Re: [PATCH v1] dir.c: don't flag the index as dirty for changes to the untracked cache

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

Ben Peart <benpeart@microsoft.com> writes:

> The untracked cache saves its current state in the UNTR index extension.
> Currently, _any_ change to that state causes the index to be flagged as dirty
> and written out to disk.  Unfortunately, the cost to write out the index can
> exceed the savings gained by using the untracked cache.  Since it is a cache
> that can be updated from the current state of the working directory, there is
> no functional requirement that the index be written out for every change to the
> untracked cache.
>
> Update the untracked cache logic so that it no longer forces the index to be
> written to disk except in the case where the extension is being turned on or
> off.  When some other git command requires the index to be written to disk, the
> untracked cache will take advantage of that to save it's updated state as well.
> This results in a performance win when looked at over common sequences of git
> commands (ie such as a status followed by add, commit, etc).
>
> After this patch, all the logic to track statistics for the untracked cache
> could be removed as it is only used by debug tracing used to debug the untracked
> cache.
>
> Signed-off-by: Ben Peart <benpeart@microsoft.com>
> ---
>

OK, so in other words (note: not a suggestion to use different
wording in the log message; just making sure I got the motivation
behind this change correctly), without this new environment
variable, changes to untracked cache alone (due to observed changes
in the filesystem) does not count as "in-core index changed so we
need to write it back to the disk".

That makes sense to me.

Is it envisioned that we want to have similar but different "testing
only" behaviour around this area?  If not, this environment variable
sounds more like "force-flush untracked cache", not "test untracked
cache", to me.

> +GIT_TEST_UNTRACKED_CACHE=true
> +export GIT_TEST_UNTRACKED_CACHE
> +
>  sync_mtime () {
>  	find . -type d -ls >/dev/null
>  }
>
> base-commit: 5be1f00a9a701532232f57958efab4be8c959a29