Re: Inconsistency in startup process's MyBackendId and procsignal array registration with ProcSignalInit() - Mailing list pgsql-hackers

On 2021/10/09 22:22, Bharath Rupireddy wrote:
> Hi,
> 
> It looks like auxiliary processes will not have a valid MyBackendId as
> they don't call InitPostgres() and SharedInvalBackendInit() unlike
> backends. But the startup process (which is an auxiliary process) in
> hot standby mode seems to be different in the way that it does have a
> valid MyBackendId as SharedInvalBackendInit() gets called from
> InitRecoveryTransactionEnvironment(). The SharedInvalBackendInit()
> usually stores the MyBackendId in the caller's PGPROC structure i.e.
> MyProc->backendId. The auxiliary processes (including the startup
> process) usually register themselves in procsignal array with
> ProcSignalInit(MaxBackends + MyAuxProcType + 1) unlike the backends
> with ProcSignalInit(MyBackendId) after SharedInvalBackendInit() in
> InitPostgres().
> 
> The problem comes when a postgres process wants to send a multiplexed
> SIGUSR1 signal (probably using SendProcSignal()) to the startup
> process after receiving its ProcSignal->psh_slot[] with its backendId
> from the PGPROC (the postgres process can get the startup process
> PGPROC structure from AuxiliaryPidGetProc()). Remember the startup
> process has registered in the procsignal array with
> ProcSignalInit(MaxBackends + MyAuxProcType + 1), not with the
> ProcSignalInit(MyBackendId) like the backends did. So, the postgres
> process, wanting to send SIGUSR1 to the startup process, refers to the
> wrong ProcSignal->psh_slot[] and may not send the signal.
> 
> Is this inconsistency of MyBackendId for a startup process a problem
> at all? Thoughts?
> 
> These are the following ways I think we can fix it, if at all some
> other hacker agrees that it is actually an issue:
> 
> 1) Fix the startup process code, probably by unregistering the
> procsignal array entry that was made with ProcSignalInit(MaxBackends +
> MyAuxProcType + 1) in AuxiliaryProcessMain() and register with
> ProcSignalInit(MyBackendId) immediately after SharedInvalBackendInit()
> calculates the MyBackendId in with SharedInvalBackendInit() in
> InitRecoveryTransactionEnvironment(). This seems risky to me as
> unregistering and registering ProcSignal array involves some barriers
> and during the registering and unregistering window, the startup
> process may miss the SIGUSR1.
> 
> 2) Ensure that the process, that wants to send the startup process
> SIGUSR1 signal, doesn't use the backendId from the startup process
> PGPROC, in which case it has to loop over all the entries of
> ProcSignal->psh_slot[] array to find the entry with the startup
> process PID. It seems easier and less riskier but only caveat is that
> the sending process shouldn't look at the backendId from auxiliary
> process PGPROC, instead it should just traverse the entire proc signal
> array to find the right slot.
> 
> 3) Add a comment around AuxiliaryPidGetProc() that says "auxiliary
> processes don't have valid backend ids, so don't use the backendId
> from the returned PGPROC".
> 
> (2) and (3) seem reasonable to me. Thoughts?

How about modifying SharedInvalBackendInit() so that it accepts
BackendId as an argument and allocates the ProcState entry of
the specified BackendId? That is, the startup process determines
that its BackendId is "MaxBackends + MyAuxProcType (=StartupProcess) + 1"
in AuxiliaryProcessMain(), and then it passes that BackendId to
SharedInvalBackendInit() in InitRecoveryTransactionEnvironment().

Maybe you need to enlarge ProcState array so that it also handles
auxiliary processes if it does not for now.


Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION