archive: optionally use zlib directly for gzip compression

As we already link to the zlib library, we can perform the compression
without even requiring gzip on the host machine.

Note: the `-n` flag that `git archive` passed to `gzip` wants to ensure
that a reproducible file is written, i.e. no filename or mtime will be
recorded in the compressed output. This is already the default for
zlib's `gzopen()` function (if the file name or mtime should be
recorded, the `deflateSetHeader()` function would have to be called
instead).

Note also that the `gzFile` datatype is defined as a pointer in
`zlib.h`, i.e. we can rely on the fact that it can be `NULL`.

At this point, this new mode is hidden behind the pseudo command
`:zlib`: assign this magic string to the `archive.tgz.command` config
setting to enable it.

Signed-off-by: Rohit Ashiwal <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: r1walz

archive-tar.c

@@ -17,6 +17,8 @@ static unsigned long offset;
 
 static int tar_umask = 002;
 
+static gzFile gzip;
+
 static int write_tar_filter_archive(const struct archiver *ar,
 				    struct archiver_args *args);
 
@@ -40,7 +42,10 @@ static int write_tar_filter_archive(const struct archiver *ar,
 
 /* writes out the whole block, or dies if fails */
 static void write_block_or_die(const char *block) {
-	write_or_die(1, block, BLOCKSIZE);
+	if (!gzip)
+		write_or_die(1, block, BLOCKSIZE);
+	else if (gzwrite(gzip, block, (unsigned) BLOCKSIZE) != BLOCKSIZE)
+		die(_("gzwrite failed"));
 }
 
 /* writes out the whole block, but only if it is full */
@@ -459,18 +464,40 @@ static int write_tar_filter_archive(const struct archiver *ar,
 	filter.use_shell = 1;
 	filter.in = -1;
 
-	if (start_command(&filter) < 0)
-		die_errno(_("unable to start '%s' filter"), argv[0]);
-	close(1);
-	if (dup2(filter.in, 1) < 0)
-		die_errno(_("unable to redirect descriptor"));
-	close(filter.in);
+	if (!strcmp(":zlib", ar->data)) {
+		struct strbuf mode = STRBUF_INIT;
+
+		strbuf_addstr(&mode, "wb");
+
+		if (args->compression_level >= 0 && args->compression_level <= 9)
+			strbuf_addf(&mode, "%d", args->compression_level);
+
+		gzip = gzdopen(fileno(stdout), mode.buf);
+		if (!gzip)
+			die(_("Could not gzdopen stdout"));
+		strbuf_release(&mode);
+	} else {
+		if (start_command(&filter) < 0)
+			die_errno(_("unable to start '%s' filter"), argv[0]);
+		close(1);
+		if (dup2(filter.in, 1) < 0)
+			die_errno(_("unable to redirect descriptor"));
+		close(filter.in);
+	}
 
 	r = write_tar_archive(ar, args);
 
-	close(1);
-	if (finish_command(&filter) != 0)
-		die(_("'%s' filter reported error"), argv[0]);
+	if (gzip) {
+		int ret = gzclose(gzip);
+		if (ret == Z_ERRNO)
+			die_errno(_("gzclose failed"));
+		else if (ret != Z_OK)
+			die(_("gzclose failed (%d)"), ret);
+	} else {
+		close(1);
+		if (finish_command(&filter) != 0)
+			die(_("'%s' filter reported error"), argv[0]);
+	}
 
 	strbuf_release(&cmd);
 	return r;