Re: [Tarantool-patches] [PATCH] vinyl: bump dump_generation in case scheduler doesn't catch up with DDL

[Aleksandr Lyapunov <alyapunov@tarantool.org>]

Thanks for the patch, lgtm.
Actually it's a bit complicated, as well vy_scheduler itself.

> It may turn out that dump_generation does not catch up with current
> generation and no other dump tasks are in progress. This may happen
> dump process is throttled due to errors. In this case generation is
> bumped but dump_generation is not (since dump is not completed). In
> turn, throttling opens a window for DDL operations. For instance, index
> dropping and creation of new one results in mentioned situation:
>
> box.snapshot() -- fails for some reason; next attempt at dumping will be
>                 -- taken in one second.
> s:drop() -- drop index to be dumped
> s = box.schema.space.create('test', {engine = 'vinyl'})
> -- create new one (its mem generation is greater than scheduler's one)
> i = s:create_index('pk')
>
> Closes #4821
> ---
> Note that current fix is only workaround amd doesn't pretend to be
> the most proper one. Alternatively, we can rollback scheduler
> generation in case dump is failed and index to be dumped is dropped.
>
> Branch:https://github.com/tarantool/tarantool/tree/np/gh-4821-scheduler-gen-assert
> Issue:https://github.com/tarantool/tarantool/issues/4821
>
>   src/box/vy_scheduler.c                        | 14 ++++-
>   .../gh-4821-ddl-during-throttled-dump.result  | 57 +++++++++++++++++++
>   ...gh-4821-ddl-during-throttled-dump.test.lua | 22 +++++++
>   test/vinyl/suite.ini                          |  2 +-
>   4 files changed, 92 insertions(+), 3 deletions(-)
>   create mode 100644 test/vinyl/gh-4821-ddl-during-throttled-dump.result
>   create mode 100644 test/vinyl/gh-4821-ddl-during-throttled-dump.test.lua
>
> diff --git a/src/box/vy_scheduler.c b/src/box/vy_scheduler.c
> index bf4c3fe58..880304b66 100644
> --- a/src/box/vy_scheduler.c
> +++ b/src/box/vy_scheduler.c
> @@ -1846,9 +1846,19 @@ retry:
>   	/*
>   	 * Dump is in progress, but all eligible LSM trees are
>   	 * already being dumped. Wait until the current round
> -	 * is complete.
> +	 * is complete. But if there's no any other tasks
> +	 * in progress, it may mean that dump_generation does
> +	 * not catch up with current generation. This may happen
> +	 * due to failed dump process (i.e. generation is bumped
> +	 * but dump_generation is not). In turn, after dump is failed,
> +	 * next dump will be throttled which opens a window for DDL
> +	 * operations. For instance, index dropping and creation of
> +	 * new one results in mentioned situation.
>   	 */
> -	assert(scheduler->dump_task_count > 0);
> +	if (scheduler->dump_task_count == 0) {
> +		scheduler->dump_generation = vy_lsm_generation(lsm);
> +		goto retry;
> +	}
>   no_task:
>   	if (worker != NULL)
>   		vy_worker_pool_put(worker);