Re: BUG #18259: Assertion in ExtendBufferedRelLocal() fails after no-space-left condition

Alexander Lakhin <exclusion(at)gmail(dot)com> 于2023年12月27日周三 15:00写道：

> Hello tender wang,
>
> 26.12.2023 19:55, tender wang write:
>
> I tried to analyze the issue, and I found that it might be caused by this
> commit:
> commit dad50f677c42de207168a3f08982ba23c9fc6720
> bufmgr: Acquire and clean victim buffer separately
>
>
> Thanks for looking into it!
>
> ...
>>
>> With debug logging added in this code within ExtendBufferedRelLocal():
>>> if (found)
>>> {
>>> BufferDesc *existing_hdr =
>>> GetLocalBufferDescriptor(hresult->id);
>>> uint32 buf_state;
>>>
>>> UnpinLocalBuffer(BufferDescriptorGetBuffer(victim_buf_hdr));
>>>
>>> existing_hdr = GetLocalBufferDescriptor(hresult->id);
>>> PinLocalBuffer(existing_hdr, false);
>>> buffers[i] = BufferDescriptorGetBuffer(existing_hdr);
>>>
>>> buf_state = pg_atomic_read_u32(&existing_hdr->state);
>>> Assert(buf_state & BM_TAG_VALID);
>>> Assert(!(buf_state & BM_DIRTY));
>>> buf_state &= BM_VALID;
>>> pg_atomic_unlocked_write_u32(&existing_hdr->state,
>>> buf_state);
>>> ...
>>> I see that it reached for the second INSERT (and NOSPC error) with
>>> existing_hdr->state == 0x2040000, but for the third INSERT I observe
>>> state == 0x0.
>>>
>>>
> I wonder, if "buf_state &= BM_VALID" is a typo here, maybe it supposed to
> be
> "buf_state &= ~BM_VALID" as in ExtendBufferedRelShared()...
>

Yeah, that's true. I analyze this issue again, and I think the root cause
is the " buf_state &= BM_VALID" .
In my report issue, buf_state & BM_VALID is true, but buf_state &
BM_TAG_VALID is false. This situation is impossible.
It can't happen that the data in the local buffer pool is valid, but
LocalBufHash has no entry.

I modified v1 patch, and attached v2 patch should fix the above issues.

Best regards,
> Alexander
>