Re: Race condition in pcp_node_info can cause it to hang

Hi Emond,

> Hi,
>
> We've hit another very rare flake in our tests, which can cause
> pcp_node_info to hang indefinitely. I've analyzed the problem with
> Claude Code, and it came to the conclusion and (quite small) fix
> below. Attached is a patch against 4.7.
>
> The problem:
> In inform_node_info() (src/pcp_con/pcp_worker.c), the PCP reply packet
> reads bi->replication_state and bi->replication_sync_state directly
> from shared memory twice: once via strlen() to compute the packet
> length, and once via pcp_write() to write the payload.
>
> The streaming-replication check worker rewrites those same
> shared-memory strings without a lock (it clears them to "" then
> repopulates them every check cycle and on state transitions,
> src/streaming_replication/pool_worker_child.c). If the string's length
> changes between the two reads, the declared wsize no longer matches
> the bytes actually written, so the PCP byte stream desynchronises. The
> client then blocks forever in pcp_read() waiting for bytes the server
> never sends.
>
> The fix:
> Snapshot the two strings into local buffers once, right after bi =
> pool_get_node_info(i),
> and use the locals for both the length and the payload ― so a single
> packet is always
> internally consistent. This matches how every other field in the
> packet is already
> handled.

Thank you for the report and fix. Yes, I agree there's a race
condition between sr checker process and pcp_node_info. I think
introducing a lock to protect bi->replication_state and
bi->replication_sync_state is overkill. The suggested fix seems to be
a right direction. Will push after current release freeze is over
(supposed to be finished by the end of today).

Regards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp