Lock around Tracker creation and destruction

The `Tracker.__enter__` and `Tracker.__exit` methods may wind up
releasing the GIL, which allows another thread to see an intermediate
state where the tracker is not fully installed. Add a lock, shared
across all trackers, to serialize access to the global state that
tracker installation and uninstallation writes to.

Signed-off-by: Matt Wozniski <[email protected]>

news/667.bugfix.rst

@@ -0,0 +1 @@
+Fix a race condition that was able to cause strange exception messages if two different threads tried to initialize Memray tracking at once.

src/memray/_memray.pyx

@@ -581,6 +581,9 @@ cdef class ProfileFunctionGuard:
         NativeTracker.forgetPythonStack()
 
 
+tracker_creation_lock = threading.Lock()
+
+
 cdef class Tracker:
     """Context manager for tracking memory allocations in a Python script.
 
@@ -690,46 +693,48 @@ cdef class Tracker:
 
     @cython.profile(False)
     def __enter__(self):
-        if NativeTracker.getTracker() != NULL:
-            raise RuntimeError("No more than one Tracker instance can be active at the same time")
-
         cdef unique_ptr[RecordWriter] writer
-        if self._writer == NULL:
-            raise RuntimeError("Attempting to use stale output handle")
-        writer = move(self._writer)
-
-        for attr in ("_name", "_ident"):
-            assert not hasattr(threading.Thread, attr)
-            setattr(
-                threading.Thread,
-                attr,
-                ThreadNameInterceptor(attr, NativeTracker.registerThreadNameById),
-            )
+        with tracker_creation_lock:
+            if NativeTracker.getTracker() != NULL:
+                raise RuntimeError("No more than one Tracker instance can be active at the same time")
+
+            if self._writer == NULL:
+                raise RuntimeError("Attempting to use stale output handle")
+            writer = move(self._writer)
+
+            for attr in ("_name", "_ident"):
+                assert not hasattr(threading.Thread, attr)
+                setattr(
+                    threading.Thread,
+                    attr,
+                    ThreadNameInterceptor(attr, NativeTracker.registerThreadNameById),
+                )
 
-        self._previous_profile_func = sys.getprofile()
-        self._previous_thread_profile_func = threading._profile_hook
-        threading.setprofile(start_thread_trace)
+            self._previous_profile_func = sys.getprofile()
+            self._previous_thread_profile_func = threading._profile_hook
+            threading.setprofile(start_thread_trace)
 
-        if "greenlet" in sys.modules:
-            NativeTracker.beginTrackingGreenlets()
+            if "greenlet" in sys.modules:
+                NativeTracker.beginTrackingGreenlets()
 
-        NativeTracker.createTracker(
-            move(writer),
-            self._native_traces,
-            self._memory_interval_ms,
-            self._follow_fork,
-            self._trace_python_allocators,
-        )
-        return self
+            NativeTracker.createTracker(
+                move(writer),
+                self._native_traces,
+                self._memory_interval_ms,
+                self._follow_fork,
+                self._trace_python_allocators,
+            )
+            return self
 
     @cython.profile(False)
     def __exit__(self, exc_type, exc_value, exc_traceback):
-        NativeTracker.destroyTracker()
-        sys.setprofile(self._previous_profile_func)
-        threading.setprofile(self._previous_thread_profile_func)
+        with tracker_creation_lock:
+            NativeTracker.destroyTracker()
+            sys.setprofile(self._previous_profile_func)
+            threading.setprofile(self._previous_thread_profile_func)
 
-        for attr in ("_name", "_ident"):
-            delattr(threading.Thread, attr)
+            for attr in ("_name", "_ident"):
+                delattr(threading.Thread, attr)
 
 
 def start_thread_trace(frame, event, arg):