Ensure write failure reports no-disk-space

A few places calling fwrite and gzwrite were not setting errno to ENOSPC
when reporting errors, as is customary; this led to some failures being
reported as
"could not write file: Success"
which makes us look silly.  Make a few of these places in pg_dump and
pg_basebackup use our customary pattern.

Backpatch-to: 9.5
Author: Justin Pryzby <pryzby@telsasoft.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/20200611153753.GU14879@telsasoft.com

src/bin/pg_basebackup/pg_basebackup.c

@@ -992,8 +992,12 @@ writeTarData(WriteTarState *state, char *buf, int r)
 #ifdef HAVE_LIBZ
 	if (state->ztarfile != NULL)
 	{
+		errno = 0;
 		if (gzwrite(state->ztarfile, buf, r) != r)
 		{
+			/* if write didn't set errno, assume problem is no disk space */
+			if (errno == 0)
+				errno = ENOSPC;
 			pg_log_error("could not write to compressed file \"%s\": %s",
 						 state->filename, get_gz_error(state->ztarfile));
 			exit(1);
@@ -1002,8 +1006,12 @@ writeTarData(WriteTarState *state, char *buf, int r)
 	else
 #endif
 	{
+		errno = 0;
 		if (fwrite(buf, r, 1, state->tarfile) != 1)
 		{
+			/* if write didn't set errno, assume problem is no disk space */
+			if (errno == 0)
+				errno = ENOSPC;
 			pg_log_error("could not write to file \"%s\": %m",
 						 state->filename);
 			exit(1);
@@ -1691,8 +1699,12 @@ ReceiveTarAndUnpackCopyChunk(size_t r, char *copybuf, void *callback_data)
 			return;
 		}
 
+		errno = 0;
 		if (fwrite(copybuf, r, 1, state->file) != 1)
 		{
+			/* if write didn't set errno, assume problem is no disk space */
+			if (errno == 0)
+				errno = ENOSPC;
 			pg_log_error("could not write to file \"%s\": %m", state->filename);
 			exit(1);
 		}
@@ -1743,8 +1755,12 @@ ReceiveBackupManifestChunk(size_t r, char *copybuf, void *callback_data)
 {
 	WriteManifestState *state = callback_data;
 
+	errno = 0;
 	if (fwrite(copybuf, r, 1, state->file) != 1)
 	{
+		/* if write didn't set errno, assume problem is no disk space */
+		if (errno == 0)
+			errno = ENOSPC;
 		pg_log_error("could not write to file \"%s\": %m", state->filename);
 		exit(1);
 	}

src/bin/pg_dump/pg_backup_directory.c

@@ -346,9 +346,15 @@ _WriteData(ArchiveHandle *AH, const void *data, size_t dLen)
 {
 	lclContext *ctx = (lclContext *) AH->formatData;
 
+	errno = 0;
 	if (dLen > 0 && cfwrite(data, dLen, ctx->dataFH) != dLen)
+	{
+		/* if write didn't set errno, assume problem is no disk space */
+		if (errno == 0)
+			errno = ENOSPC;
 		fatal("could not write to output file: %s",
 			  get_cfp_error(ctx->dataFH));
+	}
 }
 
 /*
@@ -481,9 +487,15 @@ _WriteByte(ArchiveHandle *AH, const int i)
 	unsigned char c = (unsigned char) i;
 	lclContext *ctx = (lclContext *) AH->formatData;
 
+	errno = 0;
 	if (cfwrite(&c, 1, ctx->dataFH) != 1)
+	{
+		/* if write didn't set errno, assume problem is no disk space */
+		if (errno == 0)
+			errno = ENOSPC;
 		fatal("could not write to output file: %s",
 			  get_cfp_error(ctx->dataFH));
+	}
 
 	return 1;
 }
@@ -511,9 +523,15 @@ _WriteBuf(ArchiveHandle *AH, const void *buf, size_t len)
 {
 	lclContext *ctx = (lclContext *) AH->formatData;
 
+	errno = 0;
 	if (cfwrite(buf, len, ctx->dataFH) != len)
+	{
+		/* if write didn't set errno, assume problem is no disk space */
+		if (errno == 0)
+			errno = ENOSPC;
 		fatal("could not write to output file: %s",
 			  get_cfp_error(ctx->dataFH));
+	}
 }
 
 /*