Improve pg_dump's checkSeek() function to verify the functioning of ftello

as well as fseeko, and to not assume that fseeko(fp, 0, SEEK_CUR) proves
anything.  Also improve some related comments.  Per my observation that
the SEEK_CUR test didn't actually work on some platforms, and subsequent
discussion with Robert Haas.

Back-patch to 8.4.  In earlier releases it's not that important whether
we get the hasSeek test right, but with parallel restore it matters.

src/bin/pg_dump/pg_backup_archiver.c

@@ -15,7 +15,7 @@
  *
  *
  * IDENTIFICATION
- *		$PostgreSQL: pgsql/src/bin/pg_dump/pg_backup_archiver.c,v 1.185 2010/05/15 21:41:16 tgl Exp $
+ *		$PostgreSQL: pgsql/src/bin/pg_dump/pg_backup_archiver.c,v 1.186 2010/06/28 02:07:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1755,6 +1755,11 @@ _discoverArchiveFormat(ArchiveHandle *AH)
 
 	if (strncmp(sig, "PGDMP", 5) == 0)
 	{
+		/*
+		 * Finish reading (most of) a custom-format header.
+		 *
+		 * NB: this code must agree with ReadHead().
+		 */
 		AH->vmaj = fgetc(fh);
 		AH->vmin = fgetc(fh);
 
@@ -2981,7 +2986,12 @@ ReadHead(ArchiveHandle *AH)
 	int			fmt;
 	struct tm	crtm;
 
-	/* If we haven't already read the header... */
+	/*
+	 * If we haven't already read the header, do so.
+	 *
+	 * NB: this code must agree with _discoverArchiveFormat().  Maybe find
+	 * a way to unify the cases?
+	 */
 	if (!AH->readHeader)
 	{
 		if ((*AH->ReadBufPtr) (AH, tmpMag, 5) != 5)
@@ -3000,7 +3010,6 @@ ReadHead(ArchiveHandle *AH)
 
 		AH->version = ((AH->vmaj * 256 + AH->vmin) * 256 + AH->vrev) * 256 + 0;
 
-
 		if (AH->version < K_VERS_1_0 || AH->version > K_VERS_MAX)
 			die_horribly(AH, modulename, "unsupported version (%d.%d) in file header\n",
 						 AH->vmaj, AH->vmin);
@@ -3068,27 +3077,37 @@ ReadHead(ArchiveHandle *AH)
 
 /*
  * checkSeek
- *	  check to see if fseek can be performed.
+ *	  check to see if ftell/fseek can be performed.
  */
 bool
 checkSeek(FILE *fp)
 {
-	if (fseeko(fp, 0, SEEK_CUR) != 0)
-		return false;
-	else if (sizeof(pgoff_t) > sizeof(long))
-	{
-		/*
-		 * At this point, pgoff_t is too large for long, so we return based on
-		 * whether an pgoff_t version of fseek is available.
-		 */
-#ifdef HAVE_FSEEKO
-		return true;
-#else
+	pgoff_t		tpos;
+
+	/*
+	 * If pgoff_t is wider than long, we must have "real" fseeko and not
+	 * an emulation using fseek.  Otherwise report no seek capability.
+	 */
+#ifndef HAVE_FSEEKO
+	if (sizeof(pgoff_t) > sizeof(long))
 		return false;
 #endif
-	}
-	else
-		return true;
+
+	/* Check that ftello works on this file */
+	errno = 0;
+	tpos = ftello(fp);
+	if (errno)
+		return false;
+
+	/*
+	 * Check that fseeko(SEEK_SET) works, too.  NB: we used to try to test
+	 * this with fseeko(fp, 0, SEEK_CUR).  But some platforms treat that as a
+	 * successful no-op even on files that are otherwise unseekable.
+	 */
+	if (fseeko(fp, tpos, SEEK_SET) != 0)
+		return false;
+
+	return true;
 }
 
 

src/bin/pg_dump/pg_backup_custom.c

@@ -19,7 +19,7 @@
  *
  *
  * IDENTIFICATION
- *		$PostgreSQL: pgsql/src/bin/pg_dump/pg_backup_custom.c,v 1.45 2010/06/27 19:07:24 tgl Exp $
+ *		$PostgreSQL: pgsql/src/bin/pg_dump/pg_backup_custom.c,v 1.46 2010/06/28 02:07:02 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -835,9 +835,10 @@ _CloseArchive(ArchiveHandle *AH)
 		WriteDataChunks(AH);
 
 		/*
-		 * This is not an essential operation - it is really only needed if we
-		 * expect to be doing seeks to read the data back - it may be ok to
-		 * just use the existing self-consistent block formatting.
+		 * If possible, re-write the TOC in order to update the data offset
+		 * information.  This is not essential, as pg_restore can cope in
+		 * most cases without it; but it can make pg_restore significantly
+		 * faster in some situations (especially parallel restore).
 		 */
 		if (ctx->hasSeek &&
 			fseeko(AH->FH, tpos, SEEK_SET) == 0)
@@ -914,7 +915,8 @@ _getFilePos(ArchiveHandle *AH, lclContext *ctx)
 
 			/*
 			 * Prior to 1.7 (pg7.3) we relied on the internally maintained
-			 * pointer. Now we rely on pgoff_t always. pos = ctx->filePos;
+			 * pointer. Now we rely on ftello() always, unless the file has
+			 * been found to not support it.
 			 */
 		}
 	}