(walfile);
+				if (!close_walfile(walfile, basedir, current_walfile_name))
+					/* Error message written in close_walfile() */
+					return false;
+
 				walfile = -1;
 				xlogoff = 0;