--- a/weatherlog/logger.py	Sat Sep 28 19:28:22 2019 -0400
+++ b/weatherlog/logger.py	Sat Sep 28 19:55:27 2019 -0400
@@ -121,6 +121,7 @@
     try:
         fcntl.flock(file, fcntl.LOCK_EX | fcntl.LOCK_NB)
     except BlockingIOError as ex:
+        file.close()
         raise OSError('Another copy of the logger is running.') from ex
     return file
 

--- a/weatherlog/logger_test.py	Sat Sep 28 19:28:22 2019 -0400
+++ b/weatherlog/logger_test.py	Sat Sep 28 19:55:27 2019 -0400
@@ -1,6 +1,8 @@
 import contextlib
 import datetime
+import fcntl
 import itertools
+import multiprocessing
 import pathlib
 import tempfile
 import time
@@ -154,6 +156,27 @@
             dict(sample_time=ts(31337), temp_c=666, rh_pct=999),
         ])
 
+    def test_fail_upon_lock(self):
+        bson_file = str(self.temp_path / logger.BSON_FILENAME)
+        out_queue = multiprocessing.Queue()
+        in_queue = multiprocessing.Queue()
+        # This needs to be in a separate multiprocessing.Process
+        # since flock-based file locks are per-process, not per-thread.
+        proc = multiprocessing.Process(
+            target=_lock_holder, args=(bson_file, out_queue, in_queue))
+        proc.start()
+        in_queue.get()  # Wait for the lock to be acquired.
+
+        with self.assertRaises(OSError):
+            logger.BufferedLogger(str(self.temp_path), FakeWriter())
+        out_queue.put(None)  # Notify that we're done.
+        out_queue.close()
+        proc.join()
+        proc.close()
+
+        # Test that it works after the lock is released.
+        logger.BufferedLogger(str(self.temp_path), FakeWriter()).close()
+
     def _read_last_sent(self):
         with (self.temp_path / logger.LAST_SENT_FILENAME).open('r') as infile:
             return infile.read()
@@ -163,5 +186,14 @@
             return bson.decode_all(infile.read(), logger.BSON_OPTIONS)
 
 
+def _lock_holder(path, in_queue, out_queue):
+    with open(path, 'w') as infile:
+        fcntl.flock(infile, fcntl.LOCK_SH)
+        out_queue.put(None)  # Notify that we've acquired the lock.
+        out_queue.close()
+        in_queue.get()  # Wait for the test to complete before closing.
+
+
 if __name__ == '__main__':
+    multiprocessing.set_start_method('spawn')
     unittest.main()