Remove manual tracking of file position in pg_dump/pg_backup_custom.c.

We do not really need to track the file position by hand.  We were
already relying on ftello() whenever the archive file is seekable,
while if it's not seekable we don't need the file position info
anyway because we're not going to be able to re-write the TOC.

Moreover, that tracking was buggy since it failed to account for
the effects of fseeko().  Somewhat remarkably, that seems not to
have made for any live bugs up to now.  We could fix the oversights,
but it seems better to just get rid of the whole error-prone mess.

In itself this is merely code cleanup.  However, it's necessary
infrastructure for an upcoming bug-fix patch (because that code
*does* need valid file position after fseeko).  The bug fix
needs to go back as far as v12; hence, back-patch that far.

Discussion: https://postgr.es/m/CALBH9DDuJ+scZc4MEvw5uO-=vRyR2=QF9+Yh=3hPEnKHWfS81A@mail.gmail.com

src/bin/pg_dump/pg_backup_custom.c

@@ -70,14 +70,12 @@ typedef struct
 {
 	CompressorState *cs;
 	int			hasSeek;
-	pgoff_t		filePos;
-	pgoff_t		dataStart;
 } lclContext;
 
 typedef struct
 {
 	int			dataState;
-	pgoff_t		dataPos;
+	pgoff_t		dataPos;		/* valid only if dataState=K_OFFSET_POS_SET */
 } lclTocEntry;
 
 
@@ -144,8 +142,6 @@ InitArchiveFmt_Custom(ArchiveHandle *AH)
 	AH->lo_buf_size = LOBBUFSIZE;
 	AH->lo_buf = (void *) pg_malloc(LOBBUFSIZE);
 
-	ctx->filePos = 0;
-
 	/*
 	 * Now open the file
 	 */
@@ -185,7 +181,6 @@ InitArchiveFmt_Custom(ArchiveHandle *AH)
 
 		ReadHead(AH);
 		ReadToc(AH);
-		ctx->dataStart = _getFilePos(AH, ctx);
 	}
 
 }
@@ -290,6 +285,7 @@ _StartData(ArchiveHandle *AH, TocEntry *te)
 	lclTocEntry *tctx = (lclTocEntry *) te->formatData;
 
 	tctx->dataPos = _getFilePos(AH, ctx);
+	if (tctx->dataPos >= 0)
 		tctx->dataState = K_OFFSET_POS_SET;
 
 	_WriteByte(AH, BLK_DATA);	/* Block type */
@@ -350,6 +346,7 @@ _StartBlobs(ArchiveHandle *AH, TocEntry *te)
 	lclTocEntry *tctx = (lclTocEntry *) te->formatData;
 
 	tctx->dataPos = _getFilePos(AH, ctx);
+	if (tctx->dataPos >= 0)
 		tctx->dataState = K_OFFSET_POS_SET;
 
 	_WriteByte(AH, BLK_BLOBS);	/* Block type */
@@ -551,7 +548,6 @@ _skipBlobs(ArchiveHandle *AH)
 static void
 _skipData(ArchiveHandle *AH)
 {
-	lclContext *ctx = (lclContext *) AH->formatData;
 	size_t		blkLen;
 	char	   *buf = NULL;
 	int			buflen = 0;
@@ -575,8 +571,6 @@ _skipData(ArchiveHandle *AH)
 				fatal("could not read from input file: %m");
 		}
 
-		ctx->filePos += blkLen;
-
 		blkLen = ReadInt(AH);
 	}
 
@@ -594,12 +588,10 @@ _skipData(ArchiveHandle *AH)
 static int
 _WriteByte(ArchiveHandle *AH, const int i)
 {
-	lclContext *ctx = (lclContext *) AH->formatData;
 	int			res;
 
 	if ((res = fputc(i, AH->FH)) == EOF)
 		WRITE_ERROR_EXIT;
-	ctx->filePos += 1;
 
 	return 1;
 }
@@ -615,13 +607,11 @@ _WriteByte(ArchiveHandle *AH, const int i)
 static int
 _ReadByte(ArchiveHandle *AH)
 {
-	lclContext *ctx = (lclContext *) AH->formatData;
 	int			res;
 
 	res = getc(AH->FH);
 	if (res == EOF)
 		READ_ERROR_EXIT(AH->FH);
-	ctx->filePos += 1;
 	return res;
 }
 
@@ -635,11 +625,8 @@ _ReadByte(ArchiveHandle *AH)
 static void
 _WriteBuf(ArchiveHandle *AH, const void *buf, size_t len)
 {
-	lclContext *ctx = (lclContext *) AH->formatData;
-
 	if (fwrite(buf, 1, len, AH->FH) != len)
 		WRITE_ERROR_EXIT;
-	ctx->filePos += len;
 }
 
 /*
@@ -652,11 +639,8 @@ _WriteBuf(ArchiveHandle *AH, const void *buf, size_t len)
 static void
 _ReadBuf(ArchiveHandle *AH, void *buf, size_t len)
 {
-	lclContext *ctx = (lclContext *) AH->formatData;
-
 	if (fread(buf, 1, len, AH->FH) != len)
 		READ_ERROR_EXIT(AH->FH);
-	ctx->filePos += len;
 }
 
 /*
@@ -688,7 +672,6 @@ _CloseArchive(ArchiveHandle *AH)
 		if (tpos < 0 && ctx->hasSeek)
 			fatal("could not determine seek position in archive file: %m");
 		WriteToc(AH);
-		ctx->dataStart = _getFilePos(AH, ctx);
 		WriteDataChunks(AH, NULL);
 
 		/*
@@ -862,30 +845,24 @@ _WorkerJobRestoreCustom(ArchiveHandle *AH, TocEntry *te)
 
 /*
  * Get the current position in the archive file.
+ *
+ * With a non-seekable archive file, we may not be able to obtain the
+ * file position.  If so, just return -1.  It's not too important in
+ * that case because we won't be able to rewrite the TOC to fill in
+ * data block offsets anyway.
  */
 static pgoff_t
 _getFilePos(ArchiveHandle *AH, lclContext *ctx)
 {
 	pgoff_t		pos;
 
-	if (ctx->hasSeek)
-	{
-		/*
-		 * Prior to 1.7 (pg7.3) we relied on the internally maintained
-		 * pointer.  Now we rely on ftello() always, unless the file has been
-		 * found to not support it.  For debugging purposes, print a warning
-		 * if the internal pointer disagrees, so that we're more likely to
-		 * notice if something's broken about the internal position tracking.
-		 */
 	pos = ftello(AH->FH);
 	if (pos < 0)
+	{
+		/* Not expected if we found we can seek. */
+		if (ctx->hasSeek)
 			fatal("could not determine seek position in archive file: %m");
-
-		if (pos != ctx->filePos)
-			pg_log_warning("ftell mismatch with expected position -- ftell used");
 	}
-	else
-		pos = ctx->filePos;
 	return pos;
 }
 
@@ -897,7 +874,6 @@ _getFilePos(ArchiveHandle *AH, lclContext *ctx)
 static void
 _readBlockHeader(ArchiveHandle *AH, int *type, int *id)
 {
-	lclContext *ctx = (lclContext *) AH->formatData;
 	int			byt;
 
 	/*
@@ -918,7 +894,6 @@ _readBlockHeader(ArchiveHandle *AH, int *type, int *id)
 			*id = 0;			/* don't return an uninitialized value */
 			return;
 		}
-		ctx->filePos += 1;
 	}
 
 	*id = ReadInt(AH);