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

Hi Duy,

On Fri, 18 Mar 2016, Duy Nguyen wrote:

On Fri, Mar 18, 2016 at 2:17 PM, Johannes Schindelin
[off-list ref] wrote:

quoted

On Fri, 18 Mar 2016, Duy Nguyen wrote:

quoted

quoted

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