buffer: draining any zero byte fragments (envoyproxy#9837) (#109) (#123)

Given that we allow creating zero byte fragments, it'd be good to proactively drain them. For example if someone is doing timing instrumentation and wants to know when Network::Connection data is written to the kernel, it could be useful to have a zero byte sentinel.

Risk Level: Low (I don't think anyone is adding zero byte fragments yet)
Testing: new unit test
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>
Signed-off-by: Jianfei Hu <jianfeih@google.com>
Co-authored-by: Lizan Zhou <lizan@tetrate.io>

Author: 2 people

source/common/buffer/buffer_impl.cc

@@ -213,6 +213,11 @@ void OwnedImpl::drain(uint64_t size) {
       }
     }
   }
+  // Make sure to drain any zero byte fragments that might have been added as
+  // sentinels for flushed data.
+  while (!slices_.empty() && slices_.front()->dataSize() == 0) {
+    slices_.pop_front();
+  }
 }
 
 uint64_t OwnedImpl::getRawSlices(RawSlice* out, uint64_t out_size) const {

test/common/buffer/owned_impl_test.cc

@@ -70,6 +70,24 @@ TEST_P(OwnedImplTest, AddBufferFragmentWithCleanup) {
   EXPECT_TRUE(release_callback_called_);
 }
 
+TEST_P(OwnedImplTest, AddEmptyFragment) {
+  char input[] = "hello world";
+  BufferFragmentImpl frag1(input, 11, [](const void*, size_t, const BufferFragmentImpl*) {});
+  BufferFragmentImpl frag2("", 0, [this](const void*, size_t, const BufferFragmentImpl*) {
+    release_callback_called_ = true;
+  });
+  Buffer::OwnedImpl buffer;
+  buffer.addBufferFragment(frag1);
+  EXPECT_EQ(11, buffer.length());
+
+  buffer.addBufferFragment(frag2);
+  EXPECT_EQ(11, buffer.length());
+
+  buffer.drain(11);
+  EXPECT_EQ(0, buffer.length());
+  EXPECT_TRUE(release_callback_called_);
+}
+
 TEST_P(OwnedImplTest, AddBufferFragmentDynamicAllocation) {
   char input_stack[] = "hello world";
   char* input = new char[11];