commit-graph idea: warn when disabled for incompatible repositories

> Accidentally, my first use-case was a local copy of a big repository
> (chromium source) that another developer had cloned shallow (I wasn't
> even aware of their clone being shallow at that point).
>
> After spending a little time trying to figure out why no commit-graph
> file was being created, I noticed that it worked just fine testing in
> a fresh git repo.
>
> Then I discovered the .git/shallow file in the big repo. So I did
> fetch --unshallow, and commit-graph started working. Taking a 20
> second log --graph operation down to less than a second (wooo!).
>
> I saw some recent release notes that mentions that commit-graph is
> disabled in incompatible repositories (graft, replace). I assume this
> also be the case for shallow clones.

> Here's the idea that may help others on the same path: Some warning
> output when attempting to use commit-graph when it is disabled (either
> by configuration or automatically).
>
> I think others that come across commit-graph may have tried such
> tricks (like shallow clones) to better work with their repositories,
> and it could be frustrating that commit-graph has no apparent effect.

> On 12/18/2018 9:22 AM, Thomas Ferris Nicolaisen wrote:
>> Hey there,
>
> Hi, Thomas!
>
>> Accidentally, my first use-case was a local copy of a big repository
>> (chromium source) that another developer had cloned shallow (I wasn't
>> even aware of their clone being shallow at that point).
>>
>> After spending a little time trying to figure out why no commit-graph
>> file was being created, I noticed that it worked just fine testing in
>> a fresh git repo.
>>
>> Then I discovered the .git/shallow file in the big repo. So I did
>> fetch --unshallow, and commit-graph started working. Taking a 20
>> second log --graph operation down to less than a second (wooo!).
>>
>> I saw some recent release notes that mentions that commit-graph is
>> disabled in incompatible repositories (graft, replace). I assume this
>> also be the case for shallow clones.
>
> The commit-graph feature is not designed to work well with these
> features, and the "easy" fix we have so far is to completely avoid the
> interaction. The tricky bits come in when the commit parents can
> "change" according to these other features. The commit-graph would
> need to change at the same time, and this is actually very difficult
> to get correct.
>
>> Here's the idea that may help others on the same path: Some warning
>> output when attempting to use commit-graph when it is disabled (either
>> by configuration or automatically).
>>
>> I think others that come across commit-graph may have tried such
>> tricks (like shallow clones) to better work with their repositories,
>> and it could be frustrating that commit-graph has no apparent effect.
>
> This is a good idea, and I would happily review a patch that added
> such a warning.
>
> The warning would want to be in builtin/commit-graph.c, and use the
> commit_graph_compatible() method from commit-graph.c. (You'll need to
> expose the method in the .h file.)

> diff --git a/builtin/gc.c b/builtin/gc.c
> index 871a56f1c5..702568b70d 100644
> --- a/builtin/gc.c
> +++ b/builtin/gc.c
> @@ -662,9 +662,14 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
>   	if (pack_garbage.nr > 0)
>   		clean_pack_garbage();
>
> -	if (gc_write_commit_graph)
> +	if (gc_write_commit_graph) {
> +		int verbose = !quiet && !daemonized;
> +		if (verbose && !commit_graph_compatible(the_repository))
> +			warning(_("The `gc.writeCommitGraph' setting is on, "
> +				  "but commit_graph_compatible() = false"));
>   		write_commit_graph_reachable(get_object_directory(), 0,
> -					     !quiet && !daemonized);
> +					     verbose);
> +	}

>   	if (auto_gc && too_many_loose_objects())
>   		warning(_("There are too many unreachable loose objects; "
> diff --git a/commit-graph.c b/commit-graph.c
> index 40c855f185..60915bf9aa 100644
> --- a/commit-graph.c
> +++ b/commit-graph.c
> @@ -61,7 +61,7 @@ static struct commit_graph *alloc_commit_graph(void)
>
>   extern int read_replace_refs;
>
> -static int commit_graph_compatible(struct repository *r)
> +int commit_graph_compatible(struct repository *r)
>   {
>   	if (!r->gitdir)
>   		return 0;
> diff --git a/commit-graph.h b/commit-graph.h
> index 9db40b4d3a..7c92d41a28 100644
> --- a/commit-graph.h
> +++ b/commit-graph.h
> @@ -12,6 +12,8 @@ struct commit;
>
>   char *get_commit_graph_filename(const char *obj_dir);
>
> +int commit_graph_compatible(struct repository *r);
> +
>   /*
>    * Given a commit struct, try to fill the commit struct info, including:
>    *  1. tree object