Horton

Re: Refactoring backend fork+exec code

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andres Freund <andres@anarazel.de>
Cc: Tristan Partin <tristan@neon.tech>, pgsql-hackers <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@gmail.com>
Date: 2024-01-23T19:07:08Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Avoid 0-length memcpy to NULL with EXEC_BACKEND

  2. Tighten check for --forkchild argument when spawning child process

  3. Add missing PGDLLIMPORT markings

  4. Remove assertions that some compiler say are tautological

  5. Move code for backend startup to separate file

  6. Refactor postmaster child process launching

  7. Move some functions from postmaster.c to a new source file

  8. Split registration of Win32 deadchild callback to separate function

  9. Improve log messages referring to background worker processes

  10. Move initialization of the Port struct to the child process

  11. Pass CAC as an argument to the backend process

  12. Set socket options in child process after forking

  13. Fix references to renamed function in comments

  14. Use MyBackendType in more places to check what process this is

  15. Remove MyAuxProcType, use MyBackendType instead

  16. Replace BackendIds with 0-based ProcNumbers

  17. Redefine backend ID to be an index into the proc array

  18. Remove superfluous 'pgprocno' field from PGPROC

  19. Fix incorrect comment on how BackendStatusArray is indexed

  20. Refactor how InitProcess is called

  21. Pass BackgroundWorker entry in the parameter file in EXEC_BACKEND mode

  22. Refactor CreateSharedMemoryAndSemaphores

  23. Silence Valgrind complaint with EXEC_BACKEND

  24. Adjust obsolete comment explaining set_stack_base().

  25. Print lwlock stats also for aux processes, when built with LWLOCK_STATS

  26. Move canAcceptConnections check from ProcessStartupPacket to caller.

  27. Fix crash on syslogger startup

  28. Refactor ListenSocket array.

  29. Use FD_CLOEXEC on ListenSockets

  30. Don't leak descriptors into subprograms.

  31. Use WaitEventSet API for postmaster's event loop.

  32. Rearrange postmaster's startup sequence for better syslogger results.

  33. Fix and enhance the assertion of no palloc's in a critical section.

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)