pgsql: aio: io_uring: Fix danger of completion getting reused before be

aio: io_uring: Fix danger of completion getting reused before being read

We called io_uring_cqe_seen(..., cqe) before reading cqe->res. That allows the
completion to be reused, which in turn could lead to cqe->res being
overwritten. The window for that is very narrow and the likelihood of it
happening is very low, as we should never actually utilize all CQEs, but the
consequences would be bad.

This bug was reported to me privately.

Backpatch-through: 18
Discussion: https://postgr.es/m/bwo3e5lj2dgi2wzq4yvbyzu7nmwueczvvzioqsqo6azu6lm5oy@pbx75g2ach3p

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7f1b3a4cea563d791d8a83e5c482f1ed8306ee6a

Modified Files
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src/backend/storage/aio/method_io_uring.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)