Fix a bug in simple motion search

This change fixed a bug revealed by b/311294795.
In simple motion search, the reference buffer pointer needs to be
restored after the search. Otherwise, it causes problems while the
reference frame scaling happens. This CL fixes the bug.

Bug: b/311294795
Change-Id: I093722d5888de3cc6a6542de82a6ec9d601f897d
(cherry picked from commit 50ed636)

test/encode_api_test.cc

@@ -863,6 +863,37 @@ TEST(EncodeAPI, Buganizer312875957PredBufferStride) {
   encoder.Encode(false);
 }
 
+// This is a test case from clusterfuzz: based on b/311294795
+// Encode a few frames with multiple change config calls
+// with different frame sizes.
+TEST(EncodeAPI, Buganizer311294795) {
+  VP9Encoder encoder(1);
+
+  // Set initial config.
+  encoder.Configure(12, 1678, 620, VPX_VBR, VPX_DL_REALTIME);
+
+  // Encode first frame.
+  encoder.Encode(false);
+
+  // Change config.
+  encoder.Configure(16, 632, 620, VPX_VBR, VPX_DL_GOOD_QUALITY);
+
+  // Encode 2nd frame with new config
+  encoder.Encode(true);
+
+  // Change config.
+  encoder.Configure(16, 1678, 342, VPX_VBR, VPX_DL_GOOD_QUALITY);
+
+  // Encode 3rd frame with new config.
+  encoder.Encode(false);
+
+  // Change config.
+  encoder.Configure(0, 1574, 618, VPX_VBR, VPX_DL_REALTIME);
+  // Encode more frames with new config.
+  encoder.Encode(false);
+  encoder.Encode(false);
+}
+
 class EncodeApiGetTplStatsTest
     : public ::libvpx_test::EncoderTest,
       public ::testing::TestWithParam<const libvpx_test::CodecFactory *> {

vp9/encoder/vp9_encodeframe.c

@@ -3443,11 +3443,17 @@ static void simple_motion_search(const VP9_COMP *const cpi, MACROBLOCK *const x,
   MV ref_mv_full = { ref_mv.row >> 3, ref_mv.col >> 3 };
   MV best_mv = { 0, 0 };
   int cost_list[5];
+  struct buf_2d backup_pre[MAX_MB_PLANE] = { { 0, 0 } };
 
-  if (scaled_ref_frame)
+  if (scaled_ref_frame) {
     yv12 = scaled_ref_frame;
-  else
+    // As reported in b/311294795, the reference buffer pointer needs to be
+    // saved and restored after the search. Otherwise, it causes problems while
+    // the reference frame scaling happens.
+    for (int i = 0; i < MAX_MB_PLANE; i++) backup_pre[i] = xd->plane[i].pre[0];
+  } else {
     yv12 = get_ref_frame_buffer(cpi, ref);
+  }
 
   assert(yv12 != NULL);
   if (!yv12) return;
@@ -3464,6 +3470,11 @@ static void simple_motion_search(const VP9_COMP *const cpi, MACROBLOCK *const x,
   x->mv_limits = tmp_mv_limits;
   mi->mv[0].as_mv = best_mv;
 
+  // Restore reference buffer pointer.
+  if (scaled_ref_frame) {
+    for (int i = 0; i < MAX_MB_PLANE; i++) xd->plane[i].pre[0] = backup_pre[i];
+  }
+
   set_ref_ptrs(cm, xd, mi->ref_frame[0], mi->ref_frame[1]);
   xd->plane[0].dst.buf = pred_buf;
   xd->plane[0].dst.stride = 64;