On Fri, Feb 08, 2019 at 02:51:13PM -0500, Ben Peart wrote:
> From: Ben Peart <benpeart@microsoft.com>
> 
> Add a post-indexchanged hook that is invoked after the index is written in
> do_write_locked_index().
> 
> This hook is meant primarily for notification, and cannot affect
> the outcome of git commands that trigger the index write.
> 
> Signed-off-by: Ben Peart <benpeart@microsoft.com>

First, I think the tests should be merged into this commit. That's what
we typically do.

I'm also going to bikeshed slightly and suggest "post-index-changed",
since we normally use dashes between words in our hook names.

> diff --git a/cache.h b/cache.h
> index 27fe635f62..46eb862d3e 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -338,7 +338,9 @@ struct index_state {
>  	struct cache_time timestamp;
>  	unsigned name_hash_initialized : 1,
>  		 initialized : 1,
> -		 drop_cache_tree : 1;
> +		 drop_cache_tree : 1,
> +		 updated_workdir : 1,
> +		 updated_skipworktree : 1;

How important is it that we expose whether the skip-worktree bit is
changed? I can understand if we expose the workdir is updated, since
that's a thing a general user of this hook is likely to be interested
in. However, I'm not sure that for a general-purpose hook, the
skip-worktree bit is interesting.

> diff --git a/read-cache.c b/read-cache.c
> index 0e0c93edc9..0fcfa8a075 100644
> --- a/read-cache.c
> +++ b/read-cache.c
> @@ -17,6 +17,7 @@
>  #include "commit.h"
>  #include "blob.h"
>  #include "resolve-undo.h"
> +#include "run-command.h"
>  #include "strbuf.h"
>  #include "varint.h"
>  #include "split-index.h"
> @@ -2999,8 +3000,17 @@ static int do_write_locked_index(struct index_state *istate, struct lock_file *l
>  	if (ret)
>  		return ret;
>  	if (flags & COMMIT_LOCK)
> -		return commit_locked_index(lock);
> -	return close_lock_file_gently(lock);
> +		ret = commit_locked_index(lock);
> +	else
> +		ret = close_lock_file_gently(lock);
> +
> +	run_hook_le(NULL, "post-indexchanged",
> +			istate->updated_workdir ? "1" : "0",
> +			istate->updated_skipworktree ? "1" : "0", NULL);

I have, in general, some concerns about this API. First, I think we need
to consider that if we're going to expose various bits of information,
we might in the future want to expose more such bits. If so, adding
integer parameters is not likely to be a good way to do this. It's hard
to remember and if a binary is used as the hook, it may not always
handle additional arguments gracefully like shell scripts tend to.

If we're not going to expose the skip-worktree bit, then I suppose one
argument is fine. Otherwise, it might be better to expose key-value
pairs on stdin instead, or something like that.

Finally, I have questions about performance. What's the overhead of
determining whether the hook exists in this code path when there isn't
one? Since the index is frequently used, and can be written out as an
optimization by some commands, it would be nice to keep overhead low if
the hook isn't present.
-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204