Re: [PATCH 18/19] index-helper: autorun

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Hi Duy,

On Fri, 18 Mar 2016, Duy Nguyen wrote:

> On Fri, Mar 18, 2016 at 2:17 PM, Johannes Schindelin
> <Johannes.Schindelin@xxxxxx> wrote:
> >
> > On Fri, 18 Mar 2016, Duy Nguyen wrote:
> >
> >> > Well, the way I read the code it is possible that:
> >> >
> >> > 1. Git process 1 starts, reading the index
> >> > 2. Git process 2 starts, poking the index-helper
> >> > 3. The index-helper updates the .pid file (why not set a bit in the shared
> >> >    memory?) with a prefix "W"
> >> > 4. Git process 2 reads the .pid file and waits for the "W" to go away
> >> >    (what if index-helper is not fast enough to write the "W"?)
> >> > 5. Git process 1 access the index, happily oblivious that it is being
> >> >    updated and the data is in an inconsistent state
> >>
> >> No, if process 1 reads the index file, then that file will remain
> >> consistent/unchanged all the time. index-helper is not allowed to
> >> touch that file at all.
> >>
> >> The process 2 gets the index content from shm (cached by the index
> >> helper), verifies that it's good (with the signature at the end of the
> >> shm). If watchman is used, process 2 can also read the list of
> >> modified files from another shm, combine it with the in-core index,
> >> then write it down the normal way. Only then process 1 (or process 3)
> >> can see the new index content from the file.
> >
> > So how do you deal with releasing the shared memory instances that are
> > essentially created for every index update?
> 
> When index-helper reads the index file and realizes the file has been
> updated (based on trailing SHA-1), it unlink()s the old shm and
> prepares new shm. If no process is accessing old shm, it's gone. If
> not, it stays until nobody elses uses it. shm on Windows behaves the
> same way, I believe.

Ah. On Windows, you do not even have to unlink: once the last user's gone,
the shared memory's gone, too.

Ciao,
Dscho
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