transaction_timeout is not enforced during ClientWrite

Hi,

On one of the PostgreSQL instances we manage, we saw a transaction
remain open for more than 15min with transaction_timeout set to 50s.
The backend was waiting on ClientWrite. We couldn't tell whether the
client or the network caused the stall, but I think the server should
have closed the connection when the transaction timeout expired.

ProcessClientWriteInterrupt() only handles ProcDiePending while the
socket write is blocked. I agree with the reasoning in the earlier
discussion [1] that canceling a query mid-write and then keeping the
connection open can leave the protocol out of sync. Since we decided
that transaction_timeout should terminate the session with FATAL [2],
is there any design reason to defer handling it while a client write
is blocked?

The attached patch lets ProcessClientWriteInterrupt() use the existing
termination path for a pending transaction timeout. In
ProcessInterrupts(), it moves transaction timeout handling immediately
after ProcDiePending so a pending query cancel cannot raise a
recoverable error first.

Thoughts?

---
Regards,
Jingtang

[1] https://www.postgresql.org/message-id/CA%2BTgmoZiZQF5YFLTM_S7TWOy6UKO0639sxDWYOQMYgvHfZ_rMA%40mail.gmail.com
[2] https://www.postgresql.org/message-id/ACC3312D-8218-4F30-8350-E8634BF1C5DC%40yandex-team.ru