maria-developers team mailing list archive

[Sergei Golubchik <serg@xxxxxxxxxxx>]

Hi, Kentoku!

On Jul 04, Kentoku wrote:
> revision-id: 23b70146c16 (mariadb-10.4.5-21-g23b70146c16)
> parent(s): 24773bf3802
> author: Kentoku <kentokushiba@xxxxxxxxx>
> committer: Kentoku <kentokushiba@xxxxxxxxx>
> timestamp: 2019-06-10 13:59:18 +0900
> message:
> 
> MDEV-6268 SPIDER table with no COMMENT clause causes queries to wait forever
> Add mysql_mutex_timedlock() and add the following parameter to Spider
> - spider_internal_lock_wait_timeout
>   The timeout when Spider tries to get internal locks.
>   0 or more : the tomeout. (second)
>   The default value is -1

please, don't forget to leave an empty line after the first line in a 
commit comment. That's how various git tools (github included) separate
"commit subject" from the "commit comment body".

Git itself doesn't care, but it's a convention that some git-related
tools rely on.

Now, about the fix.

Svoj made a general obervation on Slack that it's much better to
eliminate or detect deadlocks, instead of wait them out with timeouts.

And here we were able to find a rather cheap solution that would avoid
such deadlocks in Spider. Here, how it can work:

* On startup Spider calculates the unique spider instance ID by
  combining my_gethwaddr() and getpid(). With delimiters it might look
  something like "-d46d6d7345c9:23050-". This is done only once on
  startup.

* When Spider needs to open a table, it checks whether this THD has a
  user variable named `spider_parents` and it it does, whether this
  variable contains the above mentioned unique spider instance ID.
  This is just one strstr(), so fast, no complex parsing.
  If the string is found, Spider returns an error, doesn't open a table.

* When Spider connects to another host now it sends

      show table status from `test` like 'user'

  in the new approach it'll send

      set @spider_parents="${its current spider_parents}-d46d6d7345c9:23050-"; show table status from `test` like 'user'

  this is all in one packet, so no additional round-trips.
  Here `${its current spider_parents}` is the value of @spider_parents
  on the connecting host.

This is it, it'll prevent self-connects, both direct and indirect via a
chain of spider tables.

Because @spider_parents variable is user-visible, better to use a
different name for it, something that has a smaller chance of causing a
conflict. May be @insternal_spider_deadlock_prevention_parent_list :)

Regards,
Sergei
VP of MariaDB Server Engineering
and security@xxxxxxxxxxx