fix(ecstore): keep O_DIRECT for eligible reads when io_uring is enabled (rustfs/backlog#1102)
UringBackend::pread_uring opens the file buffered, so with io_uring enabled an
O_DIRECT-eligible read (RUSTFS_OBJECT_DIRECT_IO_READ_ENABLE + length >= the
threshold) silently lost O_DIRECT and polluted the page cache — a behavior
change the moment io_uring was turned on for a disk that uses O_DIRECT. The
io_uring backend is gray-off by default, so no deployment is affected, but the
gap had to close before it can be enabled anywhere O_DIRECT is in use.
Route O_DIRECT-eligible reads back through StdBackend's aligned path (which
itself falls back to buffered when the filesystem rejects O_DIRECT). A native
aligned O_DIRECT read in the driver remains part of rustfs/backlog#1102.
Adds uring_preserves_o_direct_for_eligible_reads: with BOTH flags on and the
threshold at 1, unaligned ranges still return exactly the requested bytes.
Co-authored-by: heihutu <heihutu@gmail.com>
houseme
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