Re: [Tarantool-patches] [PATCH v2 4/4] qsync: don't send negative timeouts into fiber_cond_wait_timeout

[Vladislav Shpilevoy <v.shpilevoy@tarantool.org>]

>> On 14.07.2020 16:53, Cyrill Gorcunov wrote:
>>> Basically our timeout is calculated via (a - b), where
>>> @a is a constant positive value fetched once, in turn
>>> the @b is rather a dynamic value thus the result may
>>> be negative. libev uses assert() call to catch such
>>> values when passed to timers setup. Thus lets intercept
>>> potential assert() trigger and exit early if timeout
>>> is already expired.
>>>
>>> Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
>>> ---
>>> There were a typo, so I force-updated the branch
>>>
>>>  src/box/txn_limbo.c | 8 ++++++--
>>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/src/box/txn_limbo.c b/src/box/txn_limbo.c
>>> index d5b887d36..0924952b7 100644
>>> --- a/src/box/txn_limbo.c
>>> +++ b/src/box/txn_limbo.c
>>> @@ -174,8 +174,10 @@ txn_limbo_wait_complete(struct txn_limbo *limbo, struct txn_limbo_entry *entry)
>>>  	double start_time = fiber_clock();
>>>  	while (true) {
>>>  		double deadline = start_time + replication_synchro_timeout;
>>> -		bool cancellable = fiber_set_cancellable(false);
>>>  		double timeout = deadline - fiber_clock();
>>> +		if (timeout < 0)
>>> +			goto do_rollback;
>>> +		bool cancellable = fiber_set_cancellable(false);
>>
>> I added timeout = -1; here and tried to commit a sync transaction. I
>> got timed out error, no assertions. Please, tell me, how to reproduce
>> the assertion you mention in the commit message. Otherwise I don't see
>> why would we need the < 0 check. If it is done somewhere inside
>> fiber_cond_wait_timeout anyway.
> 
> Look, here is libev code
> 
> ---
> noinline
> void
> ev_timer_start (EV_P_ ev_timer *w) EV_THROW
> {
>   if (expect_false (ev_is_active (w)))
>     return;
> 
>   ev_at (w) += mn_now;
> 
>   assert (("libev: ev_timer_start called with negative timer repeat value", w->repeat >= 0.));
> ---
> 
> To trigger this assert we have to enter idle cycle, then manually change
> replication_synchro_timeout via cfg (make it less than it was initially)
> which should lead to negative timeout.

Yeah, well. We set w->after. Not w->repeat.

> To be fair I don't know how to force it without error injection. Lets
> assume we have initial fiber clock 1 (the clocks are increasing everytime
> libev does a new polling cycle). Thus
> 
> 	// replication_synchro_timeout = 2
> 
> 	double start_time = fiber_clock();	// 1
> 	while (true) {
> 		double deadline = start_time + replication_synchro_timeout;
> 		// => 3
> 		double timeout = deadline - fiber_clock();
> 		// => 2
> 		bool cancellable = fiber_set_cancellable(false);
> 		int rc = fiber_cond_wait_timeout(&limbo->wait_cond, timeout);
> 		//
> 		// enter into resched cycle
> 		// manually change replication_synchro_timeout to 1
> 		// next cycle starts
> 
> 		double deadline = start_time + replication_synchro_timeout;
> 		// start_time didn't changed
> 		// => 1 + 1 = 2
> 		double timeout = deadline - fiber_clock();
> 		// assume several resched cycles passed, thus
> 		// fiber_clock returns 3, thus
> 		// => 2 - 3 => -1
> 	...
> 
> Am I missing something obvious here?

I still don't see how to trigger this assert. Please, show a code sample with
error injection, if you think that will help. I explicitly added line
'timeout = -1;' and all worked fine.