Make GrGLShaderBuilder check whether GrEffect advertised that it would require the dst color or frag

src/gpu/gl/GrGLShaderBuilder.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gl/GrGLShaderBuilder.h"
 #include "gl/GrGLProgram.h"
 #include "gl/GrGLUniformHandle.h"
@@ skipping 212 matching lines @@
     if (fCodeStage.inStageCode()) {
         if (out->endsWith('_')) {
             // Names containing "__" are reserved.
             out->append("x");
         }
         out->appendf("_Stage%d", fCodeStage.stageIndex());
     }
 }
 
 const char* GrGLShaderBuilder::dstColor() {
+    if (fCodeStage.inStageCode()) {
+        const GrEffectRef& effect = *fCodeStage.effectStage()->getEffect();
+        if (!effect->willReadDstColor()) {
+            GrDebugCrash("GrGLEffect asked for dst color but its generating GrEffect "
+                         "did not request access.");
+            return "";
+        }
+    }
     static const char kFBFetchColorName[] = "gl_LastFragData[0]";
     GrGLCaps::FBFetchType fetchType = fCtxInfo.caps()->fbFetchType();
     if (GrGLCaps::kEXT_FBFetchType == fetchType) {
         SkAssertResult(this->enablePrivateFeature(kEXTShaderFramebufferFetch_GLSLPrivateFeature));
         return kFBFetchColorName;
     } else if (GrGLCaps::kNV_FBFetchType == fetchType) {
         SkAssertResult(this->enablePrivateFeature(kNVShaderFramebufferFetch_GLSLPrivateFeature));
         return kFBFetchColorName;
     } else if (fDstCopySampler.isInitialized()) {
         return kDstCopyColorName;
     } else {
-        return NULL;
+        return "";
     }
 }
 
 void GrGLShaderBuilder::codeAppendf(ShaderType type, const char format[], va_list args) {
     SkString* string = NULL;
     switch (type) {
         case kVertex_ShaderType:
             string = &fVSCode;
             break;
         case kGeometry_ShaderType:
@@ skipping 195 matching lines @@
     fFSInputs.push_back();
     fFSInputs.back().setType(type);
     fFSInputs.back().setTypeModifier(GrGLShaderVar::kVaryingIn_TypeModifier);
     fFSInputs.back().setName(*fsName);
     if (fsInName) {
         *fsInName = fsName->c_str();
     }
 }
 
 const char* GrGLShaderBuilder::fragmentPosition() {
+    if (fCodeStage.inStageCode()) {
+        const GrEffectRef& effect = *fCodeStage.effectStage()->getEffect();
+        if (!effect->willReadFragmentPosition()) {
+            GrDebugCrash("GrGLEffect asked for frag position but its generating GrEffect "
+                         "did not request access.");
+            return "";
+        }
+    }
 #if 1
     if (fCtxInfo.caps()->fragCoordConventionsSupport()) {
         if (!fSetupFragPosition) {
             SkAssertResult(this->enablePrivateFeature(kFragCoordConventions_GLSLPrivateFeature));
             fFSInputs.push_back().set(kVec4f_GrSLType,
                                       GrGLShaderVar::kIn_TypeModifier,
                                       "gl_FragCoord",
                                       GrGLShaderVar::kDefault_Precision,
                                       GrGLShaderVar::kUpperLeft_Origin);
             fSetupFragPosition = true;
@@ skipping 236 matching lines @@
     for (const AttributePair* attrib = this->getEffectAttributes().begin();
          attrib != attribEnd;
          ++attrib) {
         if (attrib->fIndex == attributeIndex) {
             return &attrib->fName;
         }
     }
 
     return NULL;
 }