Re: Using WaitEventSet in the postmaster

On Thu, Jan 12, 2023 at 7:57 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> On Thu, Jan 12, 2023 at 7:27 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > skink seems to have found a problem:
> >
> > ==2011873== VALGRINDERROR-BEGIN
> > ==2011873== Syscall param epoll_wait(events) points to unaddressable byte(s)
> > ==2011873==    at 0x4D8DC73: epoll_wait (epoll_wait.c:30)
> > ==2011873==    by 0x55CA49: WaitEventSetWaitBlock (latch.c:1527)
> > ==2011873==    by 0x55D591: WaitEventSetWait (latch.c:1473)
> > ==2011873==    by 0x4F2B28: ServerLoop (postmaster.c:1729)
> > ==2011873==    by 0x4F3E85: PostmasterMain (postmaster.c:1452)
> > ==2011873==    by 0x42643C: main (main.c:200)
> > ==2011873==  Address 0x7b1e620 is 1,360 bytes inside a recently re-allocated block of size 8,192 alloc'd
> > ==2011873==    at 0x48407B4: malloc (vg_replace_malloc.c:381)
> > ==2011873==    by 0x6D9D30: AllocSetContextCreateInternal (aset.c:446)
> > ==2011873==    by 0x4F2D9B: PostmasterMain (postmaster.c:614)
> > ==2011873==    by 0x42643C: main (main.c:200)
> > ==2011873==
> > ==2011873== VALGRINDERROR-END
>
> Repro'd here on Valgrind.  Oh, that's interesting.  WaitEventSetWait()
> wants to use an internal buffer of the size given to the constructor
> function, but passes the size of the caller's output buffer to
> epoll_wait() and friends.  Perhaps it should use Min(nevents,
> set->nevents_space).  I mean, I should have noticed that, but I think
> that's arguably a pre-existing bug in the WES code, or at least an
> unhelpful interface.  Thinking...

Yeah.  This stops valgrind complaining here.