[PATCH v3 8/8] reftable/merged: transfer ownership of records when iterating

When iterating over records with the merged iterator we put the records into a priority queue before yielding them to the caller. This means that we need to allocate the contents of these records before we can pass them over to the caller.

The handover to the caller is quite inefficient though because we first deallocate the record passed in by the caller and then copy over the new record, which requires us to reallocate memory.

Refactor the code to instead transfer ownership of the new record to the caller. So instead of reallocating all contents, we now release the old record and then copy contents of the new record into place.

The following benchmark of `git show-ref --quiet` in a repository with around 350k refs shows a clear improvement. Before:

    HEAP SUMMARY:
        in use at exit: 21,163 bytes in 193 blocks
      total heap usage: 708,058 allocs, 707,865 frees, 36,783,255 bytes allocated

After:

    HEAP SUMMARY:
        in use at exit: 21,163 bytes in 193 blocks
      total heap usage: 357,007 allocs, 356,814 frees, 24,193,602 bytes allocated

This shows that we now have roundabout a single allocation per record that we're yielding from the iterator. Ideally, we'd also get rid of this allocation so that the number of allocations doesn't scale with the number of refs anymore. This would require some larger surgery though because the memory is owned by the priority queue before transferring it over to the caller.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 reftable/merged.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)