Check that WithContext is constructed and destructed on the same thread. (#206)

This only happens in debug mode (!NDEBUG).
Add a death test for it.

Author: g-easy

opencensus/context/internal/with_context.cc

@@ -14,24 +14,42 @@
 
 #include "opencensus/context/with_context.h"
 
+#include <cassert>
 #include <utility>
 
 #include "opencensus/context/context.h"
 
 namespace opencensus {
 namespace context {
 
-WithContext::WithContext(const Context& ctx) : swapped_context_(ctx) {
+WithContext::WithContext(const Context& ctx)
+    : swapped_context_(ctx)
+#ifndef NDEBUG
+      ,
+      original_context_(Context::InternalMutableCurrent())
+#endif
+{
   using std::swap;
   swap(*Context::InternalMutableCurrent(), swapped_context_);
 }
 
-WithContext::WithContext(Context&& ctx) : swapped_context_(std::move(ctx)) {
+WithContext::WithContext(Context&& ctx)
+    : swapped_context_(std::move(ctx))
+#ifndef NDEBUG
+      ,
+      original_context_(Context::InternalMutableCurrent())
+#endif
+{
   using std::swap;
   swap(*Context::InternalMutableCurrent(), swapped_context_);
 }
 
 WithContext::~WithContext() {
+#ifndef NDEBUG
+  assert(original_context_ == Context::InternalMutableCurrent() &&
+         "WithContext must be destructed on the same thread as it was "
+         "constructed.");
+#endif
   using std::swap;
   swap(*Context::InternalMutableCurrent(), swapped_context_);
 }

opencensus/context/internal/with_context_test.cc

@@ -15,6 +15,7 @@
 #include "opencensus/context/with_context.h"
 
 #include <iostream>
+#include <thread>
 
 #include "gtest/gtest.h"
 #include "opencensus/context/context.h"
@@ -34,4 +35,21 @@ TEST(WithContextTest, WithContextMovable) {
   opencensus::context::WithContext wc(std::move(ctx));
 }
 
+#ifndef NDEBUG
+TEST(WithContextDeathTest, DestructorOnWrongThread) {
+  opencensus::context::Context ctx = opencensus::context::Context::Current();
+  EXPECT_DEATH_IF_SUPPORTED(
+      {
+        auto* wc = new opencensus::context::WithContext(ctx);
+        std::thread t([wc]() {
+          // Running the destructor in a different thread corrupts its
+          // thread-local Context. In debug mode, it assert()s.
+          delete wc;
+        });
+        t.join();
+      },
+      "must be destructed on the same thread");
+}
+#endif
+
 }  // namespace

opencensus/context/with_context.h

@@ -40,6 +40,9 @@ class WithContext {
   WithContext& operator=(WithContext&&) = delete;
 
   Context swapped_context_;
+#ifndef NDEBUG
+  const Context* original_context_;
+#endif
 };
 
 }  // namespace context