Re: [PATCH] builtin/commit.c: prevent bad commits

[Jeff King <peff@peff.net>]

On Sun, Apr 01, 2018 at 05:06:50PM +0300, Dan Aloni wrote:

> These commits which have hashes starting with the hex string 'bad',
> always give me the chills. Why should a perfectly good commit be
> jinxed?
> 
> Statistically, one of 4096 commits may be 'bad'. This change adds a
> '--prevent-bad' switch to the commit command in order to prevent such
> commit hashes from being generated. Internally, the commit is retried
> with a slight commit meta-data modification - a newline is added to the
> end of the commit message. The meta-data change results in a different
> hash, that if we are lucky enough (4095/4096 chance) may not be 'bad'.
> 
> Note that this change does not affect actual software quality maintained
> using Git. Thus, it is recommended keep testing all generated versions
> regardless of commit hash jinxes.

Cute.

A while back we had patches to generate "desirable" commit prefixes, and
they were focused on making the brute-forcing as fast as possible. I
don't think I've seen this reverse case, but it's much easier: we can be
fairly slow since the probability of hitting the bad case repeatedly is
low.

> @@ -1583,12 +1585,34 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
>  		append_merge_tag_headers(parents, &tail);
>  	}
>  
> -	if (commit_tree_extended(sb.buf, sb.len, &active_cache_tree->oid,
> -				 parents, &oid, author_ident.buf, sign_commit,
> -				 extra)) {
> -		rollback_index_files();
> -		die(_("failed to write commit object"));
> +	for (;;) {
> +		char *oid_hex;
> +		struct commit_list *copy_parents;
> +
> +		copy_parents = copy_commit_list(parents);
> +
> +		if (commit_tree_extended(sb.buf, sb.len, &active_cache_tree->oid,
> +					 parents, &oid, author_ident.buf, sign_commit,
> +					 extra)) {
> +			rollback_index_files();
> +			die(_("failed to write commit object"));
> +		}

You could still have a "bad" tree. :)

-Peff