Save python import cost in folly/python get_fiber_manager

Summary:
Save python import cost in folly/python get_fiber_manager.

In my testing, using call_once here is safe even in multi-threaded and forked environment.
Though I wouldn't be surprised if a corner case is found in the future, as I couldn't find a definite answer about the safety of memoization Python C-API import calls.

Reviewed By: fried

Differential Revision: D9542909

fbshipit-source-id: c62a08ebf67bd4eb82f1bf4b7657c4a3a6daf4db

CMakeLists.txt

@@ -115,11 +115,13 @@ REMOVE_MATCHES_FROM_LISTS(files hfiles
 list(REMOVE_ITEM files
   ${FOLLY_DIR}/experimental/JSONSchemaTester.cpp
   ${FOLLY_DIR}/experimental/io/HugePageUtil.cpp
+  ${FOLLY_DIR}/python/fibers.cpp
   ${FOLLY_DIR}/python/GILAwareManualExecutor.cpp
 )
 list(REMOVE_ITEM hfiles
   ${FOLLY_DIR}/detail/SlowFingerprint.h
   ${FOLLY_DIR}/detail/FingerprintPolynomial.h
+  ${FOLLY_DIR}/python/fibers.h
   ${FOLLY_DIR}/python/GILAwareManualExecutor.h
 )
 

folly/python/fibers.cpp

+/*
+ * Copyright 2018-present Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/python/fibers.h>
+#include <folly/fibers/CallOnce.h>
+#include <folly/python/fiber_manager_api.h>
+
+namespace folly {
+namespace python {
+
+folly::fibers::FiberManager* getFiberManager(
+    const folly::fibers::FiberManager::Options& opts) {
+  static folly::fibers::once_flag flag;
+  // Use call_once because Python performance is really poor,
+  // just to check if a module was already imported
+  folly::call_once(flag, [&]() {
+    // Use main context, because import can load arbitrary number of files,
+    // which is incompatible with fiber stack size restrictions
+    folly::fibers::runInMainContext([&]() {
+      import_folly__fiber_manager();
+      if (PyErr_Occurred() != nullptr) {
+        throw std::logic_error("Fail to import cython fiber_manager");
+      }
+    });
+  });
+  return get_fiber_manager(opts);
+}
+
+} // namespace python
+} // namespace folly

folly/python/fibers.h

@@ -23,16 +23,12 @@
 #include <Python.h>
 #include <folly/Function.h>
 #include <folly/fibers/FiberManagerInternal.h>
-#include <folly/python/fiber_manager_api.h>
 
 namespace folly {
 namespace python {
 
-inline folly::fibers::FiberManager* getFiberManager(
-    const folly::fibers::FiberManager::Options& opts = {}) {
-  import_folly__fiber_manager();
-  return get_fiber_manager(opts);
-}
+folly::fibers::FiberManager* getFiberManager(
+    const folly::fibers::FiberManager::Options& opts = {});
 
 /**
  * Helper function with similar callback/userData parameters as bridgeFuture.