Split out the global ShutdownSocketSet singleton

Summary:
[Folly] Split out the global `ShutdownSocketSet` singleton into its own library to keep the dependencies of `ShutdownSocketSet` lighter-weight.

In general, globals should be an opt-in sort of thing anyway.

Reviewed By: elsteveogrande

Differential Revision: D6790009

fbshipit-source-id: c68fa2bbaeaf3ff205e0c3b1c739d7561e940e64

folly/io/GlobalShutdownSocketSet.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/io/GlobalShutdownSocketSet.h>
+
+#include <folly/Singleton.h>
+
+namespace folly {
+
+namespace {
+struct PrivateTag {};
+} // namespace
+
+static Singleton<ShutdownSocketSet, PrivateTag> singleton;
+
+std::shared_ptr<ShutdownSocketSet> tryGetShutdownSocketSet() {
+  return singleton.try_get();
+}
+ReadMostlySharedPtr<ShutdownSocketSet> tryGetShutdownSocketSetFast() {
+  return singleton.try_get_fast();
+}
+
+} // namespace folly

folly/io/GlobalShutdownSocketSet.h

+/*
+ * 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.
+ */
+
+#pragma once
+
+#include <memory>
+
+#include <folly/experimental/ReadMostlySharedPtr.h>
+#include <folly/io/ShutdownSocketSet.h>
+
+namespace folly {
+
+std::shared_ptr<ShutdownSocketSet> tryGetShutdownSocketSet();
+ReadMostlySharedPtr<ShutdownSocketSet> tryGetShutdownSocketSetFast();
+
+} // namespace folly

folly/io/ShutdownSocketSet.cpp

@@ -22,26 +22,16 @@
 #include <glog/logging.h>
 
 #include <folly/FileUtil.h>
-#include <folly/Singleton.h>
 #include <folly/portability/Sockets.h>
 
 namespace folly {
 
-namespace {
-struct PrivateTag {};
-folly::Singleton<folly::ShutdownSocketSet, PrivateTag> singleton;
-} // namespace
-
 ShutdownSocketSet::ShutdownSocketSet(int maxFd)
     : maxFd_(maxFd),
       data_(static_cast<std::atomic<uint8_t>*>(
           folly::checkedCalloc(size_t(maxFd), sizeof(std::atomic<uint8_t>)))),
       nullFile_("/dev/null", O_RDWR) {}
 
-std::shared_ptr<ShutdownSocketSet> ShutdownSocketSet::getInstance() {
-  return singleton.try_get();
-}
-
 void ShutdownSocketSet::add(int fd) {
   // Silently ignore any fds >= maxFd_, very unlikely
   DCHECK_GE(fd, 0);

folly/io/ShutdownSocketSet.h

@@ -39,10 +39,6 @@ class ShutdownSocketSet : private boost::noncopyable {
    */
   explicit ShutdownSocketSet(int maxFd = 1 << 18);
 
-  // Singleton instance used by all thrift servers.
-  // May return nullptr on startup/shutdown.
-  static std::shared_ptr<ShutdownSocketSet> getInstance();
-
   /**
    * Add an already open socket to the list of sockets managed by
    * ShutdownSocketSet. You MUST close the socket by calling