Re: [PATCH] fsmonitor: only enable it in non-bare repositories

[Jeff Hostetler <git@jeffhostetler.com>]

On 5/3/21 1:56 PM, Ævar Arnfjörð Bjarmason wrote:
> On Mon, May 03 2021, Jeff Hostetler wrote:
> 
>> On 5/3/21 9:58 AM, Ævar Arnfjörð Bjarmason wrote:
>>> On Thu, Apr 29 2021, Johannes Schindelin via GitGitGadget wrote:
>>>
>>>> From: Johannes Schindelin <johannes.schindelin@gmx.de>
>>>>
>>>> The entire point of the FSMonitor is to monitor the worktree changes in
>>>> a more efficient manner than `lstat()`ing all worktree files every time
>>>> we refresh the index.
>>>>
>>>> But if there is no worktree, FSMonitor has nothing to monitor.
>>>>
>>>> So let's ignore if an FSMonitor is configured (e.g. in `~/.gitconfig`)
>>>> and we're running in a repository without worktree.
>>>>
>>>> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
>>>> ---
>> ...
>>>>
>>>> base-commit: 14d50074ff19e68e7a8d718b22d138882087bbc9
>>> This is surely a correct fix for now, but wouldn't it in the future
>>> also
>>> be useful to run it in bare repositories e.g. to be able cache lookups
>>> for non-existing loose objects?
>>>
>>
>> No, the FSMonitor feature only expects data for paths within the
>> working directory.  (And is independent of whether the FS change
>> data is provided by my fsmonitor--daemon or provided by a hook-based
>> provider, such as Watchman.)  The FSMonitor feature uses that data to
>> shortcut scans of the working directory.
> 
> Indeed, hence "in the future". I'm not suggesting that it'll do anything
> useful by watching anything in the .git directory now, but that it might
> be an interesting thing to explore.
> 
>> There is no interaction with the contents of the .git/objects
>> directory and I'm not sure how that would work.
> 
> We'd watch .git/objects and .git/objects/{aa..ff}, then when about to
> check for a loose object we'd avoid hitting the FS.
> 
> I don't know how useful that is post-61c7711cfea (sha1-file: use loose
> object cache for quick existence check, 2018-11-12), but e.g. on NFS
> this sort of thing still mattered. I had a "bigger hammer" approach with
> [1] that ran (and still does, I believe) on a big corporate
> installation.
> 
> More generally, if you strace .git access during repo operations you'll
> find we're doing all sorts of existence checks etc. all the time. Loose
> objects, refs, seeing what packs there are (better with the MIDX, but do
> we still fall back?) etc. If we had up-to-date inotify/fsmonitor info we
> could ask the daemon about it.
> 
> 1. https://lore.kernel.org/git/20181028225023.26427-5-avarab@gmail.com/
> 

Interesting.

That's certainly something to look into later.  I know there is
code in the object lookup code to rescan/reload the packfiles or
loose objects (under the assumption that another process just created
a new packfile (and after our process loaded the packed-git list)).
Such a daemon might be helpful to improve something like that.

But I can't think about any of that right now.  I'd like to finish the
current fsmonitor--daemon patch series and let it settle down
before starting to think about an orthogonal use case such as this.

Thanks
Jeff