[native] Save 1 memcpy in parsing message body (#24941)

## Description
In our HTTP callbacks, we parse the vector of iobufs into a string;
however, we assume that each iobuf is singularly chained. This is not
guaranteed to be the case.
This diff resolves two issues:

- Correctly consumes the vector of iobufs by taking each buffer chain
into account

- Saves 1 memcopy. The previous code path would copy each iobuf into a
stream, and then use.c_str(), which also is a copy
([https://en.cppreference.com/w/cpp/io/basic_ostringstream/str](https://l.facebook.com/l.php?u=https%3A%2F%2Fen.cppreference.com%2Fw%2Fcpp%2Fio%2Fbasic_ostringstream%2Fstr&h=AT2RH-TyloQE5MPfdcxYizn__by-8eZd_7-9saGWz05YOZfunpxrYWFq0lHU6WVhx1zpPwW_rnjI9dtanJo1HqBKf5SDv96S4UZBgEOE02Jzb2MevW9chO9bBLeyNU4g7pEs9yMea4EnPqVa9h_K4QrRx_c)).
This diff will just copy once into the string buffer



## Test Plan
Added Unit test

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Author: yuandagits

presto-native-execution/presto_cpp/main/common/Utils.cpp

@@ -14,6 +14,7 @@
 
 #include "presto_cpp/main/common/Utils.h"
 #include <fmt/format.h>
+#include <folly/io/Cursor.h>
 #include <sys/resource.h>
 #include "velox/common/process/ThreadDebugInfo.h"
 
@@ -66,12 +67,20 @@ void installSignalHandler() {
 
 std::string extractMessageBody(
     const std::vector<std::unique_ptr<folly::IOBuf>>& body) {
-  // TODO Avoid copy
-  std::ostringstream oss;
-  for (auto& buf : body) {
-    oss << std::string(
-        reinterpret_cast<const char*>(buf->data()), buf->length());
+  std::string ret;
+  size_t bodySize = 0;
+  for (const auto& buf : body) {
+    bodySize += buf->computeChainDataLength();
   }
-  return oss.str();
+  ret.resize(bodySize);
+
+  size_t offset = 0;
+  for (const auto& buf : body) {
+    folly::io::Cursor cursor(buf.get());
+    size_t chainLength = buf->computeChainDataLength();
+    cursor.pull(ret.data() + offset, chainLength);
+    offset += chainLength;
+  }
+  return ret;
 }
 } // namespace facebook::presto::util

presto-native-execution/presto_cpp/main/common/tests/CommonTest.cpp

@@ -133,3 +133,15 @@ TEST(UtilsTest, general) {
   EXPECT_EQ("2021-05-20T19:18:27.001Z", util::toISOTimestamp(1621538307001l));
   EXPECT_EQ("2021-05-20T19:18:27.000Z", util::toISOTimestamp(1621538307000l));
 }
+
+TEST(UtilsTest, extractMessageBody) {
+  std::vector<std::unique_ptr<folly::IOBuf>> body;
+  body.push_back(folly::IOBuf::copyBuffer("body1"));
+  body.push_back(folly::IOBuf::copyBuffer("body2"));
+  body.push_back(folly::IOBuf::copyBuffer("body3"));
+  auto iobuf = folly::IOBuf::copyBuffer("body4");
+  iobuf->appendToChain(folly::IOBuf::copyBuffer("body5"));
+  body.push_back(std::move(iobuf));
+  auto messageBody = util::extractMessageBody(body);
+  EXPECT_EQ(messageBody, "body1body2body3body4body5");
+}