midx: avoid potential rename-while-mapped on Windows

(This topic is based on tb/repack-write-midx).

While looking through some failures in t5319 when Git is compiled with SANITIZE=leak, I noticed a curious pattern that basically looks like:

    struct multi_pack_index *m = load_multi_pack_index(object_dir, 0);
    /* ... */
    write_midx_internal();

This can cause problems on platforms where a memory mapped file cannot be overridden via a rename, because load_multi_pack_index() builds a new MIDX from scratch each time it is called without adding that MIDX to the object store. But write_midx_internal() loads a MIDX from the object store, causing the same region to be mapped twice.

The details are fully spelled out in the second patch, but the gist is that while write_midx_internal() unmaps *its* copy of the MIDX, the caller's copy is still mapped in memory.

Strangely, I could not get this to produce a failure on Windows, even though I would have expected one. I asked around off-list, and it sounds like it's possible this behavior has changed, or Windows' mmap-equivalent may not lock files that fit within a single page.

In any case, we let's side step the issue entirely by only interacting with MIDXs that can be found from the object store.

Taylor Blau (4):
midx.c: extract MIDX lookup by object_dir midx.c: lookup MIDX by object directory during expire midx.c: lookup MIDX by object directory during repack midx.c: guard against commit_lock_file() failures

 midx.c | 42 +++++++++++++++++++++++-------------------
 1 file changed, 23 insertions(+), 19 deletions(-)

2.33.0.96.g73915697e6