Fix pg_dump's errno checking for zlib I/O

Some error reports were reporting strerror(errno), which for some error
conditions coming from zlib are wrong, resulting in confusing reports
such as
  pg_restore: [compress_io] could not read from input file: Success
which makes no sense.  To correctly extract the error message we need to
use gzerror(), so let's do that.

This isn't as comprehensive or as neat as I would like, but at least it
should improve things in many common cases.  The zlib abstraction in
compress_io does not seem to be applied consistently enough; we could
perhaps improve that, but it seems master-only material, not a bug fix
for back-patching.

This problem goes back all the way, but I decided to apply back to 9.4
only, because older branches don't contain commit 14ea89366 which this
change depends on.

Authors: Vladimir Kunschikov, Álvaro Herrera
Discussion: https://postgr.es/m/1498120508308.9826@infotecs.ru

src/bin/pg_dump/compress_io.c

@@ -592,8 +592,14 @@ cfread(void *ptr, int size, cfp *fp)
 	{
 		ret = gzread(fp->compressedfp, ptr, size);
 		if (ret != size && !gzeof(fp->compressedfp))
+		{
+			int		errnum;
+			const char *errmsg = gzerror(fp->compressedfp, &errnum);
+
 			exit_horribly(modulename,
-						  "could not read from input file: %s\n", strerror(errno));
+						  "could not read from input file: %s\n",
+						  errnum == Z_ERRNO ? strerror(errno) : errmsg);
+		}
 	}
 	else
 #endif
@@ -695,6 +701,22 @@ cfeof(cfp *fp)
 		return feof(fp->uncompressedfp);
 }
 
+const char *
+get_cfp_error(cfp *fp)
+{
+#ifdef HAVE_LIBZ
+	if (fp->compressedfp)
+	{
+		int			errnum;
+		const char *errmsg = gzerror(fp->compressedfp, &errnum);
+
+		if (errnum != Z_ERRNO)
+			return errmsg;
+	}
+#endif
+	return strerror(errno);
+}
+
 #ifdef HAVE_LIBZ
 static int
 hasSuffix(const char *filename, const char *suffix)

src/bin/pg_dump/compress_io.h

@@ -65,5 +65,6 @@ extern int	cfgetc(cfp *fp);
 extern char *cfgets(cfp *fp, char *buf, int len);
 extern int	cfclose(cfp *fp);
 extern int	cfeof(cfp *fp);
+extern const char *get_cfp_error(cfp *fp);
 
 #endif

src/bin/pg_dump/pg_backup_directory.c

@@ -352,7 +352,9 @@ _WriteData(ArchiveHandle *AH, const void *data, size_t dLen)
 	lclContext *ctx = (lclContext *) AH->formatData;
 
 	if (dLen > 0 && cfwrite(data, dLen, ctx->dataFH) != dLen)
-		WRITE_ERROR_EXIT;
+		exit_horribly(modulename, "could not write to output file: %s\n",
+					  get_cfp_error(ctx->dataFH));
+
 
 	return;
 }
@@ -490,7 +492,8 @@ _WriteByte(ArchiveHandle *AH, const int i)
 	lclContext *ctx = (lclContext *) AH->formatData;
 
 	if (cfwrite(&c, 1, ctx->dataFH) != 1)
-		WRITE_ERROR_EXIT;
+		exit_horribly(modulename, "could not write to output file: %s\n",
+					  get_cfp_error(ctx->dataFH));
 
 	return 1;
 }
@@ -519,7 +522,8 @@ _WriteBuf(ArchiveHandle *AH, const void *buf, size_t len)
 	lclContext *ctx = (lclContext *) AH->formatData;
 
 	if (cfwrite(buf, len, ctx->dataFH) != len)
-		WRITE_ERROR_EXIT;
+		exit_horribly(modulename, "could not write to output file: %s\n",
+					  get_cfp_error(ctx->dataFH));
 
 	return;
 }

src/bin/pg_dump/pg_backup_tar.c

@@ -555,8 +555,14 @@ _tarReadRaw(ArchiveHandle *AH, void *buf, size_t len, TAR_MEMBER *th, FILE *fh)
 			{
 				res = GZREAD(&((char *) buf)[used], 1, len, th->zFH);
 				if (res != len && !GZEOF(th->zFH))
+				{
+					int		errnum;
+					const char *errmsg = gzerror(th->zFH, &errnum);
+
 					exit_horribly(modulename,
-								  "could not read from input file: %s\n", strerror(errno));
+								  "could not read from input file: %s\n",
+								  errnum == Z_ERRNO ? strerror(errno) : errmsg);
+				}
 			}
 			else
 			{