Simplify replacement code for preadv and pwritev

Enterprise / PostgreSQL - Thomas Munro [postgresql.org] - 5 August 2022 02:04 UTC

preadv() and pwritev() are not standardized by POSIX, but appeared in NetBSD in 1999 and were adopted by at least OpenBSD, FreeBSD, DragonFlyBSD, Linux, AIX, illumos and macOS. We don't use them much yet, but an active proposal uses them heavily.

In 15, we had two replacement implementations for other OSes: one based on lseek() + -v function if available for true vector I/O, and the other based on a loop over p- function.

The former would be an obstacle to hypothetical future multi-threaded code sharing file descriptors, while the latter would not, since commit cf112c12. Furthermore, the number of targeted systems that could benefit from the former's potential upside has dwindled to just one niche OS, since macOS added the functions and we de-supported HP-UX. That doesn't seem like a good trade-off.

Therefore, drop the lseek()-based variant, and also the pg_ prefix now that the file position portability hazard is gone.

At the time of writing, the only systems in our build farm that lack native preadv/pwritev and thus use fallback code are:

- Solaris (but not illumos)
- macOS before release 11.0
- Windows

With this commit, the above systems will now use the *same* fallback code, the version that loops over pread()/pwrite(). Windows already used that (though a later proposal may include true vector I/O for Windows), so this decision really only affects Solaris, until it gets around to adding these system calls.

Also remove some useless includes while here.

a0dc827112 Simplify replacement code for preadv and pwritev.
configure | 2 +-
configure.ac | 2 --
src/backend/storage/file/fd.c | 4 ++--
src/include/pg_config.h.in | 6 ------
src/include/port/pg_iovec.h | 12 ++++--------
src/port/preadv.c | 17 +----------------
src/port/pwritev.c | 17 +----------------
src/tools/msvc/Solution.pm | 2 --
8 files changed, 9 insertions(+), 53 deletions(-)

Upstream: git.postgresql.org


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