Re: Can a background worker exist without shared memory access for EXEC_BACKEND cases?

On Tue, Aug 4, 2020 at 10:20 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Tue, Aug 4, 2020 at 7:27 AM Bharath Rupireddy
> <bharath.rupireddyforpostgres@gmail.com> wrote:
> > I could get these points earlier in my initial analysis. In fact, I
> > could figure out the flow on Windows, how these parameters are shared
> > using a shared file(CreateFileMapping(), MapViewOfFile()), and the
> > shared file name being passed as an argv[2] to the child process, and
> > the way child process uses this file name to read the backend
> > parameters in read_backend_variables().
>
> Doesn't that happen even if the background worker isn't declared to
> use BGWORKER_SHMEM_ACCESS? See StartBackgroundWorker(): IIUC, we start
> with shared memory access, then afterwards detach.
>

Yes, the bg worker starts with shared memory access even with no
BGWORKER_SHMEM_ACCESS and later it gets detached in
StartBackgroundWorker() with PGSharedMemoryDetach().

if ((worker->bgw_flags & BGWORKER_SHMEM_ACCESS) == 0)
{
        dsm_detach_all();
        PGSharedMemoryDetach();
 }

In EXEC_BACKEND cases, right after PGSharedMemoryDetach(), the bg
worker will no longer be able to access the backend parameters, see
below(I tried this on my Ubuntu machine with a bgworker with no
BGWORKER_SHMEM_ACCESS flag and defined EXEC_BACKEND macro in
pg_config_manual.h) :

(gdb) p *MyLatch
Cannot access memory at address 0x7fd60424a6b4
(gdb) p *ShmemVariableCache
Cannot access memory at address 0x7fd58427bf80
(gdb) p ProcStructLock
$10 = (slock_t *) 0x7fd60429bd00 <error: Cannot access memory at
address 0x7fd60429bd00>
(gdb) p *AuxiliaryProcs
Cannot access memory at address 0x7fd60424cc60
(gdb) p *ProcGlobal
Cannot access memory at address 0x7fd604232880

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com