StdBackend opened, stat'd, and access-checked the shard file on every
positioned read, so each small-object GET paid N x (open + access + fstat)
syscalls even for hot shards. UringBackend already caches descriptors
behind a generation-guarded, rlimit-bounded moka cache with full
rename/delete/heal invalidation; StdBackend had no equivalent.
Port that cache to the default backend:
- StdBackend gains an `fd_cache: Option<FdCache>` (Linux only, mirroring
UringBackend), built in `new()` behind `RUSTFS_LOCAL_FD_CACHE` (default
on) and the same `rlimit_allows_fd_cache` guard.
- pread_bytes consults the cache on the buffered path: a hit reuses the
descriptor via `dup` (one syscall, no path resolution or permission
re-check) and skips volume access; a miss opens as before, snapshots the
invalidation generation, and hands the freshly opened descriptor back
for `insert_if_fresh`, which refuses to cache if a heal/delete bumped the
generation mid-open (rustfs/backlog#1176). O_DIRECT reads keep opening
their own aligned descriptors.
- The DirectReadCopy branch switches from seek+read_exact to
`FileExt::read_exact_at`: a `dup`'d cached descriptor shares the source
descriptor's open-file offset, so a positioned read (like the mmap path's
offset argument) keeps concurrent cache hits on the same shard correct.
- The four `LocalIoBackend` invalidation methods now drop stale entries on
StdBackend. LocalDisk already calls them on rename_data/rename_file/
delete/delete_volume/close, so no new call sites are needed.
- Two tests mirror the io_uring ones: a heal rename must be hidden until
invalidate_cached_fds_under runs, and a repeated read caches exactly one
descriptor that prefix invalidation drops.
Behavior is byte-for-byte unchanged on a miss and on non-Linux; the cache
is auto-disabled only when RLIMIT_NOFILE is too low. macOS cargo check
--lib and --tests pass; Linux compile deferred to CI.
Co-authored-by: heihutu <[email protected]>
