Explain crash when exception is thrown from Scope Guard

Summary:
Print a message to `std::cerr` when the current program is about to call `std::terminate` because a `folly::ScopeGuard` callback threw an exception. This goes to `std::terminate` in the (common) cases when the `ScopeGuard` destructor is `noexcept`

This gives the user a small clue as to what just happened, since the default diagnostics for this on some platforms do not help at all.

Reviewed By: nbronson

Differential Revision: D4061096

fbshipit-source-id: c3b534d4a36b095e08e46f375251b6fd416ccd68

folly/Makefile.am

@@ -483,6 +483,7 @@ libfolly_la_SOURCES = \
 	portability/Unistd.cpp \
 	Random.cpp \
 	SafeAssert.cpp \
+	ScopeGuard.cpp \
 	SharedMutex.cpp \
 	Shell.cpp \
 	MicroLock.cpp \

folly/ScopeGuard.cpp

+/*
+ * Copyright 2016 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 "ScopeGuard.h"
+
+#include <iostream>
+
+/*static*/ void folly::ScopeGuardImplBase::warnAboutToCrash() noexcept {
+  // Ensure the availability of std::cerr
+  std::ios_base::Init ioInit;
+  std::cerr
+      << "This program will now terminate because a folly::ScopeGuard callback "
+         "threw an \nexception.\n";
+}

folly/ScopeGuard.h

@@ -22,6 +22,7 @@
 #include <type_traits>
 #include <utility>
 
+#include <folly/Portability.h>
 #include <folly/Preprocessor.h>
 #include <folly/detail/UncaughtExceptionCounter.h>
 
@@ -75,6 +76,17 @@ class ScopeGuardImplBase {
     dismissed_ = true;
   }
 
+  template <typename T>
+  FOLLY_ALWAYS_INLINE static void runAndWarnAboutToCrashOnException(
+      T& function) {
+    try {
+      function();
+    } catch (...) {
+      warnAboutToCrash();
+      throw;
+    }
+  }
+
  protected:
   ScopeGuardImplBase() noexcept : dismissed_(false) {}
 
@@ -88,6 +100,9 @@ class ScopeGuardImplBase {
   }
 
   bool dismissed_;
+
+ private:
+  static void warnAboutToCrash() noexcept;
 };
 
 template <typename FunctionType>
@@ -151,7 +166,9 @@ class ScopeGuardImpl : public ScopeGuardImplBase {
 
   void* operator new(std::size_t) = delete;
 
-  void execute() noexcept { function_(); }
+  void execute() noexcept {
+    runAndWarnAboutToCrashOnException(function_);
+  }
 
   FunctionType function_;
 };
@@ -185,7 +202,7 @@ namespace detail {
  * If the parameter is false, then the function is executed if no new uncaught
  * exceptions are present at the end of the scope.
  *
- * Used to implement SCOPE_FAIL and SCOPE_SUCCES below.
+ * Used to implement SCOPE_FAIL and SCOPE_SUCCESS below.
  */
 template <typename FunctionType, bool executeOnException>
 class ScopeGuardForNewException {
@@ -205,7 +222,11 @@ class ScopeGuardForNewException {
 
   ~ScopeGuardForNewException() noexcept(executeOnException) {
     if (executeOnException == exceptionCounter_.isNewUncaughtException()) {
-      function_();
+      if (executeOnException) {
+        ScopeGuardImplBase::runAndWarnAboutToCrashOnException(function_);
+      } else {
+        function_();
+      }
     }
   }