Fix decoding incomplete PNG images

If process_data is unable to read (and therefore process) as many bytes
as it expects, process the bytes read before returning false.

Fixes differences in Gold.

Add a test that verifies that it is okay to call png_process_data with
0 bytes. (We could special case 0, but libpng already checks for 0.)

Change-Id: Id500b9305ee3bb6a1a7e8fc70d4e723cb4742b55
Reviewed-on: https://skia-review.googlesource.com/14144
Commit-Queue: Leon Scroggins <scroggo@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/src/codec/SkPngCodec.cpp b/src/codec/SkPngCodec.cpp
index 6b6ac99f57..1f69159ee6 100644
--- a/src/codec/SkPngCodec.cpp
+++ b/src/codec/SkPngCodec.cpp
@@ -132,10 +132,11 @@ static inline bool process_data(png_structp png_ptr, png_infop info_ptr,
         SkStream* stream, void* buffer, size_t bufferSize, size_t length) {
     while (length > 0) {
         const size_t bytesToProcess = std::min(bufferSize, length);
-        if (stream->read(buffer, bytesToProcess) < bytesToProcess) {
+        const size_t bytesRead = stream->read(buffer, bytesToProcess);
+        png_process_data(png_ptr, info_ptr, (png_bytep) buffer, bytesRead);
+        if (bytesRead < bytesToProcess) {
             return false;
         }
-        png_process_data(png_ptr, info_ptr, (png_bytep) buffer, bytesToProcess);
         length -= bytesToProcess;
     }
     return true;