Remove use of default SerialExecutor

Summary: Remove all uses of the default SerialExecutor by changing callsites to be explicit, and removing the default parameter. This will allow us to change the default to a safer option in a subsequent diff.

Reviewed By: yfeldblum

Differential Revision: D28842180

fbshipit-source-id: 93027dcf8b19c44380534dabd731651780dac90e

folly/executors/SerialExecutor.h

@@ -57,9 +57,7 @@ class SerialExecutor : public SequencedExecutor {
   SerialExecutor(SerialExecutor&&) = delete;
   SerialExecutor& operator=(SerialExecutor&&) = delete;
 
-  static KeepAlive<SerialExecutor> create(
-      KeepAlive<Executor> parent =
-          getKeepAliveToken(getUnsafeMutableGlobalCPUExecutor().get()));
+  static KeepAlive<SerialExecutor> create(KeepAlive<Executor> parent);
 
   class Deleter {
    public:

folly/executors/test/SerialExecutorTest.cpp

@@ -179,7 +179,8 @@ TEST(SerialExecutor, RecursiveAddInline) {
 }
 
 TEST(SerialExecutor, ExecutionThrows) {
-  auto executor = SerialExecutor::create();
+  auto executor = SerialExecutor::create(folly::getKeepAliveToken(
+      folly::getUnsafeMutableGlobalCPUExecutor().get()));
 
   // an empty Func will throw std::bad_function_call when invoked,
   // but SerialExecutor should catch that exception