Re: [PATCHES] Infrastructure changes for recovery (v8)

On Tue, Nov 18, 2008 at 12:39 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>
> On Mon, 2008-11-17 at 16:18 +0200, Heikki Linnakangas wrote:
>> Simon Riggs wrote:
>> > @@ -3845,6 +3850,52 @@ sigusr1_handler(SIGNAL_ARGS)
>> >
>> >         PG_SETMASK(&BlockSig);
>> >
>> > +       if (CheckPostmasterSignal(PMSIGNAL_RECOVERY_START))
>> > +       {
>> > +               Assert(pmState == PM_STARTUP);
>> > +
>> > +               /*
>> > +                * Go to shutdown mode if a shutdown request was pending.
>> > +                */
>> > +               if (Shutdown > NoShutdown)
>> > +               {
>> > +                       pmState = PM_WAIT_BACKENDS;
>> > +                       /* PostmasterStateMachine logic does the rest */
>> > +               }
>> > +               else
>> > +               {
>> > +                       /*
>> > +                        * Startup process has entered recovery
>> > +                        */
>> > +                       pmState = PM_RECOVERY;
>>
>>
>> Hmm, I smell a race condition here:
>>
>> 1. Startup process goes into consistent state, and signals postmaster
>> 2. Startup process finishes WAL replay and dies
>> 3. Postmaster wakes up in reaper(), noting that the startup process
>> dies, and goes into PM_RUN mode.
>> 4. The above signal handler for postmaster is run, causing an assertion
>> failure, or putting postmaster back into PM_RECOVERY mode if assertions
>> are disabled.
>>
>> Highly unlikely in practice, given how much code needs to run in the
>> startup process between signaling the postmaster and exiting, but it
>> seems theoretically possible. Do we care, and if we do, how can we fix it?
>
> Might be possible - it does depend on the sequence of actions its true.
> Agree not likely to happen, except as the result of another bug.
>
> I'll change it to a test for
>
> if (pmState == PM_STARTUP)
>        pmState = PM_RECOVERY;

Likewise, should we also change the assertion against the pid of the
background process (BgWriterPID, PgStatPID)?

Regards,

-- 
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center