Replace IOBufQueue in retransmission buffer.

Summary:
`IOBufQueue` has some facilities for fast appending to the tail. This is not useful for us in the retransmission buffer usecase, and probably not at all. Flushing the tail cache from the `IOBufQueue` is expensive when we have to shuffle around the retransmission buffer queue on removal.

This diff replaces `IOBufQueue` with a bespoke version that only has some of the functionality.

This also changes the dependent peek APIs to use `IOBuf`s directly.

Reviewed By: siyengar, yangchi

Differential Revision: D18126437

fbshipit-source-id: a2fec0f45a72459855700c605bfd0d863a9067b7

quic/common/test/BufUtilTest.cpp

@@ -0,0 +1,113 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
+
+#include <gtest/gtest.h>
+
+#include <quic/common/BufUtil.h>
+
+using namespace std;
+using namespace folly;
+using namespace quic;
+
+#define SCL(x) (x), sizeof(x) - 1
+
+namespace {
+
+void checkConsistency(const BufQueue& queue) {
+  size_t len = queue.front() ? queue.front()->computeChainDataLength() : 0;
+  EXPECT_EQ(len, queue.chainLength());
+}
+
+} // namespace
+
+TEST(BufQueue, Append) {
+  BufQueue queue;
+  queue.append(IOBuf::copyBuffer(SCL("Hello")));
+  BufQueue queue2;
+  queue2.append(IOBuf::copyBuffer(SCL(", ")));
+  queue2.append(IOBuf::copyBuffer(SCL("World")));
+  checkConsistency(queue);
+  checkConsistency(queue2);
+  queue.append(queue2.move());
+  checkConsistency(queue);
+  checkConsistency(queue2);
+  const IOBuf* chain = queue.front();
+  EXPECT_NE((IOBuf*)nullptr, chain);
+  EXPECT_EQ(12, chain->computeChainDataLength());
+  EXPECT_EQ(nullptr, queue2.front());
+}
+
+TEST(BufQueue, Append2) {
+  BufQueue queue;
+  queue.append(IOBuf::copyBuffer(SCL("Hello")));
+  BufQueue queue2;
+  queue2.append(IOBuf::copyBuffer(SCL(", ")));
+  queue2.append(IOBuf::copyBuffer(SCL("World")));
+  checkConsistency(queue);
+  checkConsistency(queue2);
+}
+
+TEST(BufQueue, AppendStringPiece) {
+  std::string s("Hello, World");
+  BufQueue queue;
+  BufQueue queue2;
+  queue.append(IOBuf::copyBuffer(s.data(), s.length()));
+  queue2.append(IOBuf::copyBuffer(s));
+  checkConsistency(queue);
+  checkConsistency(queue2);
+  const IOBuf* chain = queue.front();
+  const IOBuf* chain2 = queue2.front();
+  EXPECT_EQ(s.length(), chain->computeChainDataLength());
+  EXPECT_EQ(s.length(), chain2->computeChainDataLength());
+  EXPECT_EQ(0, memcmp(chain->data(), chain2->data(), s.length()));
+}
+
+TEST(BufQueue, Split) {
+  BufQueue queue;
+  queue.append(IOBuf::copyBuffer(SCL("Hello")));
+  queue.append(IOBuf::copyBuffer(SCL(",")));
+  queue.append(IOBuf::copyBuffer(SCL(" ")));
+  queue.append(IOBuf::copyBuffer(SCL("")));
+  queue.append(IOBuf::copyBuffer(SCL("World")));
+  checkConsistency(queue);
+  EXPECT_EQ(12, queue.front()->computeChainDataLength());
+
+  unique_ptr<IOBuf> prefix(queue.split(1));
+  checkConsistency(queue);
+  EXPECT_EQ(1, prefix->computeChainDataLength());
+  EXPECT_EQ(11, queue.front()->computeChainDataLength());
+  prefix = queue.split(2);
+  checkConsistency(queue);
+  EXPECT_EQ(2, prefix->computeChainDataLength());
+  EXPECT_EQ(9, queue.front()->computeChainDataLength());
+  prefix = queue.split(3);
+  checkConsistency(queue);
+  EXPECT_EQ(3, prefix->computeChainDataLength());
+  EXPECT_EQ(6, queue.front()->computeChainDataLength());
+  prefix = queue.split(1);
+  checkConsistency(queue);
+  EXPECT_EQ(1, prefix->computeChainDataLength());
+  EXPECT_EQ(5, queue.front()->computeChainDataLength());
+  prefix = queue.split(5);
+  checkConsistency(queue);
+  EXPECT_EQ(5, prefix->computeChainDataLength());
+  EXPECT_EQ((IOBuf*)nullptr, queue.front());
+
+  queue.append(IOBuf::copyBuffer(SCL("Hello,")));
+  queue.append(IOBuf::copyBuffer(SCL(" World")));
+  checkConsistency(queue);
+  EXPECT_THROW({ prefix = queue.split(13); }, std::underflow_error);
+  checkConsistency(queue);
+}
+
+TEST(BufQueue, SplitZero) {
+  BufQueue queue;
+  queue.append(IOBuf::copyBuffer(SCL("Hello world")));
+  auto buf = queue.split(0);
+  EXPECT_EQ(buf->computeChainDataLength(), 0);
+}