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>

cache.h

@@ -785,7 +785,24 @@ extern char *sha1_pack_name(const unsigned char *sha1);
  */
 extern char *sha1_pack_index_name(const unsigned char *sha1);
 
-extern const char *find_unique_abbrev(const unsigned char *sha1, int);
+/*
+ * Return an abbreviated sha1 unique within this repository's object database.
+ * The result will be at least `len` characters long, and will be NUL
+ * terminated.
+ *
+ * The non-`_r` version returns a static buffer which will be overwritten by
+ * subsequent calls.
+ *
+ * The `_r` variant writes to a buffer supplied by the caller, which must be at
+ * least `GIT_SHA1_HEXSZ + 1` bytes. The return value is the number of bytes
+ * written (excluding the NUL terminator).
+ *
+ * Note that while this version avoids the static buffer, it is not fully
+ * reentrant, as it calls into other non-reentrant git code.
+ */
+extern const char *find_unique_abbrev(const unsigned char *sha1, int len);
+extern int find_unique_abbrev_r(char *hex, const unsigned char *sha1, int len);
+
 extern const unsigned char null_sha1[GIT_SHA1_RAWSZ];
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
@@ -1067,6 +1084,18 @@ extern int for_each_abbrev(const char *prefix, each_abbrev_fn, void *);
 extern int get_sha1_hex(const char *hex, unsigned char *sha1);
 extern int get_oid_hex(const char *hex, struct object_id *sha1);
 
+/*
+ * Convert a binary sha1 to its hex equivalent. The `_r` variant is reentrant,
+ * and writes the NUL-terminated output to the buffer `out`, which must be at
+ * least `GIT_SHA1_HEXSZ + 1` bytes, and returns a pointer to out for
+ * convenience.
+ *
+ * The non-`_r` variant returns a static buffer, but uses a ring of 4
+ * buffers, making it safe to make multiple calls for a single statement, like:
+ *
+ *   printf("%s -> %s", sha1_to_hex(one), sha1_to_hex(two));
+ */
+extern char *sha1_to_hex_r(char *out, const unsigned char *sha1);
 extern char *sha1_to_hex(const unsigned char *sha1);	/* static buffer result! */
 extern char *oid_to_hex(const struct object_id *oid);	/* same static buffer as sha1_to_hex */