Re: [Tarantool-patches] [PATCH] test: stabilize flaky fiber memory leak detection

[Nikita Pettik <korablev@tarantool.org>]

On 29 Jan 20:03, Alexander Turenko wrote:
> After #4736 regression fix (in fact it just reverts the new logic in
> small) it is possible again that a fiber's region may hold a memory for
> a while, but release it eventually. When the used memory exceeds 128 KiB
> threshold, fiber_gc() puts 'garbage' slabs back to slab_cache and
> subtracts them from region_used() metric. But until this point those
> slabs are accounted in region_used() and so in fiber.info() metrics.
> 
> This commit fixes flakiness of test cases of the following kind:
> 
>  | fiber.info()[fiber.self().id()].memory.used -- should be zero
>  | <...workload...>
>  | fiber.info()[fiber.self().id()].memory.used -- should be zero
> 
> The problem is that the first `<...>.memory.used` value may be non-zero.
> It depends of previous tests that were executed on this tarantool
> instance. It is resolved by restarting of a tarantool instance before
> such test cases to ensure that there are no 'garbage' slabs in a current
> fiber's region.

Hm, why not simply save value of ..memory.used before workload, value
after workload and compare them:

reg_sz_before = fiber.info()[fiber.self().id()].memory.used
...
reg_sz_after = fiber.info()[fiber.self().id()].memory.used

assert(reg_sz_before == reg_sz_after);

So that make sure workload returns all occupied memory. This may fail
only in case allocated memory > 4kb, but examples in this particular
case definitely don't require so many memory (as you noted below).

> Note: This works only if a test case reserves only one slab at the
> moment: otherwise some memory may be hold after the case (and so a
> memory check after a workload will fail). However it seems that our
> cases are small enough to don't trigger this situation.
> 
> Call of region_free() would be enough, but we have no Lua API for it.
> 
> Fixes #4750.
> ---
>