DM: catch -r foo -w foo.

dm/DMWriteTask.cpp

 #include "DMWriteTask.h"
 
 #include "DMUtil.h"
 #include "SkColorPriv.h"
 #include "SkCommandLineFlags.h"
 #include "SkImageDecoder.h"
 #include "SkImageEncoder.h"
 #include "SkString.h"
 #include "SkUnPreMultiply.h"
 
@@ skipping 73 matching lines @@
     // Remove the suffix and tack on a .png.
     filename.remove(filename.size() - suffixLength, suffixLength);
     filename.append(".png");
 
     //SkDebugf("dir %s, filename %s\n", dir.c_str(), filename.c_str());
 
     return SkOSPath::SkPathJoin(dir.c_str(), filename.c_str());
 }
 
 bool WriteTask::Expectations::check(const Task& task, SkBitmap bitmap) const {
+    if (!FLAGS_writePath.isEmpty() && 0 == strcmp(FLAGS_writePath[0], fRoot)) {
+        SkDebugf("We seem to be reading and writing %s concurrently.  This won't work.\n", fRoot);
+        return false;
+    }
+
     // PNG is stored unpremultiplied, and going from premul to unpremul to premul is lossy.  To
     // skirt this problem, we decode the PNG into an unpremul bitmap, convert our bitmap to unpremul
     // if needed, and compare those.  Each image goes once from premul to unpremul, never back.
     const SkString path = path_to_expected_image(fRoot, task);
 
     SkAutoTUnref<SkStreamRewindable> stream(SkStream::NewFromFile(path.c_str()));
     if (NULL == stream.get()) {
         SkDebugf("Could not read %s.\n", path.c_str());
         return false;
     }
@@ skipping 39 matching lines @@
         for (size_t i = 0; i < bitmap.getSize()/4; i++) {
             dst[i] = SkUnPreMultiply::UnPreMultiplyPreservingByteOrder(src[i]);
         }
         bitmap.swap(unpremul);
     }
 
     return BitmapsEqual(expected, bitmap);
 }
 
 }  // namespace DM