fix: detect external file edits that preserve mtime and length (#1582)

An external write that keeps a file's byte length and mtime was missed twice: the Maven launch fingerprint reused the stale class snapshot, and watcher events for clean open documents were dropped by the (mtime, length) stamp check.

The launch fingerprint now records a blake3 content hash for project files (toolchain inputs such as the JDK's lib/modules keep the stamp only) and bumps its version. Watcher events for clean documents compare the disk bytes with what saving the buffer would write; blind focus scans keep the stamp dedup, and forced refreshes after IDE disk operations still reload clean buffers.

Co-authored-by: qtvbwfn <5848026+qtvbwfn@users.noreply.github.com>

Cargo.lock

@@ -4121,6 +4121,7 @@ dependencies = [
 name = "ide-build"
 version = "0.1.37"
 dependencies = [
+ "blake3",
  "dirs 6.0.0",
  "dunce",
  "ide-core",

crates/ide-build/Cargo.toml

@@ -6,6 +6,7 @@ edition.workspace = true
 license.workspace = true
 
 [dependencies]
+blake3.workspace = true
 dunce.workspace = true
 ide-core.workspace = true
 zip.workspace = true

crates/ide-build/src/compile/fingerprint.rs

@@ -2,18 +2,49 @@
 use super::*;
 use std::fs;
 use std::hash::{Hash, Hasher};
+use std::io::Read;
 use std::time::SystemTime;
 
+/// `(byte length, mtime, blake3 hex)`; the hash is recorded for files inside the project only.
+type Stamp = (u64, SystemTime, Option<String>);
+
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub struct LaunchFingerprint {
     version: u32,
     parameters: String,
-    files: BTreeMap<PathBuf, Option<(u64, SystemTime)>>,
+    /// Per input: a [`Stamp`] for regular files, `None` for
+    /// directories and inputs that cannot be resolved. The content hash catches external
+    /// edits that preserve both length and mtime (timestamp-preserving tools, coarse
+    /// timestamp granularity), which `(length, mtime)` alone would miss. Only files inside
+    /// the project are hashed; toolchain inputs such as the JDK's `lib/modules` keep the stamp.
+    files: BTreeMap<PathBuf, Option<Stamp>>,
+}
+
+/// `blake3` hex of a file's bytes, streamed in chunks so memory stays bounded. An
+/// unreadable file records a sentinel so one permission problem does not drop the whole
+/// fingerprint (that would just force a normal rebuild).
+fn content_hash(path: &Path) -> IdeResult<String> {
+    let mut file = match fs::File::open(path) {
+        Ok(file) => file,
+        Err(e) if e.kind() == std::io::ErrorKind::NotFound => return Ok("missing".into()),
+        Err(e) => return Err(e.into()),
+    };
+    let mut hasher = blake3::Hasher::new();
+    let mut buffer = [0; 64 * 1024];
+    loop {
+        let count = file.read(&mut buffer)?;
+        if count == 0 {
+            break;
+        }
+        hasher.update(&buffer[..count]);
+    }
+    Ok(hasher.finalize().to_hex().to_string())
 }
 
 fn scan(
+    root: &Path,
     path: &Path,
-    files: &mut BTreeMap<PathBuf, Option<(u64, SystemTime)>>,
+    files: &mut BTreeMap<PathBuf, Option<Stamp>>,
     ancestors: &mut HashSet<PathBuf>,
     cancel: &CancellationToken,
 ) -> IdeResult<()> {
@@ -36,11 +67,12 @@ fn scan(
         // Directory mtimes change on irrelevant JDT writes. File paths detect additions/deletions.
         files.insert(path.to_path_buf(), None);
         for entry in fs::read_dir(path)? {
-            scan(&entry?.path(), files, ancestors, cancel)?;
+            scan(root, &entry?.path(), files, ancestors, cancel)?;
         }
         ancestors.remove(&canonical);
     } else if meta.is_file() {
-        files.insert(path.to_path_buf(), Some((meta.len(), meta.modified()?)));
+        let hash = if path.starts_with(root) { Some(content_hash(path)?) } else { None };
+        files.insert(path.to_path_buf(), Some((meta.len(), meta.modified()?, hash)));
     } else {
         return Err(IdeError::internal("unsupported launch input file"));
     }
@@ -145,11 +177,11 @@ pub fn launch_fingerprint(
             .map_err(|e| IdeError::internal(e.to_string()))?;
     let mut files = BTreeMap::new();
     for path in inputs {
-        scan(&path, &mut files, &mut HashSet::new(), cancel)?;
+        scan(root, &path, &mut files, &mut HashSet::new(), cancel)?;
     }
     // Store project inputs relative to the project, including each file's relative path.
     let files = files.into_iter().map(|(p, stamp)| (p.strip_prefix(root).unwrap_or(&p).to_path_buf(), stamp)).collect();
-    Ok(Some(LaunchFingerprint { version: 1, parameters, files }))
+    Ok(Some(LaunchFingerprint { version: 2, parameters, files }))
 }
 
 #[cfg(test)]
@@ -231,6 +263,25 @@ mod tests {
         }
     }
 
+    /// An external tool that keeps the byte length and the mtime (timestamp-preserving
+    /// writes, coarse-granularity filesystems) still changes the content: the fingerprint
+    /// must differ via the content hash, not via `(length, mtime)`.
+    #[test]
+    fn external_edit_preserving_length_and_mtime_still_invalidates() {
+        let dir = tempfile::tempdir().unwrap();
+        let root = dir.path();
+        let model = fixture(root);
+        let source = root.join("lib/src/main/kotlin/Channel.kt");
+        let before = fingerprint(root, &model);
+        assert!(before.is_some());
+        let mtime = fs::metadata(&source).unwrap().modified().unwrap();
+        // Same 5-byte length, different content.
+        write(root, "lib/src/main/kotlin/Channel.kt", "valve");
+        fs::File::options().write(true).open(&source).unwrap().set_modified(mtime).unwrap();
+        let after = fingerprint(root, &model);
+        assert_ne!(before, after, "a same-length, same-mtime content change must invalidate the fingerprint");
+    }
+
     #[test]
     fn profile_options_jdk_goal_and_wrapper_invalidate_without_using_class_outputs() {
         let dir = tempfile::tempdir().unwrap();

crates/ide-host/src/ops/java/cache.rs

@@ -242,6 +242,29 @@ mod tests {
         assert!(build().is_err());
         assert_eq!(fs::read_to_string(root.join("calls")).unwrap().lines().count(), 5);
     }
+    /// An external tool (e.g. an AI code editor) rewrites a source file with the same byte
+    /// length and preserves its mtime. The `(length, mtime)` fingerprint reads that as
+    /// "unchanged" and would reuse the stale snapshot; the content hash must catch it.
+    #[test]
+    fn external_edit_preserving_length_and_mtime_rebuilds_the_snapshot() {
+        let dir = tempfile::tempdir().unwrap();
+        let root = dir.path();
+        let model = fixture(root);
+        let cache = root.join("cache");
+        let cancel = CancellationToken::new();
+        let mut session = Session::new(cache.clone());
+        let first = session.build(root, &model, root, false, None, &cancel).unwrap();
+        assert_eq!(fs::read_to_string(first.snapshot.join("0/Main.class")).unwrap(), "first");
+
+        let source = root.join("src/main/kotlin/Main.kt");
+        let mtime = fs::metadata(&source).unwrap().modified().unwrap();
+        fs::write(&source, "other").unwrap(); // 5 bytes, same length as "first"
+        fs::File::options().write(true).open(&source).unwrap().set_modified(mtime).unwrap();
+        let rebuilt = session.build(root, &model, root, false, None, &cancel).unwrap();
+        assert_ne!(rebuilt.snapshot, first.snapshot, "a same-length, same-mtime content change must rebuild");
+        assert_eq!(fs::read_to_string(rebuilt.snapshot.join("0/Main.class")).unwrap(), "other");
+    }
+
     #[test]
     fn session_drop_removes_superseded_snapshots_and_preserves_records_across_reopen() {
         let dir = tempfile::tempdir().unwrap();

crates/ide-workspace/src/documents.rs

@@ -868,17 +868,18 @@ impl Documents {
                     .then(|| st.path.clone())
             })
             .collect();
-        self.reload_from_disk(&paths, force_clean);
+        self.reload_from_disk(&paths, force_clean, true);
     }
 
     /// Re-reads from disk after an external change. Clean documents reload
     /// silently; dirty ones are left alone and reported via `IdeEvent::Notify`.
     /// Changes matching our own last save are ignored.
     pub fn on_disk_changed(&self, paths: &[PathBuf]) {
-        self.reload_from_disk(paths, false);
+        self.reload_from_disk(paths, false, false);
     }
 
-    fn reload_from_disk(&self, paths: &[PathBuf], force_clean: bool) {
+    /// `dedup`: blind scans skip files whose `(mtime, length)` stamp still matches.
+    fn reload_from_disk(&self, paths: &[PathBuf], force_clean: bool, dedup: bool) {
         for path in paths {
             let Some(id) = self.by_path(path) else { continue };
             let result = self.with_doc(id, |st| {
@@ -887,7 +888,23 @@ impl Documents {
                 }
                 let Ok(meta) = std::fs::metadata(&st.path) else { return Ok(()) };
                 let mtime = ide_vfs::mtime_ms(&meta);
-                if mtime == st.disk_mtime && meta.len() == st.disk_len && (!force_clean || st.dirty) {
+                let unchanged = mtime == st.disk_mtime && meta.len() == st.disk_len;
+                // Blind scans trust the stamp. OS events are authoritative, but they also
+                // echo our own saves: a clean document whose stamp matches is re-checked by
+                // bytes — an external write that preserved both length and mtime differs
+                // from what saving this buffer would write, our own save's echo does not.
+                // Native-to-ascii properties documents and dirty documents keep the stamp
+                // dedup: the former's disk bytes legitimately differ from the buffer text,
+                // the latter warn once per disk state.
+                if unchanged
+                    && (!force_clean || st.dirty)
+                    && (dedup
+                        || st.dirty
+                        || st.native_to_ascii
+                        || ide_vfs::encode(&st.doc.text_for_save(), &st.encoding, st.has_bom)
+                            .map(|bytes| bytes == std::fs::read(&st.path).unwrap_or_default())
+                            .unwrap_or(true))
+                {
                     return Ok(());
                 }
                 if st.dirty {

crates/ide-workspace/tests/documents.rs

@@ -403,6 +403,46 @@ fn external_changes() {
     f.docs.on_disk_changed(&[f.root.join("other.txt")]);
 }
 
+/// An IDE operation that rewrites a file with the same length and mtime still refreshes
+/// the clean buffer.
+#[test]
+fn own_disk_change_preserving_length_and_mtime_reloads_a_clean_document() {
+    let f = fixture();
+    let path = f.root.join("stamp.txt");
+    fs::write(&path, "first").unwrap();
+    let id = f.docs.open(&path).unwrap().id;
+    let mtime = fs::metadata(&path).unwrap().modified().unwrap();
+    fs::write(&path, "other").unwrap();
+    fs::File::options().write(true).open(&path).unwrap().set_modified(mtime).unwrap();
+    f.docs.sync_after_disk_change();
+    assert_eq!(f.docs.snapshot(id).unwrap().text, "other");
+}
+
+/// An external write that keeps the byte length and the mtime (timestamp-preserving
+/// tools, coarse-granularity filesystems) leaves the `(mtime, length)` stamp equal:
+/// the OS event is still authoritative, so a clean document reloads.
+#[test]
+fn external_change_preserving_length_and_mtime_reloads_a_clean_document() {
+    let f = fixture();
+    let path = f.root.join("stamp.txt");
+    fs::write(&path, "first").unwrap(); // 5 bytes
+    let id = f.docs.open(&path).unwrap().id;
+    let mtime = fs::metadata(&path).unwrap().modified().unwrap();
+
+    fs::write(&path, "other").unwrap(); // 5 bytes, different content
+    fs::File::options().write(true).open(&path).unwrap().set_modified(mtime).unwrap();
+    f.docs.on_disk_changed(std::slice::from_ref(&path));
+    let snap = f.docs.snapshot(id).unwrap();
+    assert_eq!(snap.text, "other");
+    assert!(!snap.dirty);
+
+    // The reload recorded the new stamp: repeating the same event changes nothing.
+    let before = f.events.lock().len();
+    f.docs.on_disk_changed(std::slice::from_ref(&path));
+    assert_eq!(f.events.lock().len(), before);
+    assert_eq!(f.docs.snapshot(id).unwrap().text, "other");
+}
+
 #[test]
 fn encodings_and_read_only() {
     let f = fixture();