reftable/stack: fix race in up-to-date check

reftable/stack: fix race in up-to-date check

In 6fdfaf15a0 (reftable/stack: use stat info to avoid re-reading stack
list, 2024-01-11) we have introduced a new mechanism to avoid re-reading
the table list in case stat(3P) figures out that the stack didn't change
since the last time we read it.

While this change significantly improved performance when writing many
refs, it can unfortunately lead to false negatives in very specific
scenarios. Given two processes A and B, there is a feasible sequence of
events that cause us to accidentally treat the table list as up-to-date
even though it changed:

  1. A reads the reftable stack and caches its stat info.

  2. B updates the stack, appending a new table to "tables.list". This
     will both use a new inode and result in a different file size, thus
     invalidating A's cache in theory.

  3. B decides to auto-compact the stack and merges two tables. The file
     size now matches what A has cached again. Furthermore, the
     filesystem may decide to recycle the inode number of the file we
     have replaced in (2) because it is not in use anymore.

  4. A reloads the reftable stack. Neither the inode number nor the
     file size changed. If the timestamps did not change either then we
     think the cached copy of our stack is up-to-date.

In fact, the commit introduced three related issues:

  - Non-POSIX compliant systems may not report proper `st_dev` and
    `st_ino` values in stat(3P), which made us rely solely on the
    file's potentially coarse-grained mtime and ctime.

  - `stat_validity_check()` and friends may end up not comparing
    `st_dev` and `st_ino` depending on the "core.checkstat" config,
    again reducing the signal to the mtime and ctime.

  - `st_ino` can be recycled, rendering the check moot even on
    POSIX-compliant systems.

Given that POSIX defines that "The st_ino and st_dev fields taken
together uniquely identify the file within the system", these issues led
to the most important signal to establish file identity to be ignored or
become useless in some cases.

Refactor the code to stop using `stat_validity_check()`. Instead, we
manually stat(3P) the file descriptors to make relevant information
available. On Windows and MSYS2 the result will have both `st_dev` and
`st_ino` set to 0, which allows us to address the first issue by not
using the stat-based cache in that case. It also allows us to make sure
that we always compare `st_dev` and `st_ino`, addressing the second
issue.

The third issue of inode recycling can be addressed by keeping the file
descriptor of "files.list" open during the lifetime of the reftable
stack. As the file will still exist on disk even though it has been
unlinked it is impossible for its inode to be recycled as long as the
file descriptor is still open.

This should address the race in a POSIX-compliant way. The only real
downside is that this mechanism cannot be used on non-POSIX-compliant
systems like Windows. But we at least have the second-level caching
mechanism in place that compares contents of "files.list" with the
currently loaded list of tables.

This new mechanism performs roughly the same as the previous one that
relied on `stat_validity_check()`:

  Benchmark 1: update-ref: create many refs (HEAD~)
    Time (mean ± σ):      4.754 s ±  0.026 s    [User: 2.204 s, System: 2.549 s]
    Range (min … max):    4.694 s …  4.802 s    20 runs

  Benchmark 2: update-ref: create many refs (HEAD)
    Time (mean ± σ):      4.721 s ±  0.020 s    [User: 2.194 s, System: 2.527 s]
    Range (min … max):    4.691 s …  4.753 s    20 runs

  Summary
    update-ref: create many refs (HEAD~) ran
      1.01 ± 0.01 times faster than update-ref: create many refs (HEAD)

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>