[PATCH v2 15/22] ui-snapshot.c: tidy up memory management in write_archive_type

[john at keeping.me.uk (John Keeping)]

- Use a strbuf instead of a fixed-size buffer
- Free the argv_array when we're done with it

Note that since write_archive modifies the argv array passed to it we
copy the argv_array values into a new array of char* and then free the
original argv_array structure and the new array without worrying about
what the values now look like.

Signed-off-by: John Keeping <john at keeping.me.uk>
---
Changes since v1:
- Fix freeing the argv array to avoid double frees.

 ui-snapshot.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/ui-snapshot.c b/ui-snapshot.c
index a47884e..1b361c1 100644
--- a/ui-snapshot.c
+++ b/ui-snapshot.c
@@ -15,14 +15,33 @@
 static int write_archive_type(const char *format, const char *hex, const char *prefix)
 {
 	struct argv_array argv = ARGV_ARRAY_INIT;
+	const char **nargv;
+	int result;
 	argv_array_push(&argv, "snapshot");
 	argv_array_push(&argv, format);
 	if (prefix) {
+		struct strbuf buf = STRBUF_INIT;
+		strbuf_addstr(&buf, prefix);
+		strbuf_addch(&buf, '/');
 		argv_array_push(&argv, "--prefix");
-		argv_array_push(&argv, fmt("%s/", prefix));
+		argv_array_push(&argv, buf.buf);
+		strbuf_release(&buf);
 	}
 	argv_array_push(&argv, hex);
-	return write_archive(argv.argc, argv.argv, NULL, 1, NULL, 0);
+	/*
+	 * Now we need to copy the pointers to arguments into a new
+	 * structure because write_archive will rearrange its arguments
+	 * which may result in duplicated/missing entries causing leaks
+	 * or double-frees in argv_array_clear.
+	 */
+	nargv = xmalloc(sizeof(char *) * (argv.argc + 1));
+	/* argv_array guarantees a trailing NULL entry. */
+	memcpy(nargv, argv.argv, sizeof(char *) * (argv.argc + 1));
+
+	result = write_archive(argv.argc, nargv, NULL, 1, NULL, 0);
+	argv_array_clear(&argv);
+	free(nargv);
+	return result;
 }
 
 static int write_tar_archive(const char *hex, const char *prefix)
-- 
1.8.2.692.g17a9715