make render_pictures properly handle empty expectations file

tools/image_expectations.h

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef image_expectations_DEFINED
 #define image_expectations_DEFINED
 
 #include "SkBitmap.h"
 #include "SkJSONCPP.h"
+#include "SkOSFile.h"
 #include "SkRefCnt.h"
 
 namespace sk_tools {
 
     /**
      * The digest of an image (either an image we have generated locally, or an image expectation).
      *
      * Currently, this is always a uint64_t hash digest of an SkBitmap.
      */
     class ImageDigest : public SkRefCnt {
@@ skipping 51 matching lines @@
         SkAutoTUnref<ImageDigest> fImageDigestRef;
     };
 
     /**
      * Collects ImageDigests of actually rendered images, perhaps comparing to expectations.
      */
     class ImageResultsAndExpectations {
     public:
         /**
          * Adds expectations from a JSON file, returning true if successful.
+         *
+         * If the file exists but is empty, it succeeds, and there will be no expectations.
+         * If the file does not exist, this will fail.
+         *
+         * Reasoning:
+         * Generating expectations the first time can be a tricky chicken-and-egg
+         * proposition.  "I need actual results to turn into expectations... but the only
+         * way to get actual results is to run the tool, and the tool won't run without
+         * expectations!"
+         * We could make the tool run even if there is no expectations file at all, but it's
+         * better for the tool to fail if the expectations file is not found--that will tell us
+         * quickly if files are not being copied around as they should be.
+         * Creating an empty file is an easy way to break the chicken-and-egg cycle and generate
+         * the first real expectations.
          */
         bool readExpectationsFile(const char *jsonPath);
 
         /**
          * Adds this image to the summary of results.
          *
          * @param sourceName name of the source file that generated this result
          * @param fileName relative path to the image output file on local disk
          * @param digest description of the image's contents
          * @param tileNumber if not NULL, pointer to tile number
@@ skipping 15 matching lines @@
         /**
          * Writes the summary (as constructed so far) to a file.
          *
          * @param filename path to write the summary to
          */
         void writeToFile(const char *filename) const;
 
     private:
 
         /**
-         * Read the file contents from jsonPath and parse them into jsonRoot.
+         * Read the file contents from filePtr and parse them into jsonRoot.
+         *
+         * It is up to the caller to close filePtr after this is done.
          *
          * Returns true if successful.
          */
-        static bool Parse(const char *jsonPath, Json::Value *jsonRoot);
+        static bool Parse(SkFILE* filePtr, Json::Value *jsonRoot);
 
         Json::Value fActualResults;
         Json::Value fExpectedJsonRoot;
         Json::Value fExpectedResults;
     };
 
 } // namespace sk_tools
 
 #endif  // image_expectations_DEFINED