//! `WritebackFile` — a `File` wrapper whose reason for existing is the
//! bounded-cache writeback pipeline.
//!
//! Why: large sequential writes (sweep, patch, mux on UHD-scale output)
//! left to the kernel's default writeback policy accumulate hundreds of
//! megabytes of dirty pages and then burst-flush, stalling subsequent
//! writes for seconds at a time. `WritebackFile` drives a continuous
//! [`super::writeback::WritebackPipeline`] that on Linux issues
//! incremental `sync_file_range` + `posix_fadvise(DONTNEED)` calls at
//! 32 MB granularity so dirty pages drain at the same rate they're
//! produced. macOS and Windows fall through to a no-op pipeline — their
//! default cache policies have not been shown to exhibit the same
//! pathology for this access pattern.
//!
//! It implements `Write` and `Seek` so any call site that wrote to a
//! plain `File` through those traits (sweep, patch, mux) can swap in
//! `WritebackFile` without touching the body of the loop. The wrapper
//! also tracks the current file position to feed the pipeline with
//! progress + seek boundaries.
//!
//! See `super::writeback::linux` for the underlying pathology and the
//! strategy.
//!
//! ## Platform split
//!
//! The platform-specific pieces of this wrapper — extent preallocation
//! (Linux `fallocate(KEEP_SIZE)`, macOS `F_PREALLOCATE`, Windows
//! `SetFileValidData`) and the durable-flush primitive (Linux/macOS
//! `fsync`/`F_FULLFSYNC` wrapped in a bounded syscall, Windows
//! `FlushFileBuffers`) — live in per-OS sibling modules. The dispatch
//! happens once at the bottom of this file via cfg-gated `mod` decls.
//! No inline `#[cfg(target_os = "...")]` in the business-logic above.
//!
//! ## Write path
//!
//! Writes are direct passthrough to the underlying `File` (no writer
//! thread, no ring, no batching). Empirically the Phase-2.5
//! writer-thread architecture introduced a ~60% mux throughput
//! regression on NFS bidirectional workloads; reverting the write path
//! to direct passthrough restores the 0.20.7 baseline. The writeback
//! pipeline still runs (it's called inline from `write` / `write_all` /
//! `seek`) so the bounded-cache invariant on Linux is preserved.
//!
//! ## Halt-safety
//!
//! `sync_all` runs the per-OS durable-flush primitive, which on
//! Linux/macOS is wrapped in [`crate::io::bounded::bounded_syscall`]
//! with a 60 s deadline. A wedged NFS server cannot trap the muxer
//! indefinitely on the final fsync.

#[cfg(target_os = "linux")]
mod linux;
#[cfg(target_os = "macos")]
mod macos;
#[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
mod other;
#[cfg(target_os = "windows")]
mod windows;

#[cfg(target_os = "linux")]
use linux as platform;
#[cfg(target_os = "macos")]
use macos as platform;
#[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
use other as platform;
#[cfg(target_os = "windows")]
use windows as platform;

use std::fs::{File, OpenOptions};
use std::io::{self, Seek, SeekFrom, Write};
use std::path::Path;

use super::writeback::WritebackPipeline;

/// Granularity at which the Linux writeback pipeline issues
/// `sync_file_range` / `posix_fadvise(DONTNEED)` pairs.
///
/// iter7 (2026-05-17): 32 → 64 MiB. iter6 (8 MiB) regressed -8.2 MB/s
/// vs iter4 (32 MiB), confirming smaller=slower for this workload.
/// Trying bigger to see if fewer-but-larger syncs reduces overhead
/// further. 0.21.14 tried 128 MiB and was reverted — we don't know
/// whether that was measurement noise or a real TCP-buffer / NFS
/// commit issue. 64 MiB is the middle test point.
const WRITEBACK_CHUNK_BYTES: u64 = 64 * 1024 * 1024;

pub(crate) struct WritebackFile {
    file: File,
    pipeline: WritebackPipeline,
    pos: u64,
}

impl WritebackFile {
    /// Wrap an open `File`. The current OS file position is queried
    /// once so the pipeline starts tracking from wherever the file
    /// already is (typically 0 for fresh files; non-zero for resumed
    /// or appended files).
    pub(crate) fn new(mut file: File) -> io::Result<Self> {
        let pos = file.stream_position()?;
        let pipeline = WritebackPipeline::new(&file, pos, WRITEBACK_CHUNK_BYTES);
        Ok(Self {
            file,
            pipeline,
            pos,
        })
    }

    /// Create a new file at `path` (truncating any existing contents)
    /// and wrap it. Convenience for the common
    /// `File::create(path)` + `WritebackFile::new(file)` pair so callers
    /// don't have to assemble a `File` first.
    ///
    /// Callers that know the target output size should prefer
    /// [`Self::create_with_size_hint`] so the kernel can pre-reserve
    /// extents.
    #[allow(dead_code)]
    pub(crate) fn create(path: &Path) -> io::Result<Self> {
        let file = File::create(path)?;
        Self::new(file)
    }

    /// Like [`Self::create`] but pre-reserves `size_bytes` of disk
    /// space via the platform's extent-preallocation primitive (Linux
    /// `fallocate(KEEP_SIZE)`, macOS `F_PREALLOCATE`, Windows
    /// `SetFileValidData` stub). The reported file size is unchanged
    /// (writes still grow the file naturally) — only the on-disk extent
    /// allocation is preallocated, which reduces extent fragmentation
    /// on large sequential writes (mux output, especially on slow
    /// storage / NFS).
    ///
    /// On platforms without an extent-preallocation primitive this is
    /// equivalent to `create` — the size hint is dropped after a debug
    /// log.
    pub(crate) fn create_with_size_hint(path: &Path, size_bytes: u64) -> io::Result<Self> {
        let file = File::create(path)?;
        platform::preallocate(&file, size_bytes);
        Self::new(file)
    }

    /// Open an existing file at `path` for writing (no truncation) and
    /// wrap it. Mirrors `File::open` semantics for the writable case
    /// — used by patch / resume paths that mutate an existing ISO in
    /// place.
    pub(crate) fn open(path: &Path) -> io::Result<Self> {
        let file = OpenOptions::new().write(true).open(path)?;
        Self::new(file)
    }

    /// Drain in-flight writeback then issue a full fsync. Use this in
    /// place of `File::sync_all`.
    ///
    /// The final durable flush is wrapped in
    /// [`crate::io::bounded::bounded_syscall`] (per the per-OS module)
    /// with a 60 s deadline on Linux/macOS — a wedged NFS server cannot
    /// trap the calling thread indefinitely. On timeout the page cache
    /// is left to the kernel's normal flush-on-close path — best
    /// effort, but bounded.
    pub(crate) fn sync_all(&mut self) -> io::Result<()> {
        self.pipeline.finalize();
        platform::durable_sync(&self.file)
    }
}

impl Write for WritebackFile {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        let n = self.file.write(buf)?;
        self.pos += n as u64;
        self.pipeline.note_progress(self.pos);
        Ok(n)
    }

    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
        self.file.write_all(buf)?;
        self.pos += buf.len() as u64;
        self.pipeline.note_progress(self.pos);
        Ok(())
    }

    fn flush(&mut self) -> io::Result<()> {
        self.file.flush()
    }
}

impl Seek for WritebackFile {
    fn seek(&mut self, from: SeekFrom) -> io::Result<u64> {
        let p = self.file.seek(from)?;
        // Only treat seeks that actually move the position as
        // boundaries — sweep does a redundant `seek(Current(pos))`
        // before every write, and we don't want that to drain the
        // pipeline on every iteration.
        if p != self.pos {
            // Diagnostic for the NFS mux hang: the MKV format requires
            // the muxer to seek back occasionally (cluster size
            // patching, Cues index write, Segment header backpatch).
            // Each such seek invalidates the writeback chunk tracking
            // and forces a finalize → WAIT_AFTER on the in-flight
            // chunk. Logging the seek delta lets us correlate hang
            // offsets with specific muxer operations.
            let from_pos = self.pos;
            let to_pos = p;
            let delta: i64 = (to_pos as i64).wrapping_sub(from_pos as i64);
            tracing::debug!(
                target: "mux",
                "WritebackFile seek from={from_pos} to={to_pos} delta={delta}"
            );
            self.pipeline.handle_seek(p);
            self.pos = p;
        }
        Ok(p)
    }
}

impl Drop for WritebackFile {
    fn drop(&mut self) {
        // Run the pipeline's tail finalize so the last in-flight chunk
        // gets its `WAIT_AFTER` + `posix_fadvise(DONTNEED)`. Without
        // this, callers that drop a `WritebackFile` without calling
        // `sync_all` (panic, early-return, idiomatic `let _ = w;`)
        // leave the trailing chunk in cache; the kernel still flushes
        // on close, but the bounded-cache invariant fails at the tail.
        // We deliberately do *not* call `self.file.sync_all()` here —
        // close already triggers a flush, and an `fsync` from `Drop`
        // would silently swallow its `io::Error` anyway. `finalize` is
        // idempotent so an explicit `sync_all` followed by drop is
        // still safe.
        self.pipeline.finalize();
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::io::Read;

    fn read_back(path: &Path) -> Vec<u8> {
        let mut f = File::open(path).unwrap();
        let mut v = Vec::new();
        f.read_to_end(&mut v).unwrap();
        v
    }

    #[test]
    fn write_then_drop_persists_bytes() {
        let dir = tempfile::tempdir().unwrap();
        let p = dir.path().join("a.bin");
        {
            let mut w = WritebackFile::create(&p).unwrap();
            w.write_all(b"hello world").unwrap();
            // Drop drains the pipeline tail.
        }
        assert_eq!(read_back(&p), b"hello world");
    }

    #[test]
    fn sync_all_drains_and_flushes() {
        let dir = tempfile::tempdir().unwrap();
        let p = dir.path().join("b.bin");
        let mut w = WritebackFile::create(&p).unwrap();
        for _ in 0..32 {
            w.write_all(&[0x5au8; 1024]).unwrap();
        }
        // After sync_all, the bytes MUST be visible to a separate
        // reader. The pipeline has been finalised and durable-sync has
        // run.
        w.sync_all().unwrap();
        let bytes = read_back(&p);
        assert_eq!(bytes.len(), 32 * 1024);
        assert!(bytes.iter().all(|&b| b == 0x5a));
        drop(w);
    }

    #[test]
    fn seek_then_patch_roundtrip() {
        // Write A; seek back; patch with B; read back; the patch lands
        // at the right offset.
        let dir = tempfile::tempdir().unwrap();
        let p = dir.path().join("c.bin");
        let mut w = WritebackFile::create(&p).unwrap();
        let big = vec![b'A'; 4096];
        w.write_all(&big).unwrap();
        // Seek back to offset 1000 and overwrite 8 bytes.
        w.seek(SeekFrom::Start(1000)).unwrap();
        w.write_all(b"PATCHED!").unwrap();
        w.sync_all().unwrap();
        drop(w);
        let bytes = read_back(&p);
        assert_eq!(bytes.len(), 4096);
        assert_eq!(&bytes[1000..1008], b"PATCHED!");
        // Bytes outside the patch are still 'A'.
        assert_eq!(bytes[999], b'A');
        assert_eq!(bytes[1008], b'A');
    }

    #[test]
    fn flush_is_observed_in_order() {
        // `Write::flush` should not panic or reorder; verify the bytes
        // land in order through interleaved flushes.
        let dir = tempfile::tempdir().unwrap();
        let p = dir.path().join("f.bin");
        let mut w = WritebackFile::create(&p).unwrap();
        w.write_all(b"one").unwrap();
        w.flush().unwrap();
        w.write_all(b"two").unwrap();
        w.flush().unwrap();
        w.write_all(b"three").unwrap();
        w.sync_all().unwrap();
        drop(w);
        assert_eq!(read_back(&p), b"onetwothree");
    }
}