fix: enforce complete external edit result contract

crates/fs-operation/src/lib.rs

@@ -337,6 +337,11 @@ pub trait FsAccessPolicy: Send + Sync {
         None
     }
 
+    /// Bound provider-owned Edit result cardinality before mutation.
+    fn max_edit_replacements(&self) -> Option<usize> {
+        None
+    }
+
     /// Atomically replace or create an already-authorized writable file.
     /// Capability providers override this to keep parent traversal descriptor-
     /// confined through the final rename.
@@ -521,6 +526,33 @@ mod tests {
         }
     }
 
+    struct CreateRaceAccess {
+        root: PathBuf,
+        destination: PathBuf,
+    }
+
+    impl FsAccessPolicy for CreateRaceAccess {
+        fn is_readable(&self, path: &Path) -> bool {
+            path.starts_with(&self.root)
+        }
+
+        fn is_writable(&self, path: &Path) -> bool {
+            path.starts_with(&self.root)
+        }
+
+        fn read_metadata(
+            &self,
+            _logical: &Path,
+            _resolved: &Path,
+        ) -> std::io::Result<std::fs::Metadata> {
+            std::fs::write(&self.destination, b"created concurrently")?;
+            Err(std::io::Error::new(
+                std::io::ErrorKind::NotFound,
+                "simulated absence before create race",
+            ))
+        }
+    }
+
     fn grep_request(path: &str, pattern: &str) -> GrepRequest {
         GrepRequest {
             pattern: pattern.to_string(),
@@ -1175,6 +1207,34 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn no_clobber_create_race_is_reported_as_conflict() {
+        let root = tempfile::tempdir().unwrap();
+        let destination = root.path().join("race.txt");
+        let root_path = root.path().canonicalize().unwrap();
+        let access = CreateRaceAccess {
+            root: root_path.clone(),
+            destination: destination.clone(),
+        };
+
+        let error = run_write(
+            &root_path,
+            WriteRequest {
+                path: FsPath::new("race.txt").unwrap(),
+                content: b"worker content".to_vec(),
+                expected_hash: None,
+            },
+            &access,
+        )
+        .unwrap_err();
+
+        assert!(matches!(error, FsError::Conflict(path) if path == "race.txt"));
+        assert_eq!(
+            std::fs::read_to_string(destination).unwrap(),
+            "created concurrently"
+        );
+    }
+
     #[cfg(target_os = "linux")]
     #[test]
     fn descriptor_confined_atomic_write_rejects_symlinks_and_survives_root_rename() {

crates/fs-operation/src/local.rs

@@ -231,7 +231,13 @@ pub fn run_write(
         }
         access
             .atomic_write_file(&path, &target, &request.content, AtomicWriteMode::CreateNew)
-            .map_err(|error| map_io(&logical, error))?;
+            .map_err(|error| {
+                if error.kind() == std::io::ErrorKind::AlreadyExists {
+                    FsError::Conflict(logical.as_str().to_string())
+                } else {
+                    map_io(&logical, error)
+                }
+            })?;
     }
     Ok(WriteResult {
         bytes_written: request.content.len(),
@@ -276,6 +282,14 @@ pub fn run_edit(
         )));
     }
     let replacement_count = if request.replace_all { occurrences } else { 1 };
+    if let Some(limit) = access.max_edit_replacements()
+        && replacement_count > limit
+    {
+        return Err(FsError::InvalidArgument(format!(
+            "{} exceeds provider Edit replacement limit {limit}",
+            logical.as_str()
+        )));
+    }
     let removed_bytes = request
         .old_string
         .len()

crates/workdir/src/external.rs

@@ -22,7 +22,7 @@ pub const MAX_EXTERNAL_PROVIDER_FRAME_BYTES: usize = 8 * 1024 * 1024;
 const MAX_PROTOCOL_ID_BYTES: usize = 128;
 const MAX_EXTERNAL_PATH_BYTES: usize = 4 * 1024;
 const MAX_EXTERNAL_PATTERN_BYTES: usize = 4 * 1024;
-const MAX_EXTERNAL_RESULT_ITEMS: usize = 10_000;
+pub const MAX_EXTERNAL_RESULT_ITEMS: usize = 10_000;
 const MAX_EXTERNAL_GREP_CONTEXT: usize = 100;
 const MAX_EXTERNAL_SCOPE_RULES: usize = 64;
 pub const MAX_EXTERNAL_WRITE_BYTES: usize = 512 * 1024;

crates/workdir/src/external_local_tests.rs

@@ -641,8 +641,15 @@ async fn external_read_write_uses_pinned_root_for_conflict_and_commit() {
 async fn external_read_write_rejects_oversized_preimages_and_edit_results_before_commit() {
     let root = tempfile::tempdir().unwrap();
     let limit = crate::external::MAX_EXTERNAL_WRITE_BYTES;
-    std::fs::write(root.path().join("oversized.txt"), vec![b'x'; limit + 1]).unwrap();
+    let oversized_content = vec![b'x'; limit + 1];
+    std::fs::write(root.path().join("oversized.txt"), &oversized_content).unwrap();
     std::fs::write(root.path().join("growth.txt"), vec![b'x'; limit]).unwrap();
+    let high_replacement_content = vec![b'x'; crate::external::MAX_EXTERNAL_RESULT_ITEMS + 1];
+    std::fs::write(
+        root.path().join("high-replacements.txt"),
+        &high_replacement_content,
+    )
+    .unwrap();
     let session = LocalWorkdirSession::external_read_write(
         Workdir::new("external-workdir-bounded-rw"),
         root.path(),
@@ -672,6 +679,10 @@ async fn external_read_write_rejects_oversized_preimages_and_edit_results_before
     .await
     .unwrap_err();
     assert!(matches!(write_error, WorkdirError::InvalidArgument(_)));
+    assert_eq!(
+        std::fs::read(root.path().join("oversized.txt")).unwrap(),
+        oversized_content
+    );
     let edit_error = WorkdirSession::edit(
         &session,
         EditRequest {
@@ -686,10 +697,8 @@ async fn external_read_write_rejects_oversized_preimages_and_edit_results_before
     .unwrap_err();
     assert!(matches!(edit_error, WorkdirError::InvalidArgument(_)));
     assert_eq!(
-        std::fs::metadata(root.path().join("oversized.txt"))
-            .unwrap()
-            .len(),
-        (limit + 1) as u64
+        std::fs::read(root.path().join("oversized.txt")).unwrap(),
+        oversized_content
     );
 
     let growth_observed = WorkdirSession::read(
@@ -720,6 +729,38 @@ async fn external_read_write_rejects_oversized_preimages_and_edit_results_before
         std::fs::read(root.path().join("growth.txt")).unwrap(),
         vec![b'x'; limit]
     );
+
+    let high_replacement_observed = WorkdirSession::read(
+        &session,
+        ReadRequest {
+            path: WorkdirPath::new("high-replacements.txt").unwrap(),
+            offset: 0,
+            limit: 1,
+            max_bytes: 1,
+        },
+    )
+    .await
+    .unwrap();
+    let high_replacement_error = WorkdirSession::edit(
+        &session,
+        EditRequest {
+            path: WorkdirPath::new("high-replacements.txt").unwrap(),
+            old_string: "x".to_string(),
+            new_string: "y".to_string(),
+            replace_all: true,
+            expected_hash: high_replacement_observed.content_hash,
+        },
+    )
+    .await
+    .unwrap_err();
+    assert!(matches!(
+        high_replacement_error,
+        WorkdirError::InvalidArgument(_)
+    ));
+    assert_eq!(
+        std::fs::read(root.path().join("high-replacements.txt")).unwrap(),
+        high_replacement_content
+    );
 }
 
 #[cfg(target_os = "linux")]

crates/workdir/src/local.rs

@@ -352,6 +352,11 @@ impl fs_operation::FsAccessPolicy for ScopeAccess {
             .then_some(crate::external::MAX_EXTERNAL_WRITE_BYTES)
     }
 
+    fn max_edit_replacements(&self) -> Option<usize> {
+        self.reject_symlinks
+            .then_some(crate::external::MAX_EXTERNAL_RESULT_ITEMS)
+    }
+
     fn atomic_write_file(
         &self,
         _logical: &Path,