Add the machinery to GrDrawTarget to enable topological sorting

src/gpu/GrContext.cpp

Index: src/gpu/GrContext.cpp
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index dfbe2fc46ad1ec1028f3f14642c1629f0a42c9bd..401159ccb7825fb7f11048ea5cb81375e49bb297 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -45,7 +45,7 @@
 #include "SkTLazy.h"
 #include "SkTLS.h"
 #include "SkTraceEvent.h"
-
+#include "SkTTopoSort.h"
 
 #include "batches/GrBatch.h"
 
@@ -95,9 +95,45 @@ void GrDrawingManager::reset() {
 }
 
 void GrDrawingManager::flush() {
+
+    struct TopoSortTraits {
+        static void Output(GrDrawTarget* dt, int /* index */) {
+            dt->setFlag(GrDrawTarget::kWasOutput_Flag);
+        }
+        static bool WasOutput(const GrDrawTarget* dt) {
+            return dt->isSetFlag(GrDrawTarget::kWasOutput_Flag);
+        }
+        static void SetTempMark(GrDrawTarget* dt) {
+            dt->setFlag(GrDrawTarget::kTempMark_Flag);
+        }
+        static void ResetTempMark(GrDrawTarget* dt) {
+            dt->resetFlag(GrDrawTarget::kTempMark_Flag);
+        }
+        static bool IsTempMarked(const GrDrawTarget* dt) {
+            return dt->isSetFlag(GrDrawTarget::kTempMark_Flag);
+        }
+        static int NumDependencies(const GrDrawTarget* dt) {
+            return dt->numDependencies();
+        }
+        static GrDrawTarget* Dependency(GrDrawTarget* dt, int index) {
+            return dt->dependency(index);
+        }
+    };
+
+    SkDEBUGCODE(bool result =) SkTTopoSort<GrDrawTarget, TopoSortTraits>(&fDrawTargets);
+    SkASSERT(result);
+
     for (int i = 0; i < fDrawTargets.count(); ++i) {
         fDrawTargets[i]->flush();
     }
+
+#ifndef ENABLE_MDB
+    // When MDB is disabled we keep reusing the same drawTarget
+    if (fDrawTargets.count()) {
+        SkASSERT(fDrawTargets.count() == 1);
+        fDrawTargets[0]->resetFlag(GrDrawTarget::kWasOutput_Flag);
+    }
+#endif
 }
 
 GrTextContext* GrDrawingManager::textContext(const SkSurfaceProps& props,
@@ -131,8 +167,8 @@ GrTextContext* GrDrawingManager::textContext(const SkSurfaceProps& props,
 GrDrawTarget* GrDrawingManager::newDrawTarget(GrRenderTarget* rt) {
     SkASSERT(fContext);
 
-    // When MDB is disabled we always just return the single drawTarget
 #ifndef ENABLE_MDB
+    // When MDB is disabled we always just return the single drawTarget
     if (fDrawTargets.count()) {
         SkASSERT(fDrawTargets.count() == 1);
         // DrawingManager gets the creation ref - this ref is for the caller