[PATCH v2 0/4] Wait for the index lock instead of failing

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

From: Jun Wu <quark@meta.com>

When another process holds the index lock, commands like "git add",
"git checkout" and "git commit" fail immediately, causing user
frustration. This series lets commands that must write the index to wait
briefly for the index lock (core.indexLockTimeout, default 1s), to be
more user friendly.

"another process that holds the index lock" is often "git status" run
periodically by an IDE, or a tool. Today, such tools have 2 choices:
- "git status": risks making "checkout", "commit" fail more often.
- "git --no-optional-locks status": because "status" no longer writes
  the refreshed stat info, "git --no-optional-locks status" will become
  slower and slower over time, assuming no other commands refresh the
  index.

This series aims for the end state that:
- Tool authors can use background "git status" just fine, and "status"
  won't become slower. They can still use "--no-optional-locks", with
  no behavior changes.
- End users can run "git commit/checkout" just fine, without seeing
  command failures due to the index lock.

Conservatively, the new wait behavior disables itself for commands with
write race (typically "read (index) -> lock -> (no reload) -> write"),
so those racy commands stay unchanged, instead of getting a larger race
window (between "read" and "lock"). This series fixes the write races
for "commit" and "reset" so the two commands, together with other
commands that already properly lock before reading, all get the new
wait behavior. The trade-off is that "commit" now lstat() the working
tree under the index lock.

Changes since v1:
- Explain details more clearly.
- New patch 1: make "git status"'s non-waiting behavior explicit.
- New patch 4: fix "reset" write race, similar to "commit".
- Patch 3 and 4: use "write race" as motivation.

Jun Wu (4):
  repository: add repo_try_hold_locked_index()
  repository: add core.indexLockTimeout
  commit: fix write race
  reset: fix write race

 Documentation/config/core.adoc |  6 +++++
 builtin/commit.c               | 11 +++------
 builtin/describe.c             |  4 ++--
 builtin/diff.c                 |  2 +-
 builtin/merge.c                |  2 +-
 builtin/rebase.c               |  2 +-
 builtin/reset.c                |  7 +++---
 repository.c                   | 33 ++++++++++++++++++++++++++
 repository.h                   | 11 +++++++++
 sequencer.c                    |  2 +-
 t/meson.build                  |  1 +
 t/t0031-lockfile-pid.sh        |  4 ++++
 t/t0032-index-lock-timeout.sh  | 42 ++++++++++++++++++++++++++++++++++
 t/t7400-submodule-basic.sh     |  1 +
 wt-status.c                    |  2 +-
 15 files changed, 112 insertions(+), 18 deletions(-)
 create mode 100755 t/t0032-index-lock-timeout.sh


base-commit: c51408889c3b372814bc20b9431c3b74d0d268eb
-- 
2.56.0