v0.17.12: mapfile time-batched persistence — unblock NFS staging

Pre-0.17.12 every Mapfile::record() persisted the full mapfile via
tempfile-create + write + atomic-rename. On local LVM that's
microseconds; on NFS each rename is multiple RPCs through the
unraid user-share fuse layer, dragging a Black Mass UHD rip from
~11 MB/s on local to ~1.5 MB/s on NFS — the mapfile path alone burned
multiple seconds of wall time per real-world second of work.

Mapfile now batches the rename to once per second:

- record() always updates in-memory state and stats; only fires
  write_to_disk when last_flushed.elapsed() >= FLUSH_INTERVAL (1 s).
- New flush() API forces a persist; called by sweep_pipeline's
  consumer at end-of-sweep and by Disc::patch at end-of-patch,
  after the file's sync_all.
- Drop impl best-effort flushes so an early-return / unwind doesn't
  silently lose pending state.

Crash-safety changes from "lose at most one block" to "lose at most
1 s of recorded progress" — the ISO file's payload bytes are unaffected;
only the mapfile's authority over which sectors are already-good is at
risk, and a resume re-reads anything Pass 1 had already covered.

Measured on the BU40N test bed against Black Mass UHD inner zone:
- NFS staging: 1.5 MB/s → 16.48 MB/s (10.9× recovery)
- Local LVM staging: 11.09 MB/s → 11.83 MB/s (+6.7 % bonus)

Internal round_trip_load test now flushes before reading back from
disk. External patch / copy tests are unaffected: patch and
sweep_pipeline flush at completion before returning.

src/disc/mapfile.rs

@@ -17,11 +17,22 @@
 //!
 //! Status chars: `?` non-tried · `*` non-trimmed · `/` non-scraped · `-` unreadable · `+` finished.
 //!
-//! The mapfile is flushed to disk on every `record()` call so a crashed
-//! rip loses at most one block of recorded state.
+//! The mapfile is flushed to disk at most once per `FLUSH_INTERVAL`
+//! during `record()` calls, plus on explicit `flush()` and on `Drop`.
+//! This bounds atomic-rename RPC rate on networked staging (e.g. NFS)
+//! where per-record persists otherwise serialize the rip pipeline.
 
 use std::io::{self, Write};
 use std::path::{Path, PathBuf};
+use std::time::{Duration, Instant};
+
+/// Minimum interval between mapfile persists. `record()` updates in-memory
+/// state every call but only writes to disk when this interval has elapsed
+/// since the last persist (or when `flush()` is called explicitly, or on
+/// `Drop`). Bounds RPC rate on NFS staging where atomic-rename per record
+/// otherwise dominates throughput. On crash the worst-case progress loss
+/// is one interval's worth of records.
+const FLUSH_INTERVAL: Duration = Duration::from_millis(1000);
 
 /// Status of a byte range in the mapfile. ddrescue-compatible.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -100,8 +111,11 @@ pub struct MapStats {
     pub main_lost_ms: f64,
 }
 
-/// Write-through mapfile. Every `record()` persists to disk immediately
-/// so a crash during rip loses at most one block.
+/// Time-batched mapfile. `record()` keeps in-memory state up-to-date on
+/// every call; persists to disk at most once per `FLUSH_INTERVAL`.
+/// Explicit `flush()` and `Drop` guarantee state is on disk after a sweep
+/// or patch finishes. On hard crash the worst-case loss is one flush
+/// interval of records — the file's payload bytes are unaffected.
 pub struct Mapfile {
     path: PathBuf,
     entries: Vec<MapEntry>,
@@ -110,6 +124,12 @@ pub struct Mapfile {
     /// Incrementally maintained stats — updated on every `record()` call
     /// so `stats()` is O(1) instead of O(n).
     stats: MapStats,
+    /// True when in-memory state has changed but `write_to_disk` has not
+    /// yet captured it.
+    dirty: bool,
+    /// Wall-clock timestamp of the last successful `write_to_disk` (or
+    /// the moment the mapfile was constructed, whichever is later).
+    last_flushed: Instant,
 }
 
 impl Mapfile {
@@ -117,7 +137,7 @@ impl Mapfile {
     /// Writes to disk immediately so a resume can pick up even if the caller
     /// never records anything.
     pub fn create(path: &Path, total_size: u64, version: &str) -> io::Result<Self> {
-        let mf = Self {
+        let mut mf = Self {
             path: path.to_path_buf(),
             entries: vec![MapEntry {
                 pos: 0,
@@ -132,8 +152,13 @@ impl Mapfile {
                 bytes_nontried: total_size,
                 ..Default::default()
             },
+            dirty: false,
+            last_flushed: Instant::now(),
         };
+        // Eager initial persist so a resume can pick this up even if
+        // `record()` is never called.
         mf.write_to_disk()?;
+        mf.last_flushed = Instant::now();
         Ok(mf)
     }
 
@@ -200,6 +225,8 @@ impl Mapfile {
             total_size,
             version,
             stats,
+            dirty: false,
+            last_flushed: Instant::now(),
         })
     }
 
@@ -268,7 +295,24 @@ impl Mapfile {
         // path during sweep/patch, it just returns the cached value.
         self.stats = Self::compute_stats(&merged, self.total_size);
         self.entries = merged;
-        self.write_to_disk()?;
+        self.dirty = true;
+        if self.last_flushed.elapsed() >= FLUSH_INTERVAL {
+            self.write_to_disk()?;
+            self.dirty = false;
+            self.last_flushed = Instant::now();
+        }
+        Ok(())
+    }
+
+    /// Persist any pending in-memory changes to disk. No-op if clean.
+    /// Callers (sweep/patch finalisation) invoke this after their last
+    /// `record()` to guarantee state is durable before returning.
+    pub fn flush(&mut self) -> io::Result<()> {
+        if self.dirty {
+            self.write_to_disk()?;
+            self.dirty = false;
+            self.last_flushed = Instant::now();
+        }
         Ok(())
     }
 
@@ -363,6 +407,16 @@ impl Mapfile {
     }
 }
 
+impl Drop for Mapfile {
+    /// Best-effort flush on drop so a sweep / patch that returns early
+    /// (or unwinds) doesn't lose its in-memory state. Errors here are
+    /// swallowed because Drop has no way to surface them; explicit
+    /// `flush()` on the success path gives callers proper error handling.
+    fn drop(&mut self) {
+        let _ = self.flush();
+    }
+}
+
 fn parse_hex(s: &str) -> io::Result<u64> {
     let s = s.strip_prefix("0x").unwrap_or(s);
     u64::from_str_radix(s, 16).map_err(|_| {
@@ -463,6 +517,8 @@ mod tests {
         let mut mf = Mapfile::create(&p, 1000, "test").unwrap();
         mf.record(100, 200, SectorStatus::Finished).unwrap();
         mf.record(500, 100, SectorStatus::Unreadable).unwrap();
+        // record() batches; explicit flush before reading back from disk.
+        mf.flush().unwrap();
         let loaded = Mapfile::load(&p).unwrap();
         assert_eq!(loaded.entries(), mf.entries());
         let _ = std::fs::remove_file(&p);

src/disc/mod.rs

@@ -2956,6 +2956,7 @@ impl Disc {
                 "patch: sync_all failed for non-regular file; ignoring"
             );
         }
+        map.flush().map_err(|e| Error::IoError { source: e })?;
 
         // Log final ISO file size for write verification
         if let Ok(metadata) = std::fs::metadata(path) {

src/disc/sweep_pipeline.rs

@@ -199,7 +199,8 @@ fn consumer_loop(
         }
     }
 
-    // Final flush — drain the writeback pipeline + fsync the ISO.
+    // Final flush — drain the writeback pipeline + fsync the ISO,
+    // then persist any pending mapfile state.
     if first_error.is_none() {
         if let Err(e) = inputs.file.sync_all() {
             if inputs.is_regular {
@@ -208,6 +209,9 @@ fn consumer_loop(
             // Non-regular outputs (/dev/null, pipes) always fail
             // sync_all; that's not a real error.
         }
+        if let Err(e) = inputs.map.flush() {
+            first_error = Some(Error::IoError { source: e });
+        }
     }
 
     ConsumerSummary {