Re: Buffer locking is special (hints, checksums, AIO writes)

Various buildfarm animals are complaining about fcb9c977a,
similarly to this from calliphoridae [1]:

[969/2355] ccache gcc -Isrc/backend/postgres_lib.a.p -Isrc/include -I../pgsql/src/include -I/usr/include/libxml2 -fdiagnostics-color=never -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O2 -g -fno-strict-aliasing -fwrapv -fexcess-precision=standard -D_GNU_SOURCE -Wmissing-prototypes -Wpointer-arith -Werror=vla -Wendif-labels -Wmissing-format-attribute -Wimplicit-fallthrough=3 -Wcast-function-type -Wshadow=compatible-local -Wformat-security -Wdeclaration-after-statement -Wmissing-variable-declarations -Wno-format-truncation -Wno-stringop-truncation -O1 -ggdb -g3 -fno-omit-frame-pointer -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -DCOPY_PARSE_PLAN_TREES -DRAW_EXPRESSION_COVERAGE_TEST -fPIC -isystem /usr/include/mit-krb5 -pthread -DBUILDING_DLL -MD -MQ src/backend/postgres_lib.a.p/storage_buffer_bufmgr.c.o -MF src/backend/postgres_lib.a.p/storage_buffer_bufmgr.c.o.d -o src/backend/postgres_lib.a.p/storage_buffer_bufmgr.c.o -c ../pgsql/src/backend/storage/buffer/bufmgr.c
In file included from ../pgsql/src/include/pgstat.h:24,
from ../pgsql/src/backend/storage/buffer/bufmgr.c:52:
In function \342\200\230pgstat_report_wait_start\342\200\231,
inlined from \342\200\230BufferLockAcquire\342\200\231 at ../pgsql/src/backend/storage/buffer/bufmgr.c:5833:3:
../pgsql/src/include/utils/wait_event.h:75:49: warning: \342\200\230wait_event\342\200\231 may be used uninitialized [-Wmaybe-uninitialized]
75 | *(volatile uint32 *) my_wait_event_info = wait_event_info;
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
../pgsql/src/backend/storage/buffer/bufmgr.c: In function \342\200\230BufferLockAcquire\342\200\231:
../pgsql/src/backend/storage/buffer/bufmgr.c:5781:33: note: \342\200\230wait_event\342\200\231 was declared here
5781 | uint32 wait_event;
| ^~~~~~~~~~

Apparently they do not find the coding in this switch persuasive:

switch (mode)
{
case BUFFER_LOCK_EXCLUSIVE:
wait_event = WAIT_EVENT_BUFFER_EXCLUSIVE;
break;
case BUFFER_LOCK_SHARE_EXCLUSIVE:
wait_event = WAIT_EVENT_BUFFER_SHARE_EXCLUSIVE;
break;
case BUFFER_LOCK_SHARE:
wait_event = WAIT_EVENT_BUFFER_SHARED;
break;
case BUFFER_LOCK_UNLOCK:
pg_unreachable();

}

It's not clear to me whether that's more about not believing
pg_unreachable() or more about the lack of a default: case.
I see that this is a modification of code that existed before
fcb9c977a and wasn't being complained of, which makes it even
stranger.

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_stage_log.pl?nm=calliphoridae&dt=2026-01-15%2019%3A35%3A00&stg=build