Re: Support for N synchronous standby servers - take 2

On Wed, Feb 10, 2016 at 11:25 AM, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> Yes, I will implement regression test patch and documentation patch as well.

Cool, now that we have a clear picture of where we want to move, that would be an excellent thing to have. Having the docs in the place is clearly mandatory.

> Attached latest version patch supporting s_s_names = '*'.
> Unlike currently behaviour a bit, s_s_names can have only one '*' character.
> e.g, The following setting will get syntax error.
>
> s_s_names = '*, node1,node2'
> s_s_names = `2[node1, *, node2]`
>
> when we use '*' character as s_s_names element, we must set s_s_names
> like follows.
>
> s_s_names = '*'
> s_s_names = '2[*]'
>
> BTW, we've discussed about mini language syntax.
> IIRC, the syntax uses [] and () like,
> 'N[node1, node2, ...]', to define priority standbys.
> 'N(node1, node2, ...)', to define quorum standbys.
> And current patch behaves so.
>
> Which type of parentheses should be used for this syntax to be more clarity?
> Or other character should be used such as <>, // ?

I am personally fine with () and [] as you mention, we could even consider {}, each one of them has a different meaning mathematically..

I am not entered into a detailed review yet (waiting for the docs), but the patch looks brittle. I have been able to crash the server just by querying pg_stat_replication:
* thread #1: tid = 0x0000, 0x0000000105eb36c2 postgres`pg_stat_get_wal_senders(fcinfo=0x00007fff5a156290) + 498 at walsender.c:2783, stop reason = signal SIGSTOP
  * frame #0: 0x0000000105eb36c2 postgres`pg_stat_get_wal_senders(fcinfo=0x00007fff5a156290) + 498 at walsender.c:2783
    frame #1: 0x0000000105d4277d postgres`ExecMakeTableFunctionResult(funcexpr=0x00007fea128f3838, econtext=0x00007fea128f1b58, argContext=0x00007fea128c8ea8, expectedDesc=0x00007fea128f4710, randomAccess='\0') + 1005 at execQual.c:2211
    frame #2: 0x0000000105d70c24 postgres`FunctionNext(node=0x00007fea128f2f78) + 180 at nodeFunctionscan.c:95
* thread #1: tid = 0x0000, 0x0000000105eb36c2 postgres`pg_stat_get_wal_senders(fcinfo=0x00007fff5a156290) + 498 at walsender.c:2783, stop reason = signal SIGSTOP
    frame #0: 0x0000000105eb36c2 postgres`pg_stat_get_wal_senders(fcinfo=0x00007fff5a156290) + 498 at walsender.c:2783
   2780        /*
   2781         * Get the currently active synchronous standby.
   2782         */
-> 2783        sync_standbys = (int *) palloc(sizeof(int) * SyncRepStandbyNames->wait_num);
   2784        LWLockAcquire(SyncRepLock, LW_SHARED);
   2785        num_sync = SyncRepGetSyncStandbysPriority(SyncRepStandbyNames, sync_standbys);
   2786        LWLockRelease(SyncRepLock);
(lldb) p SyncRepStandbyNames
(SyncGroupNode *) $0 = 0x0000000000000000

--
Michael