external/webp: cherry-pick alpha decoding fix

this is fairly rare and tends to happen with uncommon encoder settings.
original commit message:

bugfix: last alpha rows were incorrectly decoded

sometimes, the last rows of the alpha plane contain more than NUM_ARGB_CACHE_ROWS
rows to process. But ExtractAlphaRows() was repeatedly calling ApplyInverseTransforms()
without updating the dec->last_row_ field, which is the starting row used as starting
point.

Fix would consist of either updating correctly dec->last_row_ before calling
ApplyInverseTransforms(). Or pass the starting row explicitly, which is simpler.

BUG=crbug.com/webp/439
Test: aosp_arm-eng aosp_arm64-eng aosp_x86-eng aosp_x86_64-eng aosp_crosshatch-userdebug build

Change-Id: I02a4c701c94e2e85d7728da41ffaa5b34d1a1b26

2 files changed, 6 insertions, 5 deletions

diff --git a/README.android b/README.android
index 0aaa2210..9c49f0f4 100644
--- a/README.android
+++ b/README.android
@@ -8,6 +8,7 @@ Local modifications:
   (e.g. bits.h) to leak into
 - Removed build files necessary for building via autoconf/automake tools
   These files are not required to build via Android.bp
+- cherry-pick 0e48d889 bugfix: last alpha rows were incorrectly decoded
 
 The Android.bp file creates WebP decoder and encoder static libraries which
 can be added to any application by adding libwebp-decode and libwebp-encode to
diff --git a/src/dec/vp8l_dec.c b/src/dec/vp8l_dec.c
index d3e27119..93615d4e 100644
--- a/src/dec/vp8l_dec.c
+++ b/src/dec/vp8l_dec.c
@@ -754,11 +754,11 @@ static WEBP_INLINE HTreeGroup* GetHtreeGroupForPos(VP8LMetadata* const hdr,
 
 typedef void (*ProcessRowsFunc)(VP8LDecoder* const dec, int row);
 
-static void ApplyInverseTransforms(VP8LDecoder* const dec, int num_rows,
+static void ApplyInverseTransforms(VP8LDecoder* const dec,
+                                   int start_row, int num_rows,
                                    const uint32_t* const rows) {
   int n = dec->next_transform_;
   const int cache_pixs = dec->width_ * num_rows;
-  const int start_row = dec->last_row_;
   const int end_row = start_row + num_rows;
   const uint32_t* rows_in = rows;
   uint32_t* const rows_out = dec->argb_cache_;
@@ -789,8 +789,7 @@ static void ProcessRows(VP8LDecoder* const dec, int row) {
     VP8Io* const io = dec->io_;
     uint8_t* rows_data = (uint8_t*)dec->argb_cache_;
     const int in_stride = io->width * sizeof(uint32_t);  // in unit of RGBA
-
-    ApplyInverseTransforms(dec, num_rows, rows);
+    ApplyInverseTransforms(dec, dec->last_row_, num_rows, rows);
     if (!SetCropWindow(io, dec->last_row_, row, &rows_data, in_stride)) {
       // Nothing to output (this time).
     } else {
@@ -1193,6 +1192,7 @@ static int DecodeImageData(VP8LDecoder* const dec, uint32_t* const data,
       VP8LFillBitWindow(br);
       dist_code = GetCopyDistance(dist_symbol, br);
       dist = PlaneCodeToDistance(width, dist_code);
+
       if (VP8LIsEndOfStream(br)) break;
       if (src - data < (ptrdiff_t)dist || src_end - src < (ptrdiff_t)length) {
         goto Error;
@@ -1553,7 +1553,7 @@ static void ExtractAlphaRows(VP8LDecoder* const dec, int last_row) {
     const int cache_pixs = width * num_rows_to_process;
     uint8_t* const dst = output + width * cur_row;
     const uint32_t* const src = dec->argb_cache_;
-    ApplyInverseTransforms(dec, num_rows_to_process, in);
+    ApplyInverseTransforms(dec, cur_row, num_rows_to_process, in);
     WebPExtractGreen(src, dst, cache_pixs);
     AlphaApplyFilter(alph_dec,
                      cur_row, cur_row + num_rows_to_process, dst, width);