packfile: fix race condition on unpack_entry()

packfile: fix race condition on unpack_entry()

The third phase of unpack_entry() performs the following sequence in a
loop, until all the deltas enumerated in phase one are applied and the
entry is fully reconstructed:

1. Add the current base entry to the delta base cache
2. Unpack the next delta
3. Patch the unpacked delta on top of the base

When the optional object reading lock is enabled, the above steps will
be performed while holding the lock. However, step 2. momentarily
releases it so that inflation can be performed in parallel for increased
performance. Because the `base` buffer inserted in the cache at 1. is
not duplicated, another thread can potentially free() it while the lock
is released at 2. (e.g. when there is no space left in the cache to
insert another entry). In this case, the later attempt to dereference
`base` at 3. will cause a segmentation fault. This problem was observed
during a multithreaded git-grep execution on a repository with large
objects.

To fix the race condition (and later segmentation fault), let's reorder
the aforementioned steps so that `base` is only added to the cache at
the end. This will prevent the buffer from being released by another
thread while it is still in use. An alternative solution which would not
require the reordering would be to duplicate `base` before inserting it
in the cache. However, as Phil Hord mentioned, memcpy()'ing large bases
can negatively affect performance: in his experiments, this alternative
approach slowed git-grep down by 10% to 20%.

Reported-by: Phil Hord <phil.hord@gmail.com>
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>