Re: [PATCH] add core.usereplacerefs config option

> We can already disable replace refs using a command line
> option or environment variable, but those are awkward to
> apply universally. Let's add a config option to do the same
> thing.
>
> That raises the question of why one might want to do so
> universally. The answer is that replace refs violate the
> immutability of objects. For instance, if you wanted to
> cache the diff between commit XYZ and its parent, then in
> theory that never changes; the hash XYZ represents the total
> state. But replace refs violate that; pushing up a new ref
> may create a completely new diff.
>
> The obvious "if it hurts, don't do it" answer is not to
> create replace refs if you're doing this kind of caching.
> But for a site hosting arbitrary repositories, they may want
> to allow users to share replace refs with each other, but
> not actually respect them on the site (because the caching
> is more important than the replace feature).
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> We've been using this patch for about 4 years at GitHub. I'm not sure
> why I never sent it upstream until now, since it's pretty trivial.
>
> I think this should interact OK with Stefan's recent c3c36d7de2
> (replace-object: check_replace_refs is safe in multi repo environment,
> 2018-04-11), because we still consider check_replace_refs before doing
> anything in lookup_replace_object(). So even if you accidentally
> _loaded_ the replace refs due to a timing issue (e.g., looking at them
> before reading config) we still wouldn't respect them as long as you've
> loaded config by the time you're actually looking at objects.
>
> I followed the existing style of t6050 here, but it looks like it could
> use some modernization (indent with spaces, and piping git command
> output losing the exit codes).
>
>   Documentation/config.txt | 5 +++++
>   config.c                 | 5 +++++
>   t/t6050-replace.sh       | 6 ++++++
>   3 files changed, 16 insertions(+)
>
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index 1cc18a828c..92b277d27b 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -908,6 +908,11 @@ core.commitGraph::
>   	Enable git commit graph feature. Allows reading from the
>   	commit-graph file.
>   
> +core.useReplaceRefs::
> +	If set to `false`, behave as if the `--no-replace-objects`
> +	option was given on the command line. See linkgit:git[1] and
> +	linkgit:git-replace[1] for more information.
> +
>   core.sparseCheckout::
>   	Enable "sparse checkout" feature. See section "Sparse checkout" in
>   	linkgit:git-read-tree[1] for more information.
> diff --git a/config.c b/config.c
> index f4a208a166..ce103ebc20 100644
> --- a/config.c
> +++ b/config.c
> @@ -1346,6 +1346,11 @@ static int git_default_core_config(const char *var, const char *value)
>   					 var, value);
>   	}
>   
> +	if (!strcmp(var, "core.usereplacerefs")) {
> +		check_replace_refs = git_config_bool(var, value);
> +		return 0;
> +	}
> +
>   	/* Add other config variables here and to Documentation/config.txt. */
>   	return 0;
>   }
> diff --git a/t/t6050-replace.sh b/t/t6050-replace.sh
> index aa3e249639..86374a9c52 100755
> --- a/t/t6050-replace.sh
> +++ b/t/t6050-replace.sh
> @@ -113,6 +113,12 @@ test_expect_success 'test GIT_NO_REPLACE_OBJECTS env variable' '
>        GIT_NO_REPLACE_OBJECTS=1 git show $HASH2 | grep "A U Thor"
>   '
>   
> +test_expect_success 'test core.usereplacerefs config option' '
> +	test_config core.usereplacerefs false &&
> +	git cat-file commit $HASH2 | grep "author A U Thor" &&
> +	git show $HASH2 | grep "A U Thor"
> +'
> +
>   cat >tag.sig <<EOF
>   object $HASH2
>   type commit