PS10083 8.4 fix apple build#5943
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The static_assert(sizeof(thread_group_t) == 512, ...) in
sql/threadpool_unix.cc has existed since PS 8.0.12 and was hand-tuned
against the Linux/glibc layout of the embedded mysql_mutex_t and PSI
structures. PS-10083 ("Add more statistics for threadpool", merged
2026-03-11 between 9.6.0 and 9.7.0) inserted two LiveStats members into
thread_group_t and adjusted the trailing 'char padding[]' from 328 to
248 to keep the Linux total at exactly 512.
LiveStats itself is platform-neutral (5 PODs, 40 bytes), but
mysql_mutex_t expands by ~128 bytes on macOS / Apple clang + libc++,
so on that platform the struct lands at 640 bytes and breaks the
build:
sql/threadpool_unix.cc:155:1: error: static assertion failed due
to requirement 'sizeof(thread_group_t) == 512'
The false-sharing guarantee comes from alignas(128); what matters is
that the struct occupies a whole number of 128-byte cache lines, not
that it is exactly 512 bytes (640 is also a multiple of 128). Replace
the equality check with 'sizeof % 128 == 0' so Linux/glibc, libc++
on macOS, and any future libstdc++ layout drift all pass while the
no-false-sharing invariant remains enforced.
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We decided to include this in #5942 |
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The static_assert(sizeof(thread_group_t) == 512, ...) in sql/threadpool_unix.cc has existed since PS 8.0.12 and was hand-tuned against the Linux/glibc layout of the embedded mysql_mutex_t and PSI structures. PS-10083 ("Add more statistics for threadpool", merged 2026-03-11 between 9.6.0 and 9.7.0) inserted two LiveStats members into thread_group_t and adjusted the trailing 'char padding[]' from 328 to 248 to keep the Linux total at exactly 512.
LiveStats itself is platform-neutral (5 PODs, 40 bytes), but mysql_mutex_t expands by ~128 bytes on macOS / Apple clang + libc++, so on that platform the struct lands at 640 bytes and breaks the build:
The false-sharing guarantee comes from alignas(128); what matters is that the struct occupies a whole number of 128-byte cache lines, not that it is exactly 512 bytes (640 is also a multiple of 128). Replace the equality check with 'sizeof % 128 == 0' so Linux/glibc, libc++ on macOS, and any future libstdc++ layout drift all pass while the no-false-sharing invariant remains enforced.