perf(delivery): reduce redundant restore file syscalls

judge/crates/sandbox/src/restore_copy/files.rs

@@ -266,22 +266,20 @@ impl Roots {
         {
             return Err(Error::Identity);
         }
-        let current = output.metadata().map_err(|_| Error::Filesystem)?;
-        let changed = (current.uid(), current.gid()) != (uid, gid);
+        // This descriptor was exclusively created by this copy. Its initial
+        // owner/mode decides the required changes; the final fstat below still
+        // checks the complete resulting file after every write and mode change.
+        let changed = (empty.uid(), empty.gid()) != (uid, gid);
         if changed {
             rustix::fs::fchown(&output, Some(Uid::from_raw(uid)), Some(Gid::from_raw(gid)))
                 .map_err(|_| Error::Filesystem)?;
         }
-        let mode_changed = changed || current.mode() & 0o7777 != mode;
+        let mode_changed = changed || empty.mode() & 0o7777 != mode;
         if mode_changed {
             rustix::fs::fchmod(&output, Mode::from_raw_mode(mode))
                 .map_err(|_| Error::Filesystem)?;
         }
-        let final_info = if mode_changed {
-            output.metadata().map_err(|_| Error::Filesystem)?
-        } else {
-            current
-        };
+        let final_info = output.metadata().map_err(|_| Error::Filesystem)?;
         member(&final_info, &row, uid, gid, mode)?;
         let witness = stamp(&final_info)?;
         drop(output);
@@ -458,6 +456,13 @@ fn verify(
             data.extend_from_slice(&buffer[..count]);
         }
         total = end;
+        // A short read already delivered every declared byte. The caller
+        // checks the same descriptor/path full nine-field witness (including
+        // exact size) before and after verification, so a changed or longer
+        // file still fails. Exact buffer multiples still require an explicit EOF read.
+        if total as u64 == row.bytes && count < buffer.len() {
+            break;
+        }
     }
     if total as u64 != row.bytes
         || hasher.is_some_and(|h| format!("{:x}", h.finalize()) != row.sha256)

judge/crates/sandbox/src/restore_copy/main.rs

@@ -527,6 +527,97 @@ mod tests {
         roots.unchanged().unwrap_or_else(|_| panic!("root anchors"));
     }
 
+    #[test]
+    fn exact_lengths_and_unchanged_create_mode_keep_complete_readback() {
+        let fixture = files::Fixture::new();
+        let roots = Arc::new(fixture.roots());
+        use std::os::unix::fs::MetadataExt;
+        let owner = fs::metadata(fixture.source()).unwrap();
+        let mut rows = Vec::new();
+        for (index, size) in [0, 13, 1024 * 1024, 1024 * 1024 + 13]
+            .into_iter()
+            .enumerate()
+        {
+            let mut data = vec![b'a'; size];
+            if let Some(last) = data.last_mut() {
+                *last = b'z';
+            }
+            let hash = format!("{:x}", Sha256::digest(&data));
+            fs::write(fixture.source().join(&hash), &data).unwrap();
+            fs::set_permissions(
+                fixture.source().join(&hash),
+                fs::Permissions::from_mode(0o600),
+            )
+            .unwrap();
+            rows.push(restore_io::RestoreFile {
+                path: format!("artifacts/length-{index}"),
+                uid: owner.uid(),
+                gid: owner.gid(),
+                mode: 0o600,
+                bytes: size as u64,
+                sha256: hash,
+            });
+        }
+        let pool = Pool::new(Arc::clone(&roots)).unwrap_or_else(|_| panic!("pool"));
+        for _ in 0..2 {
+            let verified = pool
+                .materialize(rows.clone(), &roots, now().unwrap() + 10_000_000_000)
+                .unwrap_or_else(|_| panic!("complete lengths including existing files"));
+            assert_eq!(verified.len(), 4);
+            for pending in &verified {
+                roots
+                    .final_file(pending)
+                    .unwrap_or_else(|_| panic!("final witness"));
+                let name = pending.row.path.strip_prefix("artifacts/").unwrap();
+                let data = fs::read(fixture.target().join(name)).unwrap();
+                assert_eq!(data.len() as u64, pending.row.bytes);
+                assert_eq!(format!("{:x}", Sha256::digest(&data)), pending.row.sha256);
+                assert_eq!(
+                    fs::metadata(fixture.target().join(name))
+                        .unwrap()
+                        .permissions()
+                        .mode()
+                        & 0o7777,
+                    0o600
+                );
+            }
+        }
+        drop(pool);
+        roots.unchanged().unwrap_or_else(|_| panic!("root anchors"));
+    }
+
+    #[test]
+    fn short_final_chunk_still_refuses_a_wrong_last_byte() {
+        for size in [13, 1024 * 1024 + 13] {
+            let fixture = files::Fixture::new();
+            let roots = Arc::new(fixture.roots());
+            let mut data = vec![b'a'; size];
+            let expected = format!("{:x}", Sha256::digest(&data));
+            *data.last_mut().unwrap() = b'z';
+            let source = fixture.source().join(&expected);
+            fs::write(&source, data).unwrap();
+            fs::set_permissions(&source, fs::Permissions::from_mode(0o600)).unwrap();
+            use std::os::unix::fs::MetadataExt;
+            let owner = fs::metadata(&source).unwrap();
+            let pool = Pool::new(Arc::clone(&roots)).unwrap_or_else(|_| panic!("pool"));
+            assert!(matches!(
+                pool.materialize(
+                    vec![restore_io::RestoreFile {
+                        path: "artifacts/wrong-last-byte".into(),
+                        uid: owner.uid(),
+                        gid: owner.gid(),
+                        mode: 0o600,
+                        bytes: size as u64,
+                        sha256: expected,
+                    }],
+                    &roots,
+                    now().unwrap() + 10_000_000_000
+                ),
+                Err(Error::Bytes)
+            ));
+        }
+    }
+
     #[test]
     fn request_line_requires_finite_complete_frame() {
         assert!(matches!(