double-check datetime init

Author: Person-93

pyo3-ffi/src/datetime.rs

@@ -611,10 +611,7 @@ pub unsafe fn PyDateTimeAPI() -> *mut PyDateTime_CAPI {
 pub unsafe fn PyDateTime_IMPORT() {
     // Protect against race conditions when the datetime API is concurrently
     // initialized in multiple threads.
-    if PyDateTimeAPI_impl
-        .already_called
-        .swap(true, Ordering::SeqCst)
-    {
+    if PyDateTimeAPI_impl.already_set.load(Ordering::SeqCst) {
         return;
     }
 
@@ -631,7 +628,12 @@ pub unsafe fn PyDateTime_IMPORT() {
         return;
     }
 
-    *PyDateTimeAPI_impl.ptr.get() = py_datetime_c_api;
+    // Protect against race conditions when the datetime API is concurrently
+    // initialized in multiple threads. UnsafeCell.get() cannot panic so this
+    // won't panic either.
+    if !PyDateTimeAPI_impl.already_set.swap(true, Ordering::SeqCst) {
+        *PyDateTimeAPI_impl.ptr.get() = py_datetime_c_api;
+    }
 }
 
 #[inline]
@@ -842,12 +844,12 @@ extern_libpython! {
 // Rust specific implementation details
 
 struct PyDateTimeAPISingleton {
-    already_called: AtomicBool,
+    already_set: AtomicBool,
     ptr: UnsafeCell<*mut PyDateTime_CAPI>,
 }
 unsafe impl Sync for PyDateTimeAPISingleton {}
 
 static PyDateTimeAPI_impl: PyDateTimeAPISingleton = PyDateTimeAPISingleton {
-    already_called: AtomicBool::new(false),
+    already_set: AtomicBool::new(false),
     ptr: UnsafeCell::new(ptr::null_mut()),
 };