[PATCH v2 2/7] gc: convert to using the_hash_algo

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

There's been a lot of changing of the hardcoded "40" values to
the_hash_algo->hexsz, but we've so far missed this one where we
hardcoded 38 for the loose object file length.

This is because a SHA-1 like abcde[...] gets turned into
objects/ab/cde[...]. There's no reason to suppose the same won't be
the case for SHA-256, and reading between the lines in
hash-function-transition.txt the format is planned to be the same.

In the future we may want to further modify this code for the hash
function transition. There's a potential pathological case here where
we'll only consider the loose objects for the currently active hash,
but objects for that hash will share a directory storage with the
other hash.

Thus we could theoretically have 1k SHA-1 loose objects, and say 1
million SHA-256 objects, and not notice because we're currently using
SHA-1.

So assuming that "gc" eventually learns to pack up both SHA-1 and
SHA-256 objects regardless of what the current the_hash_alg is perhaps
this check should be changed to consider all files in objects/17/
matching [0-9a-f] 38 or 62 characters in length (i.e. both SHA-1 and
SHA-256).

But none of that is something we need to worry about now, and
supporting both 38 and 62 characters depending on "the_hash_algo"
removes another case of SHA-1 hardcoding.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 builtin/gc.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/builtin/gc.c b/builtin/gc.c
index 8c2312681c..733bd7bdf4 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -156,6 +156,7 @@ static int too_many_loose_objects(void)
 	int auto_threshold;
 	int num_loose = 0;
 	int needed = 0;
+	const unsigned hexsz_loose = the_hash_algo->hexsz - 2;
 
 	dir = opendir(git_path("objects/17"));
 	if (!dir)
@@ -163,8 +164,8 @@ static int too_many_loose_objects(void)
 
 	auto_threshold = DIV_ROUND_UP(gc_auto_threshold, 256);
 	while ((ent = readdir(dir)) != NULL) {
-		if (strspn(ent->d_name, "0123456789abcdef") != 38 ||
-		    ent->d_name[38] != '\0')
+		if (strspn(ent->d_name, "0123456789abcdef") != hexsz_loose ||
+		    ent->d_name[hexsz_loose] != '\0')
 			continue;
 		if (++num_loose > auto_threshold) {
 			needed = 1;
-- 
2.21.0.360.g471c308f928