(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
