Re: [PATCH v2 3/5] index-pack: check (and optionally set) hash algo based on input file

After 454253f059 (builtin/index-pack: improve hash function abstraction
- 2018-02-01), index-pack uses the_hash_algo for hashing. If "git
index-pack" is executed without a repository, we do not know what hash
algorithm to be used and the_hash_algo in theory could be undefined.

Since there should be some information about the hash algorithm in the
input pack file, we can initialize the correct hash algorithm with that
if the_hash_algo is not yet initialized. This assumes that pack files
with new hash algorithm MUST step up pack version.

While at there, make sure the hash algorithm requested by the pack file
and configured by the repository (if we're running with a repo) are
consistent.

Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
---
 builtin/index-pack.c | 26 +++++++++++++++++++++++++-
 1 file changed, 25 insertions(+), 1 deletion(-)

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 7e3e1a461c..1144458140 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c

@@ -326,10 +326,31 @@ static const char *open_pack_file(const char *pack_name)
 		output_fd = -1;
 		nothread_data.pack_fd = input_fd;
 	}
-	the_hash_algo->init_fn(&input_ctx);
 	return pack_name;
 }
 
+static void prepare_hash_algo(uint32_t pack_version)
+{
+	const struct git_hash_algo *pack_algo;
+
+	switch (pack_version) {
+	case 2:
+	case 3:
+		pack_algo = &hash_algos[GIT_HASH_SHA1];
+		break;
+	default:
+		die("BUG: how to determine hash algo for new version?");
+	}
+
+	if (!the_hash_algo)	/* running without repo */
+		the_hash_algo = pack_algo;
+
+	if (the_hash_algo != pack_algo)
+		die(_("incompatible hash algorithm, "
+		      "configured for %s but the pack file needs %s"),
+		    the_hash_algo->name, pack_algo->name);