Add asserts for shared mutex.

src/core/SkSharedMutex.cpp

Index: src/core/SkSharedMutex.cpp
diff --git a/src/core/SkSharedMutex.cpp b/src/core/SkSharedMutex.cpp
index b9af10a2beb368e9564159bc2dfe1a1b4f4d163e..4cf63120676e658f43b1b354c22d27b08eb7d549 100644
--- a/src/core/SkSharedMutex.cpp
+++ b/src/core/SkSharedMutex.cpp
@@ -141,6 +141,23 @@ void SkSharedMutex::release() {
     }
 }
 
+#ifdef SK_DEBUG
+void SkSharedMutex::assertHeld() const {
+    int32_t queueCounts = fQueueCounts.load(sk_memory_order_relaxed);
+    // These are very loose asserts about the mutex being held exclusively.
+    SkASSERTF(0 == (queueCounts & kSharedMask),
+              "running shared: %d, exclusive: %d, waiting shared: %d",
+              (queueCounts & kSharedMask) >> kSharedOffset,
+              (queueCounts & kWaitingExclusiveMask) >> kWaitingExlusiveOffset,
+              (queueCounts & kWaitingSharedMask) >> kWaitingSharedOffset);
+    SkASSERTF((queueCounts & kWaitingExclusiveMask) > 0,
+              "running shared: %d, exclusive: %d, waiting shared: %d",
+              (queueCounts & kSharedMask) >> kSharedOffset,
+              (queueCounts & kWaitingExclusiveMask) >> kWaitingExlusiveOffset,
+              (queueCounts & kWaitingSharedMask) >> kWaitingSharedOffset);
+}
+#endif
+
 void SkSharedMutex::acquireShared() {
     int32_t oldQueueCounts = fQueueCounts.load(sk_memory_order_relaxed);
     int32_t newQueueCounts;
@@ -177,3 +194,16 @@ void SkSharedMutex::releaseShared() {
         fExclusiveQueue.signal();
     }
 }
+
+#ifdef SK_DEBUG
+void SkSharedMutex::assertHeldShared() const {
+    int32_t queueCounts = fQueueCounts.load(sk_memory_order_relaxed);
+    // A very loose assert about the mutex being shared.
+    SkASSERTF((queueCounts & kSharedMask) > 0,
+              "running shared: %d, exclusive: %d, waiting shared: %d",
+              (queueCounts & kSharedMask) >> kSharedOffset,
+              (queueCounts & kWaitingExclusiveMask) >> kWaitingExlusiveOffset,
+              (queueCounts & kWaitingSharedMask) >> kWaitingSharedOffset);
+}
+
+#endif