hex: default to the_hash_algo on zero algorithm value

There are numerous places in the codebase where we assume we can
initialize data by zeroing all its bytes.  However, when we do that with
a struct object_id, it leaves the structure with a zero value for the
algorithm, which is invalid.

We could forbid this pattern and require that all struct object_id
instances be initialized using oidclr, but this seems burdensome and
it's unnatural to most C programmers.  Instead, if the algorithm is
zero, assume we wanted to use the default hash algorithm instead.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/hex.c b/hex.c
index e7af18fe55ac65b6b394d4319d221ce577d6b342..74d256f239b1ddb136efef0d6fafaee2a57ff137 100644 (file)
--- a/hex.c
+++ b/hex.c
@@ -124,6 +124,13 @@ char *hash_to_hex_algop_r(char *buffer, const unsigned char *hash,
        char *buf = buffer;
        int i;
 
+       /*
+        * Our struct object_id has been memset to 0, so default to printing
+        * using the default hash.
+        */
+       if (algop == &hash_algos[0])
+               algop = the_hash_algo;
+
        for (i = 0; i < algop->rawsz; i++) {
                unsigned int val = *hash++;
                *buf++ = hex[val >> 4];