[PATCH 3/3] validate_headref: use get_oid_hex for detached HEADs

If a candidate HEAD isn't a symref, we check that it
contains a viable sha1. But in a post-sha1 world, we should
be checking whether it has any plausible object-id.

We can do that by switching to get_oid_hex().

Note that both before and after this patch, we only check
for a plausible object id at the start of the file, and then
call that good enough.  We ignore any content _after_ the
hex, so a string like:

  0123456789012345678901234567890123456789 foo

is accepted. Though we do put extra bytes like this into
some pseudorefs (e.g., FETCH_HEAD), we don't typically do so
with HEAD. We could tighten this up by using parse_oid_hex(),
like:

  if (!parse_oid_hex(buffer, &oid, &end) &&
      *end++ == '\n' && *end == '\0')
          return 0;

But we're probably better to remain on the loose side. We're
just checking here for a plausible-looking repository
directory, so heuristics are acceptable (if we really want
to be meticulous, we should use the actual ref code to parse
HEAD).

Signed-off-by: Jeff King <redacted>
---
 path.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/path.c b/path.c
index 2ef3a86613..5aa9244eb2 100644
--- a/path.c
+++ b/path.c

@@ -639,7 +639,7 @@ int validate_headref(const char *path)
 	struct stat st;
 	char buffer[256];
 	const char *refname;
-	unsigned char sha1[20];
+	struct object_id oid;
 	int fd;
 	ssize_t len;
 

@@ -680,7 +680,7 @@ int validate_headref(const char *path)
 	/*
 	 * Is this a detached HEAD?
 	 */
-	if (!get_sha1_hex(buffer, sha1))
+	if (!get_oid_hex(buffer, &oid))
 		return 0;
 
 	return -1;

-- 
2.14.2.988.g01c8b37dde