add reentrant variants of sha1_to_hex and find_unique_abbrev

The sha1_to_hex and find_unique_abbrev functions always
write into reusable static buffers. There are a few problems
with this:

  - future calls overwrite our result. This is especially
    annoying with find_unique_abbrev, which does not have a
    ring of buffers, so you cannot even printf() a result
    that has two abbreviated sha1s.

  - if you want to put the result into another buffer, we
    often strcpy, which looks suspicious when auditing for
    overflows.

This patch introduces sha1_to_hex_r and find_unique_abbrev_r,
which write into a user-provided buffer. Of course this is
just punting on the overflow-auditing, as the buffer
obviously needs to be GIT_SHA1_HEXSZ + 1 bytes. But it is
much easier to audit, since that is a well-known size.

We retain the non-reentrant forms, which just become thin
wrappers around the reentrant ones. This patch also adds a
strbuf variant of find_unique_abbrev, which will be handy in
later patches.

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

1 files changed, 11 insertions, 5 deletions

diff --git a/sha1_name.c b/sha1_name.c
index da6874c15e..c58b4771c0 100644
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -368,14 +368,13 @@ int for_each_abbrev(const char *prefix, each_abbrev_fn fn, void *cb_data)
 	return ds.ambiguous;
 }
 
-const char *find_unique_abbrev(const unsigned char *sha1, int len)
+int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len)
 {
 	int status, exists;
-	static char hex[41];
 
-	memcpy(hex, sha1_to_hex(sha1), 40);
+	sha1_to_hex_r(hex, sha1);
 	if (len == 40 || !len)
-		return hex;
+		return 40;
 	exists = has_sha1_file(sha1);
 	while (len < 40) {
 		unsigned char sha1_ret[20];
@@ -384,10 +383,17 @@ const char *find_unique_abbrev(const unsigned char *sha1, int len)
 		    ? !status
 		    : status == SHORT_NAME_NOT_FOUND) {
 			hex[len] = 0;
-			return hex;
+			return len;
 		}
 		len++;
 	}
+	return len;
+}
+
+const char *find_unique_abbrev(const unsigned char *sha1, int len)
+{
+	static char hex[GIT_SHA1_HEXSZ + 1];
+	find_unique_abbrev_r(hex, sha1, len);
 	return hex;
 }