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, 8 insertions, 0 deletions

diff --git a/strbuf.h b/strbuf.h
index 43f27c3a6..0f9c8a72b 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -475,6 +475,14 @@ static inline struct strbuf **strbuf_split(const struct strbuf *sb,
 extern void strbuf_list_free(struct strbuf **);
 
 /**
+ * Add the abbreviation, as generated by find_unique_abbrev, of `sha1` to
+ * the strbuf `sb`.
+ */
+extern void strbuf_add_unique_abbrev(struct strbuf *sb,
+				     const unsigned char *sha1,
+				     int abbrev_len);
+
+/**
  * Launch the user preferred editor to edit a file and fill the buffer
  * with the file's contents upon the user completing their editing. The
  * third argument can be used to set the environment which the editor is