Add debug mode to shared mutex.

src/core/SkSharedMutex.h

Index: src/core/SkSharedMutex.h
diff --git a/src/core/SkSharedMutex.h b/src/core/SkSharedMutex.h
index f3430040e38b51ee05ddb9d2915db44a30ff6ec3..9660adf0eb4084093801867033838db045cc40d3 100644
--- a/src/core/SkSharedMutex.h
+++ b/src/core/SkSharedMutex.h
@@ -11,9 +11,16 @@
 #include "SkAtomics.h"
 #include "SkSemaphore.h"
 #include "SkTypes.h"
-    
-// This is a shared lock implementation similar to pthreads rwlocks. This implementation is
-// cribbed from Preshing's article:
+
+#ifdef SK_DEBUG
+    #include "SkMutex.h"
+    #include "../private/SkUniquePtr.h"
+#endif  // SK_DEBUG
+
+// There are two shared lock implementations one debug the other is high performance. They implement
+// an interface similar to pthread's rwlocks.
+// This is a shared lock implementation similar to pthreads rwlocks. The high performance
+// implementation is cribbed from Preshing's article:
 // http://preshing.com/20150316/semaphores-are-surprisingly-versatile/
 //
 // This lock does not obey strict queue ordering. It will always alternate between readers and
@@ -29,11 +36,7 @@ public:
     void release();
 
     // Fail if exclusive is not held.
-#ifdef SK_DEBUG
     void assertHeld() const;
-#else
-    void assertHeld() const {}
-#endif
 
     // Acquire lock for shared use.
     void acquireShared();
@@ -42,17 +45,28 @@ public:
     void releaseShared();
 
     // Fail if shared lock not held.
-#ifdef SK_DEBUG
     void assertHeldShared() const;
-#else
-    void assertHeldShared() const {}
-#endif
 
 private:
-    SkAtomic<int32_t> fQueueCounts;
-    SkSemaphore fSharedQueue;
+#ifdef SK_DEBUG
+    class ThreadIDSet;
+    skstd::unique_ptr<ThreadIDSet> fCurrentShared;
+    skstd::unique_ptr<ThreadIDSet> fWaitingExclusive;
+    skstd::unique_ptr<ThreadIDSet> fWaitingShared;
+    int fSharedQueueSelect{0};
+    mutable SkMutex fMu;
+    SkSemaphore fSharedQueue[2];
     SkSemaphore fExclusiveQueue;
+#else
+    SkAtomic<int32_t> fQueueCounts;
+    SkSemaphore       fSharedQueue;
+    SkSemaphore       fExclusiveQueue;
+#endif  // SK_DEBUG
 };
 
+#ifndef SK_DEBUG
+inline void SkSharedMutex::assertHeld() const {};
+inline void SkSharedMutex::assertHeldShared() const {};
+#endif  // SK_DEBUG
 
 #endif // SkSharedLock_DEFINED