Avoid issue AOMediaCodec/libavif#2886 avifRGBImagePremultiplyAlpha() and avifRGBImageUnpremultiplyAlpha() assume four-channel pixel buffers, so they would overrun the two-channel pixel buffer in an avifRGBImage of the AVIF_RGB_FORMAT_GRAYA or AVIF_RGB_FORMAT_AGRAY format. Return AVIF_RESULT_NOT_IMPLEMENTED before this bug is fixed. The bug was reported by Harrison Green. Avoids https://github.com/AOMediaCodec/libavif/issues/2886.

diff --git a/src/alpha.c b/src/alpha.c
index f73400a..ec4976f 100644
--- a/src/alpha.c
+++ b/src/alpha.c

@@ -165,6 +165,10 @@
         return libyuvResult;
     }
 
+    if (rgb->format == AVIF_RGB_FORMAT_GRAYA || rgb->format == AVIF_RGB_FORMAT_AGRAY) {
+        return AVIF_RESULT_NOT_IMPLEMENTED;
+    }
+
     assert(rgb->depth >= 8 && rgb->depth <= 16);
 
     uint32_t max = (1 << rgb->depth) - 1;
@@ -280,6 +284,10 @@
         return libyuvResult;
     }
 
+    if (rgb->format == AVIF_RGB_FORMAT_GRAYA || rgb->format == AVIF_RGB_FORMAT_AGRAY) {
+        return AVIF_RESULT_NOT_IMPLEMENTED;
+    }
+
     assert(rgb->depth >= 8 && rgb->depth <= 16);
 
     uint32_t max = (1 << rgb->depth) - 1;

diff --git a/tests/gtest/avifalphapremtest.cc b/tests/gtest/avifalphapremtest.cc
index 5c8bf8e..a1dc2fd 100644
--- a/tests/gtest/avifalphapremtest.cc
+++ b/tests/gtest/avifalphapremtest.cc

@@ -1,6 +1,8 @@
 // Copyright 2022 Google LLC
 // SPDX-License-Identifier: BSD-2-Clause
 
+#include <string.h>
+
 #include "avif/avif.h"
 #include "aviftest_helpers.h"
 #include "gtest/gtest.h"
@@ -61,6 +63,22 @@
   }
 }
 
+TEST(AlphaMultiplyTest, GrayAImagePremultiplyAlpha) {
+  avifRGBImage rgb;
+  memset(&rgb, 0, sizeof(rgb));
+  rgb.width = 6;
+  rgb.height = 1;
+  rgb.depth = 10;
+  rgb.format = AVIF_RGB_FORMAT_GRAYA;  // 2 channels with alpha
+  ASSERT_EQ(avifRGBImageAllocatePixels(&rgb), AVIF_RESULT_OK);
+  memset(rgb.pixels, 1, (size_t)rgb.rowBytes * rgb.height);
+  // avifRGBImagePremultiplyAlpha assumes 4 channels and would overrun the
+  // 2-channel buffer. Until this bug (issue #2886) is fixed, it should return
+  // AVIF_RESULT_NOT_IMPLEMENTED.
+  EXPECT_EQ(avifRGBImagePremultiplyAlpha(&rgb), AVIF_RESULT_NOT_IMPLEMENTED);
+  avifRGBImageFreePixels(&rgb);
+}
+
 //------------------------------------------------------------------------------
 
 }  // namespace