Re: [PATCH] setup: default hash algo to GIT_HASH_DEFAULT

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

> I wonder what the "git init" command does wrt the reference storage
> format (Patrick CC'ed for this)?  I am sure we do not convert from
> reffiles to reftable on the fly at "git init" time.

I looked at setup.c:repository_format_configure() and found why
your patch regresses, as well as a latent bug in the existing code.

First, REPOSITORY_FORMAT_INIT sets .hash_algo = GIT_HASH_DEFAULT,
but read_repository_format() forces it to GIT_HASH_SHA1_LEGACY
when version == -1 (no config yet), which is why new repositories
kept getting SHA-1.

Second, unlike the ref-storage block right below it, which checks
"else if (repo_fmt->version >= 0)" once between the explicit
overrides and all default sources, the hash-algo block guards only
the GIT_DEFAULT_HASH branch (repo_fmt->version < 0), leaving "else
if (cfg.hash != GIT_HASH_UNKNOWN)" unguarded.

So init.defaultObjectFormat in ~/.gitconfig already corrupts an
existing repository on re-init, and an unguarded "else" for
GIT_HASH_DEFAULT falls into the same trap even without config.

Instead of starting the if/else if cascade like this:

	if (repo_fmt->version >= 0 &&
	    hash != GIT_HASH_UNKNOWN &&
	    hash != repo_fmt->hash_algo)
		die(_("attempt to reinitialize repository with different hash"));

we should have started it like so:

	if (repo_fmt->version >= 0) {
		if (hash != GIT_HASH_UNKNOWN && hash != repo_fmt->hash_algo)
			die(_("attempt to reinitialize repository with different hash"));
		/* otherwise happy */
	} else if ...

so that all the "set a new value from different sources like command
line, environment, ~/.gitconfig, or the built-in default" would
happen only when (repo_fmt->version < 0), probably?