archive: pass archiver struct to write_archive callback

The current archivers are very static; when you are in the
write_tar_archive function, you know you are writing a tar.
However, to facilitate runtime-configurable archivers
that will share a common write function we need to tell the
function which archiver was used.

As a convenience, we also provide an opaque data pointer in
the archiver struct so that individual archivers can put
something useful there when they register themselves.
Technically they could just use the "name" field to look in
an internal map of names to data, but this is much simpler.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive-tar.c

@@ -234,7 +234,8 @@ static int git_tar_config(const char *var, const char *value, void *cb)
 	return 0;
 }
 
-static int write_tar_archive(struct archiver_args *args)
+static int write_tar_archive(const struct archiver *ar,
+			     struct archiver_args *args)
 {
 	int err = 0;
 

archive-zip.c

@@ -261,7 +261,8 @@ static void dos_time(time_t *time, int *dos_date, int *dos_time)
 	*dos_time = t->tm_sec / 2 + t->tm_min * 32 + t->tm_hour * 2048;
 }
 
-static int write_zip_archive(struct archiver_args *args)
+static int write_zip_archive(const struct archiver *ar,
+			     struct archiver_args *args)
 {
 	int err;
 

archive.c

@@ -410,5 +410,5 @@ int write_archive(int argc, const char **argv, const char *prefix,
 	parse_treeish_arg(argv, &args, prefix);
 	parse_pathspec_arg(argv + 1, &args);
 
-	return ar->write_archive(&args);
+	return ar->write_archive(ar, &args);
 }

archive.h

@@ -17,8 +17,9 @@ struct archiver_args {
 #define ARCHIVER_WANT_COMPRESSION_LEVELS 1
 struct archiver {
 	const char *name;
-	int (*write_archive)(struct archiver_args *);
+	int (*write_archive)(const struct archiver *, struct archiver_args *);
 	unsigned flags;
+	void *data;
 };
 extern void register_archiver(struct archiver *);