From 8dca0247d20cecb7e88d075026a07e6427d2cab6 Mon Sep 17 00:00:00 2001
From: Pascal Massimino <pascal.massimino@gmail.com>
Date: Fri, 1 Apr 2016 00:59:39 -0700
Subject: [PATCH] fix bug in alpha.c that was triggering a memory error in
 incremental mode

this change will be superseded by patch #335160 eventually, but until then
let's fix the problem temporarily.

Change-Id: Iafd979c2ff6801e3f1de4614870ca854a4747b04
---
 src/dec/alpha.c | 47 ++++++++++++++++++++++++-----------------------
 1 file changed, 24 insertions(+), 23 deletions(-)

diff --git a/src/dec/alpha.c b/src/dec/alpha.c
index 7f90ddca..47c70395 100644
--- a/src/dec/alpha.c
+++ b/src/dec/alpha.c
@@ -131,30 +131,31 @@ const uint8_t* VP8DecompressAlphaRows(VP8Decoder* const dec,
     return NULL;    // sanity check.
   }
 
-  if (dec->alph_dec_ == NULL) {    // Initialize decoder.
-    assert(dec->alpha_plane_ != NULL);
-    dec->alph_dec_ = ALPHNew();
-    if (dec->alph_dec_ == NULL) return NULL;
-    if (!ALPHInit(dec->alph_dec_, dec->alpha_data_, dec->alpha_data_size_,
-                  io->width, io->height, dec->alpha_plane_)) {
-      ALPHDelete(dec->alph_dec_);
-      dec->alph_dec_ = NULL;
-      return NULL;
-    }
-    // if we allowed use of alpha dithering, check whether it's needed at all
-    if (dec->alph_dec_->pre_processing_ != ALPHA_PREPROCESSED_LEVELS) {
-      dec->alpha_dithering_ = 0;  // disable dithering
-    } else {
-      num_rows = height;          // decode everything in one pass
-    }
-  }
-
-  if (row + num_rows > height) {
-    num_rows = height - row;
-  }
-
   if (!dec->is_alpha_decoded_) {
-    int ok = ALPHDecode(dec, row, num_rows);
+    int ok;
+    if (dec->alph_dec_ == NULL) {    // Initialize decoder.
+      assert(dec->alpha_plane_ != NULL);
+      dec->alph_dec_ = ALPHNew();
+      if (dec->alph_dec_ == NULL) return NULL;
+      if (!ALPHInit(dec->alph_dec_, dec->alpha_data_, dec->alpha_data_size_,
+                    io->width, io->height, dec->alpha_plane_)) {
+        ALPHDelete(dec->alph_dec_);
+        dec->alph_dec_ = NULL;
+        return NULL;
+      }
+      // if we allowed use of alpha dithering, check whether it's needed at all
+      if (dec->alph_dec_->pre_processing_ != ALPHA_PREPROCESSED_LEVELS) {
+        dec->alpha_dithering_ = 0;  // disable dithering
+      } else {
+        num_rows = height;          // decode everything in one pass
+      }
+    }
+
+    if (row + num_rows > height) {
+      num_rows = height - row;
+    }
+
+    ok = ALPHDecode(dec, row, num_rows);
     assert(dec->alph_dec_ != NULL);
     if (!ok || dec->is_alpha_decoded_) {
       ALPHDelete(dec->alph_dec_);