clawhdf5: a dropped FileEditor releases its lock at once

FileEditor's flock belongs to the open file description. When another
thread forks to spawn a process, the child shares the locked descriptor
until it execs, so a reopen right after the drop could be refused with
Error::Locked (a one-off failure of edit_interop::editor_locks_the_file in
a parallel test run). Drop now unlocks before closing, which releases the
lock for every descriptor sharing it.

Reproducer edit_tests::drop_releases_the_lock_while_other_threads_spawn_processes
(4 threads running `true`, 2000 open/drop rounds): 1483 of 2000 reopens
refused before, 0 in 30 runs after (tank). An OFD lock would not help: it
is inherited across fork the same way and does not conflict with
libhdf5's flock. The agent store's lock file unlocks on drop too.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-agent/src/store_lock.rs

@@ -20,7 +20,7 @@ const LOCK_RETRY_DELAY: std::time::Duration = std::time::Duration::from_millis(1
 /// never leaves a stale lock behind; the empty lock file itself is harmless).
 #[derive(Debug)]
 pub(crate) struct StoreLock {
-    _file: File,
+    file: File,
 }
 
 impl StoreLock {
@@ -42,7 +42,7 @@ impl StoreLock {
         let mut attempts_left = LOCK_RETRIES;
         loop {
             match file.try_lock() {
-                Ok(()) => return Ok(Self { _file: file }),
+                Ok(()) => return Ok(Self { file }),
                 Err(TryLockError::WouldBlock) if attempts_left > 0 => {
                     attempts_left -= 1;
                     std::thread::sleep(LOCK_RETRY_DELAY);
@@ -60,6 +60,16 @@ impl StoreLock {
     }
 }
 
+impl Drop for StoreLock {
+    /// Unlocks before the file is closed: a process another thread forks
+    /// inherits the descriptor until it execs, and a `flock` lasts while any
+    /// descriptor of the open file does, so closing alone could keep the
+    /// store locked for a moment after the drop (see `FileEditor`'s `Drop`).
+    fn drop(&mut self) {
+        let _ = self.file.unlock();
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;