Re: Refactoring backend fork+exec code

On 22/01/2024 23:07, Andres Freund wrote:
> On 2024-01-10 14:35:52 +0200, Heikki Linnakangas wrote:
>> @@ -5344,31 +5344,31 @@ StartChildProcess(AuxProcType type)
>>           errno = save_errno;
>>           switch (type)
>>           {
>> -            case StartupProcess:
>> +            case B_STARTUP:
>>                   ereport(LOG,
>>                           (errmsg("could not fork startup process: %m")));
>>                   break;
>> -            case ArchiverProcess:
>> +            case B_ARCHIVER:
>>                   ereport(LOG,
>>                           (errmsg("could not fork archiver process: %m")));
>>                   break;
>> -            case BgWriterProcess:
>> +            case B_BG_WRITER:
>>                   ereport(LOG,
>>                           (errmsg("could not fork background writer process: %m")));
>>                   break;
>> -            case CheckpointerProcess:
>> +            case B_CHECKPOINTER:
>>                   ereport(LOG,
>>                           (errmsg("could not fork checkpointer process: %m")));
>>                   break;
>> -            case WalWriterProcess:
>> +            case B_WAL_WRITER:
>>                   ereport(LOG,
>>                           (errmsg("could not fork WAL writer process: %m")));
>>                   break;
>> -            case WalReceiverProcess:
>> +            case B_WAL_RECEIVER:
>>                   ereport(LOG,
>>                           (errmsg("could not fork WAL receiver process: %m")));
>>                   break;
>> -            case WalSummarizerProcess:
>> +            case B_WAL_SUMMARIZER:
>>                   ereport(LOG,
>>                           (errmsg("could not fork WAL summarizer process: %m")));
>>                   break;
> 
> Seems we should replace this with something slightly more generic one of these
> days...

The later patches in this thread will turn these into

ereport(LOG,
         (errmsg("could not fork %s process: %m", 
PostmasterChildName(type))));

>> diff --git a/src/backend/utils/activity/backend_status.c b/src/backend/utils/activity/backend_status.c
>> index 1a1050c8da1..92f24db4e18 100644
>> --- a/src/backend/utils/activity/backend_status.c
>> +++ b/src/backend/utils/activity/backend_status.c
>> @@ -257,17 +257,16 @@ pgstat_beinit(void)
>>       else
>>       {
>>           /* Must be an auxiliary process */
>> -        Assert(MyAuxProcType != NotAnAuxProcess);
>> +        Assert(IsAuxProcess(MyBackendType));
>>   
>>           /*
>>            * Assign the MyBEEntry for an auxiliary process.  Since it doesn't
>>            * have a BackendId, the slot is statically allocated based on the
>> -         * auxiliary process type (MyAuxProcType).  Backends use slots indexed
>> -         * in the range from 0 to MaxBackends (exclusive), so we use
>> -         * MaxBackends + AuxProcType as the index of the slot for an auxiliary
>> -         * process.
>> +         * auxiliary process type.  Backends use slots indexed in the range
>> +         * from 0 to MaxBackends (exclusive), and aux processes use the slots
>> +         * after that.
>>            */
>> -        MyBEEntry = &BackendStatusArray[MaxBackends + MyAuxProcType];
>> +        MyBEEntry = &BackendStatusArray[MaxBackends + MyBackendType - FIRST_AUX_PROC];
>>       }
> 
> Hm, this seems less than pretty. It's probably ok for now, but it seems like a
> better fix might be to just start assigning backend ids to aux procs or switch
> to indexing by pgprocno?

Using pgprocno is a good idea. Come to think of it, why do we even have 
a concept of backend ID that's separate from pgprocno? backend ID is 
used to index the ProcState array, but AFAICS we could use pgprocno as 
the index to that, too.

-- 
Heikki Linnakangas
Neon (https://neon.tech)