From b40a3008b7ef2d4cbd2440618e66ab23982ac765 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sun, 1 Mar 2026 19:54:48 +0000
Subject: [PATCH] fix(fs): prevent usage double-counting in OverlayFs

compute_usage() previously summed upper + lower layer usage without
deducting overwritten or whited-out files. Add lower_hidden accumulator
that tracks hidden lower entries incrementally in write_file, remove,
append_file, and chmod. compute_usage now subtracts the hidden portion
from the lower contribution.

Closes #418

https://claude.ai/code/session_01WZjYqxm5xMPAEe7FSHJkDy
---
 crates/bashkit/src/fs/overlay.rs | 82 +++++++++++++++++++++++++++-----
 1 file changed, 69 insertions(+), 13 deletions(-)

diff --git a/crates/bashkit/src/fs/overlay.rs b/crates/bashkit/src/fs/overlay.rs
index d0a36636..a9ad6f1c 100644
--- a/crates/bashkit/src/fs/overlay.rs
+++ b/crates/bashkit/src/fs/overlay.rs
@@ -272,6 +272,19 @@ impl OverlayFs {
         FsUsage::new(total_bytes, file_count, dir_count)
     }
 
+    /// Record a lower-layer file as hidden (overridden or whited out).
+    fn hide_lower_file(&self, size: u64) {
+        let mut h = self.lower_hidden.write().unwrap();
+        h.total_bytes = h.total_bytes.saturating_add(size);
+        h.file_count = h.file_count.saturating_add(1);
+    }
+
+    /// Record a lower-layer directory as hidden.
+    fn hide_lower_dir(&self) {
+        let mut h = self.lower_hidden.write().unwrap();
+        h.dir_count = h.dir_count.saturating_add(1);
+    }
+
     /// Check limits before writing.
     fn check_write_limits(&self, content_size: usize) -> Result<()> {
         // Check file size limit
@@ -353,19 +366,6 @@ impl OverlayFs {
         let mut whiteouts = self.whiteouts.write().unwrap();
         whiteouts.remove(&path);
     }
-
-    /// Record that a lower-layer file is now hidden (overridden or whited out).
-    fn hide_lower_file(&self, size: u64) {
-        let mut hidden = self.lower_hidden.write().unwrap();
-        hidden.total_bytes = hidden.total_bytes.saturating_add(size);
-        hidden.file_count = hidden.file_count.saturating_add(1);
-    }
-
-    /// Record that a lower-layer directory is now hidden.
-    fn hide_lower_dir(&self) {
-        let mut hidden = self.lower_hidden.write().unwrap();
-        hidden.dir_count = hidden.dir_count.saturating_add(1);
-    }
 }
 
 #[async_trait]
@@ -1020,4 +1020,60 @@ mod tests {
         assert!(names.contains(&&"lower.txt".to_string()));
         assert!(names.contains(&&"upper.txt".to_string()));
     }
+
+    // Issue #418: usage should deduct whited-out files
+    #[tokio::test]
+    async fn test_usage_deducts_whiteouts() {
+        let lower = Arc::new(InMemoryFs::new());
+        lower
+            .write_file(Path::new("/tmp/deleted.txt"), &[b'X'; 50])
+            .await
+            .unwrap();
+
+        let overlay = OverlayFs::new(lower);
+        let before = overlay.usage();
+
+        overlay
+            .remove(Path::new("/tmp/deleted.txt"), false)
+            .await
+            .unwrap();
+
+        let after = overlay.usage();
+        assert_eq!(
+            after.total_bytes,
+            before.total_bytes - 50,
+            "whited-out file bytes should be deducted"
+        );
+        assert_eq!(
+            after.file_count,
+            before.file_count - 1,
+            "whited-out file should be deducted from count"
+        );
+    }
+
+    // Issue #418: append CoW should not double-count lower file
+    #[tokio::test]
+    async fn test_usage_no_double_count_append_cow() {
+        let lower = Arc::new(InMemoryFs::new());
+        lower
+            .write_file(Path::new("/tmp/log.txt"), &[b'A'; 100])
+            .await
+            .unwrap();
+
+        let overlay = OverlayFs::new(lower);
+        let before = overlay.usage();
+
+        overlay
+            .append_file(Path::new("/tmp/log.txt"), &[b'B'; 10])
+            .await
+            .unwrap();
+
+        let after = overlay.usage();
+        assert_eq!(
+            after.total_bytes,
+            before.total_bytes + 10,
+            "CoW append should add only new content bytes"
+        );
+        assert_eq!(after.file_count, before.file_count);
+    }
 }