Add diagnostic printout to RTC_DCHECK_RUN_ON.

When using a SequenceChecker, this adds a bit more information
about why the check failed.

Example (The "Expects" line is new):

# Fatal error in: foo.cc, line 380
# last system error: 0
# Check failed: (&thread_checker_)->IsCurrent()
# Expects: System queue: 0x7fff69541330, TaskQueue: 0x101804370 (not current), Thread: 0x10053cdc0

Bug: none
Change-Id: I3743e1d80f369f15219de5946e9e081f998b9b17
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/176569
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31466}

rtc_base/synchronization/BUILD.gn

@@ -72,6 +72,7 @@ rtc_library("sequence_checker") {
     "..:criticalsection",
     "..:macromagic",
     "..:platform_thread_types",
+    "..:stringutils",
     "../../api/task_queue",
     "../system:rtc_export",
   ]

rtc_base/synchronization/sequence_checker.cc

@@ -9,10 +9,16 @@
  */
 #include "rtc_base/synchronization/sequence_checker.h"
 
+#include <cinttypes>
+#include <cstdint>
+#include <type_traits>
+
 #if defined(WEBRTC_MAC)
 #include <dispatch/dispatch.h>
 #endif
 
+#include "rtc_base/strings/string_builder.h"
+
 namespace webrtc {
 namespace {
 // On Mac, returns the label of the current dispatch queue; elsewhere, return
@@ -24,8 +30,28 @@ const void* GetSystemQueueRef() {
   return nullptr;
 #endif
 }
+
+template <class T,
+          typename std::enable_if<std::is_pointer<T>::value>::type* = nullptr>
+uintptr_t CastToUintPtr(T t) {
+  return reinterpret_cast<uintptr_t>(t);
+}
+
+template <class T,
+          typename std::enable_if<!std::is_pointer<T>::value>::type* = nullptr>
+uintptr_t CastToUintPtr(T t) {
+  return static_cast<uintptr_t>(t);
+}
+
 }  // namespace
 
+std::string ExpectationToString(const webrtc::SequenceChecker* checker) {
+#if RTC_DCHECK_IS_ON
+  return checker->ExpectationToString();
+#endif
+  return std::string();
+}
+
 SequenceCheckerImpl::SequenceCheckerImpl()
     : attached_(true),
       valid_thread_(rtc::CurrentThreadRef()),
@@ -62,4 +88,46 @@ void SequenceCheckerImpl::Detach() {
   // reset on the next call to IsCurrent().
 }
 
+#if RTC_DCHECK_IS_ON
+std::string SequenceCheckerImpl::ExpectationToString() const {
+  const TaskQueueBase* const current_queue = TaskQueueBase::Current();
+  const rtc::PlatformThreadRef current_thread = rtc::CurrentThreadRef();
+  const void* const current_system_queue = GetSystemQueueRef();
+  rtc::CritScope scoped_lock(&lock_);
+  if (!attached_)
+    return "Checker currently not attached.";
+
+  // NOTE: The format of thie string built here is meant to compliment the one
+  // we have inside of FatalLog() (checks.cc).
+  //
+  // Example:
+  //
+  // Expectations vs Actual:
+  // # Exp: TQ: 0000000000000000 SysQ: 00007fff69541330 Thread: 0000000113aafdc0
+  // # Act: TQ: 00007fcde7a22210 SysQ: 00007fcde78553c0 Thread: 0000700005ddc000
+  // TaskQueue doesn't match
+
+  rtc::StringBuilder message;
+  message.AppendFormat(
+      "Expectations vs Actual:\n# Exp: "
+      "TQ: %016" PRIxPTR " SysQ: %016" PRIxPTR " Thread: %016" PRIxPTR
+      "\n# Act: "
+      "TQ: %016" PRIxPTR " SysQ: %016" PRIxPTR " Thread: %016" PRIxPTR "\n",
+      CastToUintPtr(valid_queue_), CastToUintPtr(valid_system_queue_),
+      CastToUintPtr(valid_thread_), CastToUintPtr(current_queue),
+      CastToUintPtr(current_system_queue), CastToUintPtr(current_thread));
+
+  if ((valid_queue_ || current_queue) && valid_queue_ != current_queue) {
+    message << "TaskQueue doesn't match\n";
+  } else if (valid_system_queue_ &&
+             valid_system_queue_ != current_system_queue) {
+    message << "System queue doesn't match\n";
+  } else if (!rtc::IsThreadRefEqual(valid_thread_, current_thread)) {
+    message << "Threads don't match\n";
+  }
+
+  return message.Release();
+}
+#endif  // RTC_DCHECK_IS_ON
+
 }  // namespace webrtc

rtc_base/synchronization/sequence_checker.h

@@ -10,6 +10,8 @@
 #ifndef RTC_BASE_SYNCHRONIZATION_SEQUENCE_CHECKER_H_
 #define RTC_BASE_SYNCHRONIZATION_SEQUENCE_CHECKER_H_
 
+#include <type_traits>
+
 #include "api/task_queue/task_queue_base.h"
 #include "rtc_base/critical_section.h"
 #include "rtc_base/platform_thread_types.h"
@@ -34,6 +36,11 @@ class RTC_EXPORT SequenceCheckerImpl {
   // used exclusively on another thread.
   void Detach();
 
+  // Returns a string that is formatted to match with the error string printed
+  // by RTC_CHECK() when a condition is not met.
+  // This is used in conjunction with the RTC_DCHECK_RUN_ON() macro.
+  std::string ExpectationToString() const;
+
  private:
   rtc::CriticalSection lock_;
   // These are mutable so that IsCurrent can set them.
@@ -162,8 +169,19 @@ class RTC_SCOPED_LOCKABLE SequenceCheckerScope {
 #define RTC_RUN_ON(x) \
   RTC_THREAD_ANNOTATION_ATTRIBUTE__(exclusive_locks_required(x))
 
+namespace webrtc {
+std::string ExpectationToString(const webrtc::SequenceChecker* checker);
+
+// Catch-all implementation for types other than explicitly supported above.
+template <typename ThreadLikeObject>
+std::string ExpectationToString(const ThreadLikeObject*) {
+  return std::string();
+}
+
+}  // namespace webrtc
+
 #define RTC_DCHECK_RUN_ON(x)                                              \
   webrtc::webrtc_seq_check_impl::SequenceCheckerScope seq_check_scope(x); \
-  RTC_DCHECK((x)->IsCurrent())
+  RTC_DCHECK((x)->IsCurrent()) << webrtc::ExpectationToString(x)
 
 #endif  // RTC_BASE_SYNCHRONIZATION_SEQUENCE_CHECKER_H_

rtc_base/synchronization/sequence_checker_unittest.cc

@@ -31,7 +31,7 @@ class CompileTimeTestForGuardedBy {
   int CalledOnSequence() RTC_RUN_ON(sequence_checker_) { return guarded_; }
 
   void CallMeFromSequence() {
-    RTC_DCHECK_RUN_ON(&sequence_checker_) << "Should be called on sequence";
+    RTC_DCHECK_RUN_ON(&sequence_checker_);
     guarded_ = 41;
   }