Handle AudioBuffer buffers that were transferred

AudioBufferSourceHandler::RenderFromBuffer wasn't handling the case
where the source buffer had buffers that were transferred away.  Check
to see if the buffer was transferred and then just fill the
destination with zero.

Bug: 1132574
Change-Id: I92abd74f20918cb53810abf9ad00c9c4f4582baa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439361Reviewed-by: Hongchan Choi <hongchan@chromium.org>
Commit-Queue: Raymond Toy <rtoy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813388}

third_party/blink/renderer/modules/webaudio/audio_buffer_source_node.cc

@@ -296,10 +296,22 @@ bool AudioBufferSourceHandler::RenderFromBuffer(
       DCHECK_LE(write_index + frames_this_time, destination_length);
       DCHECK_LE(read_index + frames_this_time, buffer_length);
 
-      for (unsigned i = 0; i < number_of_channels; ++i)
-        memcpy(destination_channels[i] + write_index,
-               source_channels[i] + read_index,
-               sizeof(float) * frames_this_time);
+      for (unsigned i = 0; i < number_of_channels; ++i) {
+        DCHECK(destination_channels[i]);
+
+        // Note: the buffer corresponding to source_channels[i] could have been
+        // transferred so need to check for that.  If it was transferred,
+        // source_channels[i] is null.
+        if (source_channels[i]) {
+          memcpy(destination_channels[i] + write_index,
+                 source_channels[i] + read_index,
+                 sizeof(float) * frames_this_time);
+        } else {
+          // Recall that a floating-point zero is represented by 4 bytes of 0.
+          memset(destination_channels[i] + write_index, 0,
+                 sizeof(float) * frames_this_time);
+        }
+      }
 
       write_index += frames_this_time;
       read_index += frames_this_time;
@@ -348,19 +360,25 @@ bool AudioBufferSourceHandler::RenderFromBuffer(
         const float* source = source_channels[i];
         double sample;
 
-        if (read_index == read_index2 && read_index >= 1) {
-          // We're at the end of the buffer, so just linearly extrapolate from
-          // the last two samples.
-          double sample1 = source[read_index - 1];
-          double sample2 = source[read_index];
-          sample = sample2 + (sample2 - sample1) * interpolation_factor;
+        // The source channel may have been transferred so don't try to read
+        // from it if it was.  Just set the destination to 0.
+        if (source) {
+          if (read_index == read_index2 && read_index >= 1) {
+            // We're at the end of the buffer, so just linearly extrapolate from
+            // the last two samples.
+            double sample1 = source[read_index - 1];
+            double sample2 = source[read_index];
+            sample = sample2 + (sample2 - sample1) * interpolation_factor;
+          } else {
+            double sample1 = source[read_index];
+            double sample2 = source[read_index2];
+            sample = (1.0 - interpolation_factor) * sample1 +
+                     interpolation_factor * sample2;
+          }
+          destination[write_index] = clampTo<float>(sample);
         } else {
-          double sample1 = source[read_index];
-          double sample2 = source[read_index2];
-          sample = (1.0 - interpolation_factor) * sample1 +
-                   interpolation_factor * sample2;
+          destination[write_index] = 0;
         }
-        destination[write_index] = clampTo<float>(sample);
       }
       write_index++;