reftable/stack: move list lock to `struct reftable_stack`

The struct `reftable_addition` is used to modify a given stack, as such,
it also includes a `struct reftable_flock` used to obtain the lock to
the list file. While the scope of the field lies within this struct, it
doesn't allow for optimizations to be made on `struct reftable_stack`
itself.

Move the field to `struct reftable_stack`, allowing us to make a simple
optimization around avoiding a stack reload when we have already
obtained a lock. While this is currently possible in the write path, the
write path also contains multiple branches to reads which only work
on top of `struct reftable_stack`, and we would miss the optimization in
such paths.

Since the lock is now shared across all additions on the same stack, a
second `reftable_addition` that fails to acquire the already held lock
would still call `reftable_addition_close()`, which will release the
`stack->list_lock` which is still held by the first addition. To avoid
this, add a new bit field `locked` to `reftable_addition` that tracks
whether a particular addition is the one holding the lock, and only
release it in that case. Add a unit test to validate this behavior.

While here, remove an unused header file from 'reftable/stack.h'.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions