[GSoC][RFC WIP PATCH 0/3] grep: allow parallelism in zlib inflation

Threads were disabled in git-grep for non-worktree case at 53b8d93 ("grep: disable threading in non-worktree case", 12-12-2011), due to perfomance drops.

To regain performance, this series work on the object reading code so that zlib inflation may be performed in parallel. This is a good hotspot for parallelism as, in some test cases[1], it accounts for up to 48% of execution time. And besides that, inflation tasks are already independent from one another.

Grep'ing 'abcd[02]' ("Regex 1") and '(static|extern) (int|double) \*' ("Regex 2") at chromium's repository[2], I got:

     Threads |   Regex 1  |  Regex 2
    ---------|------------|-----------
        1    |  17.5815s  | 21.7217s
        2    |   9.7983s  | 11.3965s
        8    |   6.3097s  |  6.9667s
    
These are all means of 30 executions after 2 warmup runs. All tests were
executed on a i7-7700HQ with 16GB of RAM and SSD. The output was also
validated against current git-grep.

I still want to repeat the test in an HDD machine and in a repo with mainly loose objects.

There're still some open issues thought:

1) To allow parallel inflation, the `obj_read_mutex` is released at
`unpack_compressed_entry()` and `get_size_from_delta()` right before
calling `git_inflate()` (and re-acquired right after). For git-grep it
seems to be OK. But, besides git-grep, if it's wanted to read objects
and perform other operations in parallel, we probably cannot guarantee
that the lock will be held by the current thread when it gets to those
two functions. Also, as the lock does only protect this call tree, if
those other operations access the same global states through other
paths, we may also have race conditions.

2) git-grep operations on textconv and submodules are still
unprotected and should result in race conditions. I'm not sure if
having two separate mutexes for them would fix it as the latter also has
to access the object store.

Any comments on the series and open issues will be highly appreciated.

[1]: https://matheustavares.gitlab.io/posts/week-6-working-at-zlib-inflation#multithreading-zlib-inflation 
[2]: chromium’s repo at commit 03ae96f (“Add filters testing at DSF=2”,
     04-06-2019), after a 'git gc' execution.

travis build: https://travis-ci.org/matheustavares/git/builds/561200398

Matheus Tavares (3):
  object-store: make read_object_file_extended() thread-safe
  grep: remove locks on object reading operations
  grep: re-enable threads in non-worktree case

 builtin/grep.c | 20 ++++--------------
 grep.c         |  3 ---
 object-store.h |  4 ++++
 packfile.c     |  7 +++++++
 sha1-file.c    | 56 +++++++++++++++++++++++++++++++++++++++++++++-----
 5 files changed, 66 insertions(+), 24 deletions(-)