io: phase 1 buffering — read-side flatness
Three changes targeting 0.20.9's "muxer never read-stalls on NFS read latency" invariant: A. FileSectorSource gets a 32 MiB internal read-ahead buffer (READAHEAD_BUF_BYTES). Splits out from src/sector/file.rs into src/io/file_sector_source/ with per-OS open hints (Linux posix_fadvise(SEQUENTIAL), macOS fcntl(F_RDADVISE) with 64 MiB cap, Windows TODO stub, BSD/illumos no-op). Backward seeks rebuffer; partial reads at EOF return only the bytes that exist; oversize-request bypass for count > BUF_SECTORS. B. WritebackFile inline #[cfg(target_os = "linux")] blocks split into per-OS files under src/io/writeback_file/. Linux unchanged (fallocate KEEP_SIZE, fsync via bounded_syscall). macOS gets a real F_PREALLOCATE + F_FULLFSYNC impl (was a "skipped (non-linux)" debug log before). Windows is a stub (FlushFileBuffers via std sync_all; TODO for SetFileValidData). BSDs/illumos fall back to std sync_all. C. New byte_channel module — byte-bounded producer/consumer wrapping std sync_channel with Mutex/Condvar byte accounting. Sender blocks when used_bytes + item.byte_size() > capacity. HasByteSize impl for PesFrame. Default cap BYTE_CHANNEL_DEFAULT_CAPACITY = 64 MiB, sized to absorb worst-case NFS read p99 (~2 s × UHD peak compressed ~15 MB/s). The mux call site lives in autorip (out of scope here); this lands the primitive in libfreemkv for autorip to adopt. Test counts: byte_channel +6, file_sector_source +5, sector::file round-trip suite (3) preserved. passn_handler_ab.rs A/B fixture (8 profiles) still green. precommit.sh libfreemkv: fmt + clippy + test all green on Rust 1.86. No version bump; no Cargo.lock changes; no forbidden-file edits (disc/patch.rs, disc/read_error.rs, io/pipeline.rs, tests/passn_handler_ab.rs).
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//! `WritebackFile` — a `File` wrapper whose reason for existing is the
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//! bounded-cache writeback pipeline.
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//!
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//! Why: large sequential writes (sweep, patch, mux on UHD-scale output)
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//! left to the kernel's default writeback policy accumulate hundreds of
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//! megabytes of dirty pages and then burst-flush, stalling subsequent
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//! writes for seconds at a time. `WritebackFile` drives a continuous
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//! [`super::writeback::WritebackPipeline`] that on Linux issues
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//! incremental `sync_file_range` + `posix_fadvise(DONTNEED)` calls at
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//! 32 MB granularity so dirty pages drain at the same rate they're
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//! produced. macOS and Windows fall through to a no-op pipeline — their
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//! default cache policies have not been shown to exhibit the same
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//! pathology for this access pattern.
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//!
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//! It implements `Write` and `Seek` so any call site that wrote to a
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//! plain `File` through those traits (sweep, patch, mux) can swap in
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//! `WritebackFile` without touching the body of the loop. The wrapper
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//! also tracks the current file position to feed the pipeline with
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//! progress + seek boundaries.
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//!
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//! See `super::writeback::linux` for the underlying pathology and the
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//! strategy.
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//!
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//! ## Platform split
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//!
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//! The platform-specific pieces of this wrapper — extent preallocation
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//! (Linux `fallocate(KEEP_SIZE)`, macOS `F_PREALLOCATE`, Windows
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//! `SetFileValidData`) and the durable-flush primitive (Linux/macOS
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//! `fsync`/`F_FULLFSYNC` wrapped in a bounded syscall, Windows
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//! `FlushFileBuffers`) — live in per-OS sibling modules. The dispatch
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//! happens once at the bottom of this file via cfg-gated `mod` decls.
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//! No inline `#[cfg(target_os = "...")]` in the business-logic above.
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#[cfg(target_os = "linux")]
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mod linux;
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#[cfg(target_os = "macos")]
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mod macos;
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#[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
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mod other;
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#[cfg(target_os = "windows")]
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mod windows;
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#[cfg(target_os = "linux")]
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use linux as platform;
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#[cfg(target_os = "macos")]
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use macos as platform;
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#[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
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use other as platform;
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#[cfg(target_os = "windows")]
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use windows as platform;
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use std::fs::{File, OpenOptions};
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use std::io::{self, Seek, SeekFrom, Write};
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use std::path::Path;
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use super::writeback::WritebackPipeline;
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const WRITEBACK_CHUNK_BYTES: u64 = 32 * 1024 * 1024;
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pub(crate) struct WritebackFile {
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file: File,
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pipeline: WritebackPipeline,
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pos: u64,
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}
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impl WritebackFile {
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/// Wrap an open `File`. The current OS file position is queried
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/// once so the pipeline starts tracking from wherever the file
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/// already is (typically 0 for fresh files; non-zero for resumed
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/// or appended files).
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pub(crate) fn new(mut file: File) -> io::Result<Self> {
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let pos = file.stream_position()?;
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let pipeline = WritebackPipeline::new(&file, pos, WRITEBACK_CHUNK_BYTES);
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Ok(Self {
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file,
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pipeline,
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pos,
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})
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}
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/// Create a new file at `path` (truncating any existing contents)
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/// and wrap it. Convenience for the common
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/// `File::create(path)` + `WritebackFile::new(file)` pair so callers
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/// don't have to assemble a `File` first.
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///
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/// Callers that know the target output size should prefer
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/// [`Self::create_with_size_hint`] so the kernel can pre-reserve
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/// extents.
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#[allow(dead_code)]
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pub(crate) fn create(path: &Path) -> io::Result<Self> {
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let file = File::create(path)?;
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Self::new(file)
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}
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/// Like [`Self::create`] but pre-reserves `size_bytes` of disk
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/// space via the platform's extent-preallocation primitive (Linux
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/// `fallocate(KEEP_SIZE)`, macOS `F_PREALLOCATE`, Windows
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/// `SetFileValidData` stub). The reported file size is unchanged
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/// (writes still grow the file naturally) — only the on-disk extent
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/// allocation is preallocated, which reduces extent fragmentation
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/// on large sequential writes (mux output, especially on slow
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/// storage / NFS).
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///
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/// On platforms without an extent-preallocation primitive this is
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/// equivalent to `create` — the size hint is dropped after a debug
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/// log.
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pub(crate) fn create_with_size_hint(path: &Path, size_bytes: u64) -> io::Result<Self> {
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let file = File::create(path)?;
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platform::preallocate(&file, size_bytes);
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Self::new(file)
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}
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/// Open an existing file at `path` for writing (no truncation) and
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/// wrap it. Mirrors `File::open` semantics for the writable case
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/// — used by patch / resume paths that mutate an existing ISO in
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/// place.
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pub(crate) fn open(path: &Path) -> io::Result<Self> {
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let file = OpenOptions::new().write(true).open(path)?;
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Self::new(file)
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}
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/// Drain in-flight writeback then issue a full fsync. Use this in
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/// place of `File::sync_all`.
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///
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/// The final fsync is wrapped in
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/// [`crate::io::bounded::bounded_syscall`] with a 60 s deadline on
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/// platforms that have a usable bounded primitive (Linux + macOS).
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/// fsync on a wedged NFS server (or a degraded local disk) can
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/// hang the calling thread; the wrapper ensures the worst case is
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/// 60 s + log-and-continue rather than indefinite. On timeout the
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/// page cache is left to the kernel's normal flush-on-close path —
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/// best effort, but bounded.
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pub(crate) fn sync_all(&mut self) -> io::Result<()> {
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self.pipeline.finalize();
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platform::durable_sync(&self.file)
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}
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}
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impl Write for WritebackFile {
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fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
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let n = self.file.write(buf)?;
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self.pos += n as u64;
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self.pipeline.note_progress(self.pos);
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Ok(n)
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}
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fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
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self.file.write_all(buf)?;
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self.pos += buf.len() as u64;
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self.pipeline.note_progress(self.pos);
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Ok(())
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}
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fn flush(&mut self) -> io::Result<()> {
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self.file.flush()
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}
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}
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impl Seek for WritebackFile {
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fn seek(&mut self, from: SeekFrom) -> io::Result<u64> {
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let p = self.file.seek(from)?;
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// Only treat seeks that actually move the position as
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// boundaries — sweep does a redundant `seek(Current(pos))`
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// before every write, and we don't want that to drain the
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// pipeline on every iteration.
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if p != self.pos {
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// Diagnostic for the NFS 73 % mux hang: the MKV format
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// requires the muxer to seek back occasionally (cluster
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// size patching, Cues index write, Segment header
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// backpatch). Each such seek invalidates the writeback
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// chunk tracking and forces a finalize → WAIT_AFTER on
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// the in-flight chunk. Logging the seek delta lets us
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// correlate hang offsets with specific muxer operations.
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let from_pos = self.pos;
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let to_pos = p;
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let delta: i64 = (to_pos as i64).wrapping_sub(from_pos as i64);
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tracing::debug!(
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target: "mux",
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"WritebackFile seek from={from_pos} to={to_pos} delta={delta}"
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);
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self.pipeline.handle_seek(p);
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self.pos = p;
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}
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Ok(p)
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}
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}
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impl Drop for WritebackFile {
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fn drop(&mut self) {
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// Run the pipeline's tail finalize so the last in-flight chunk
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// gets its `WAIT_AFTER` + `posix_fadvise(DONTNEED)`. Without
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// this, callers that drop a `WritebackFile` without calling
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// `sync_all` (panic, early-return, idiomatic `let _ = w;`)
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// leave the trailing chunk in cache; the kernel still flushes
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// on close, but the bounded-cache invariant fails at the tail.
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// We deliberately do *not* call `self.file.sync_all()` here —
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// close already triggers a flush, and an `fsync` from `Drop`
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// would silently swallow its `io::Error` anyway. `finalize` is
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// idempotent so an explicit `sync_all` followed by drop is
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// still safe.
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self.pipeline.finalize();
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}
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}
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