Re: [tarantool-patches] [PATCH v1 1/1] box: restore region after box.snapshot()

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Sat, Nov 03, 2018 at 05:11:42PM +0300, imeevma@tarantool.org wrote:
> Before this patch region wasn't restored after box.snapshot() so
> some tests fail due to "memory leak". After this patch region will
> be restored using region_truncate(). It could be done by cleaning
> memory in case there wasn't active transaction but it seems like
> region_truncate() is more universal.
> 
> Closes #3732
> ---
> Issue: https://github.com/tarantool/tarantool/issues/3732
> Branch: https://github.com/tarantool/tarantool/tree/imeevma/gh-3732-restore_region_after_box_snapshot
> 
>  src/box/box.cc                |  2 ++
>  test/engine/snapshot.result   | 35 +++++++++++++++++++++++++++++++++++
>  test/engine/snapshot.test.lua | 17 +++++++++++++++++
>  3 files changed, 54 insertions(+)
> 
> diff --git a/src/box/box.cc b/src/box/box.cc
> index c4b4fb4..0d27aa2 100644
> --- a/src/box/box.cc
> +++ b/src/box/box.cc
> @@ -2157,6 +2157,7 @@ box_checkpoint()
>  	if (! is_box_configured)
>  		return 0;
>  	int rc = 0;
> +	size_t used = region_used(&fiber()->gc);
>  	if (box_checkpoint_is_in_progress) {
>  		diag_set(ClientError, ER_CHECKPOINT_IN_PROGRESS);
>  		return -1;
> @@ -2181,6 +2182,7 @@ end:
>  
>  	latch_unlock(&schema_lock);
>  	box_checkpoint_is_in_progress = false;
> +	region_truncate(&fiber()->gc, used);
>  	return rc;
>  }

IMHO this looks confusing, because we typically do region_truncate only
in functions/modules that actually allocate something on the region.
Here one has to go much deeper to understand what's going on (I assume
it's memtx_engine_begin_checkpoint) so chances are high somebody will
try to delete this useless code sooner or later. We could probably add a
comment here to clear things up, but if memtx_engine_begin_checkpoint is
changed one day, we will definitely forget to remove the comment that
will become obsolete.

I'd prefer if you did region_truncate where objects are actually
allocated or not use the region allocator at all - it doesn't seem to be
a hot path after all.