pgsql: bufmgr: Return whether WaitReadBuffers() needed to wait

bufmgr: Return whether WaitReadBuffers() needed to wait

Thanks to the previous commit, pgaio_wref_check_done() will now detect whether
IO has completed even if userspace has not yet consumed the kernel completion.
This knowledge can be useful for callers of WaitReadBuffers() to know whether
it needed to wait or not, e.g. for adjusting read-ahead aggressiveness or for
instrumentation.

Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
Discussion: https://postgr.es/m/CAH2-Wz%3DkMg3PNay96cHMT0LFwtxP-cQSRZTZzh1Cixxf8G%3Dzrw%40mail.gmail.com
Discussion: https://postgr.es/m/a177a6dd-240b-455a-8f25-aca0b1c08c6e@vondra.me

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/513374a47a71d64ff5c4790b7f962d3a80e8cc7c

Modified Files
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src/backend/storage/buffer/bufmgr.c | 17 ++++++++++++++++-
src/include/storage/bufmgr.h | 2 +-
2 files changed, 17 insertions(+), 2 deletions(-)