Add offset support to BitstreamBuffer.

content/common/gpu/gpu_messages.cc

Index: content/common/gpu/gpu_messages.cc
diff --git a/content/common/gpu/gpu_messages.cc b/content/common/gpu/gpu_messages.cc
index 9c90693c33adeb42fca5cf2ea48896c65fd4bb44..7512a7877195f4749bcee79ed43181272725da2f 100644
--- a/content/common/gpu/gpu_messages.cc
+++ b/content/common/gpu/gpu_messages.cc
@@ -16,6 +16,7 @@ void ParamTraits<media::BitstreamBuffer>::Write(base::Pickle* m,
                                                 const param_type& p) {
   WriteParam(m, p.id());
   WriteParam(m, static_cast<uint64_t>(p.size()));
+  WriteParam(m, static_cast<int64_t>(p.offset()));

[dcheng, 2016/03/01 01:48:43]

Does it make sense to DCHECK() that this is non-negative?

[Owen Lin, 2016/03/02 02:50:52]

I agree with Pawel that
"MapAt() will fail us if the offset is invalid, e.g. if we pass a negative value
for some reason. So I don't think we have to check for <0 explicitly."

[dcheng, 2016/03/02 05:58:20]

It's not in the contract of SharedMemory::MapAt(), and it's not actually obvious
that it will fail. In fact, here's the Windows implementation:

bool SharedMemory::MapAt(off_t offset, size_t bytes) {
  ...
  memory_ = MapViewOfFile(
      mapped_file_, read_only_ ? FILE_MAP_READ : FILE_MAP_READ | FILE_MAP_WRITE,
      static_cast<uint64_t>(offset) >> 32, static_cast<DWORD>(offset), bytes);
  ...
}

Will this fail or succeed with off_t = -1? We cast it to unsigned before we
shift, so we won't get sign extension...

   WriteParam(m, p.presentation_timestamp());
   WriteParam(m, p.key_id());
   if (!p.key_id().empty()) {
@@ -30,7 +31,9 @@ bool ParamTraits<media::BitstreamBuffer>::Read(const base::Pickle* m,
                                                param_type* r) {
   DCHECK(r);
   uint64_t size = 0;
+  int64_t offset = 0;
   if (!(ReadParam(m, iter, &r->id_) && ReadParam(m, iter, &size) &&
+        ReadParam(m, iter, &offset) &&
         ReadParam(m, iter, &r->presentation_timestamp_) &&
         ReadParam(m, iter, &r->key_id_)))
     return false;
@@ -42,6 +45,13 @@ bool ParamTraits<media::BitstreamBuffer>::Read(const base::Pickle* m,
   }
   r->size_ = checked_size.ValueOrDie();
 
+  base::CheckedNumeric<off_t> checked_offset(offset);
+  if (!checked_offset.IsValid()) {

[dcheng, 2016/03/01 01:48:43]

Similarly, does it make sense to assert that the value is non-negative?

[Owen Lin, 2016/03/02 02:50:52]

See above.

+    DLOG(ERROR) << "Invalid offset: " << offset;
+    return false;
+  }
+  r->offset_ = checked_offset.ValueOrDie();
+
   if (!r->key_id_.empty()) {
     if (!(ReadParam(m, iter, &r->iv_) && ReadParam(m, iter, &r->subsamples_)))
       return false;