test: cover mtime-stable watch fingerprints

Author: zackees

Cargo.lock

@@ -38,6 +38,7 @@ tokio-serial = "5"
 serialport = { version = "4", default-features = false }
 pyo3 = { version = "0.22", features = ["abi3-py310"] }
 tempfile = "3"
+filetime = "0.2"
 sha2 = "0.10"
 regex = "1"
 walkdir = "2"

Cargo.toml

@@ -24,3 +24,4 @@ blake3 = { workspace = true }
 tempfile = { workspace = true }
 
 [dev-dependencies]
+filetime = { workspace = true }

crates/fbuild-build/Cargo.toml

@@ -16,7 +16,7 @@ use walkdir::WalkDir;
 use crate::zccache::FingerprintWatch;
 
 pub const BUILD_FINGERPRINT_VERSION: u32 = 2;
-const WATCH_STAMP_CACHE_VERSION: u32 = 1;
+const WATCH_STAMP_CACHE_VERSION: u32 = 2;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct PersistedBuildFingerprint {
@@ -456,6 +456,10 @@ mod tests {
         .unwrap()
     }
 
+    fn set_file_mtime(path: &Path, secs: i64) {
+        filetime::set_file_mtime(path, filetime::FileTime::from_unix_time(secs, 0)).unwrap();
+    }
+
     #[test]
     fn test_hash_files_changes_when_contents_change() {
         let tmp = tempfile::TempDir::new().unwrap();
@@ -568,6 +572,60 @@ mod tests {
         assert_eq!(first, second);
     }
 
+    #[test]
+    fn test_hash_watch_set_stamps_survives_mtime_reset_without_rewrite() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let src = tmp.path().join("src");
+        fs::create_dir_all(&src).unwrap();
+        let main = src.join("main.cpp");
+        fs::write(&main, "int main() { return 1; }\n").unwrap();
+        set_file_mtime(&main, 1_700_000_000);
+
+        let watch = make_watch(&src, &tmp.path().join(".project.zccache_fp.json"));
+        let hash_count = Cell::new(0usize);
+
+        let first = hash_watch_with_counter(&watch, &hash_count);
+        set_file_mtime(&main, 1_700_000_123);
+        let second = hash_watch_with_counter(&watch, &hash_count);
+        let after_second = hash_count.get();
+        let third = hash_watch_with_counter(&watch, &hash_count);
+
+        assert_eq!(first, second);
+        assert_eq!(second, third);
+        assert_eq!(
+            after_second, 2,
+            "mtime reset should rehash once to prove byte identity"
+        );
+        assert_eq!(
+            hash_count.get(),
+            after_second,
+            "updated stamp cache should restore the stat-only fast path"
+        );
+    }
+
+    #[test]
+    fn test_hash_watch_set_stamps_detects_same_length_content_change_after_mtime_reset() {
+        let tmp = tempfile::TempDir::new().unwrap();
+        let src = tmp.path().join("src");
+        fs::create_dir_all(&src).unwrap();
+        let main = src.join("main.cpp");
+        fs::write(&main, "int main() { return 1; }\n").unwrap();
+        set_file_mtime(&main, 1_700_000_000);
+
+        let watch = make_watch(&src, &tmp.path().join(".project.zccache_fp.json"));
+
+        let first = hash_watch_set_stamps(std::slice::from_ref(&watch)).unwrap();
+        fs::write(&main, "int main() { return 2; }\n").unwrap();
+        set_file_mtime(&main, 1_700_000_123);
+        let second = hash_watch_set_stamps(std::slice::from_ref(&watch)).unwrap();
+
+        assert_eq!(
+            fs::metadata(&main).unwrap().len(),
+            "int main() { return 1; }\n".len() as u64
+        );
+        assert_ne!(first, second);
+    }
+
     #[test]
     fn test_hash_watch_set_stamps_skips_rehash_when_stamp_unchanged() {
         let tmp = tempfile::TempDir::new().unwrap();