[PATCH 0/2] Add a way to wait for index lock

[Jun Wu <quark.zju@gmail.com>]

From: Jun Wu <quark@meta.com>

An IDE that runs `git status` periodically in the background has 2
choices. Both are suboptimal:
- with `--no-optional-locks`: `status` becomes slower over time.
- without `--no-optional-locks`: conflicts with other commands like
  `add`, `commit`, often run by a user from the IDE terminal.

Patch 1 adds index lock timeout so `add` can wait instead of fail.
Patch 2 adjusts the read/lock order so `commit` can also wait.

To minimize write race, if the index is read before locking, the
wait will be skipped, as if timeout is 0. Depending on the read/lock
order, some commands (e.g. add, rm, mv, checkout, restore, read-tree,
apply, update-index) respect the timeout. Some (e.g. reset, stash,
merge, rebase, cherry-pick, am) do not. I started with the simple and
common commands. If there is interest in other commands I can take a
look too.

The default timeout is 1s (reasoning in patch 1). I'm happy to change it
to 0 if people prefer no behavior change.

Related context:
- (2017-9-22) https://lore.kernel.org/git/20170923033425.6ilykxl5pi7unlj6@sigill.intra.peff.net/
  proposed index lock timeout.
- (2017-9-27) Commit 27344d6a6c (git: add --no-optional-locks option, 2017-09-27)
  introduces `--no-optional-locks` for the same issue.
- (2019-5-3) https://lore.kernel.org/git/CACsJy8BcBk6Fsfy7o1aBTQAPeH-w+rBwLZrzw4jJ-OM_jhkxMQ@mail.gmail.com/
  mentioned `update_if_able` and "gap" between read and lock, mitigated
  in patch 1 with the `index->initialized` check.

Jun Wu (2):
  repository: add core.indexLockTimeout
  commit: lock index before reading

 Documentation/config/core.adoc |  6 ++++++
 builtin/commit.c               |  9 ++-------
 repository.c                   | 17 +++++++++++++++-
 t/meson.build                  |  1 +
 t/t0031-lockfile-pid.sh        |  4 ++++
 t/t0032-index-lock-timeout.sh  | 37 ++++++++++++++++++++++++++++++++++
 t/t7400-submodule-basic.sh     |  1 +
 7 files changed, 67 insertions(+), 8 deletions(-)
 create mode 100755 t/t0032-index-lock-timeout.sh


base-commit: 6de20f6092dcf9bdb1c8efe03db4b70c82b423dd
-- 
2.56.0