enable connection reuse in libcurl

Originally, in the `HttpClient::perform()` method, a new `CurlHandle` was created
upon every invocation, leading to subsequent calls to `curl_easy_init()` and
`curl_easy_cleanup()`, as the variable went in and out of scope while it executed
on the `asio::thread_pool`. This meant that there was no opportunity to reuse
connections from the underlying connection pool, because it was always cleaned up.

By marking the `CurlHandle` with the `thread_local` storage duration, it ensures
that the same one remains in use for each thread, thus providing the means to
reuse the established connection pool.

Fixes Netflix-Skunkworks#51.

.github/workflows/build.yml

@@ -52,7 +52,7 @@ jobs:
         run: |
           tools/generate_version.sh > spectator/version.h
           cd $BUILD_DIR
-          cmake .. -DCMAKE_BUILD_TYPE=Release
+          cmake .. -DCMAKE_BUILD_TYPE=Debug
           cmake --build .
           echo "==== ldd ===="
           ldd bin/spectatord_main || true

build.sh

@@ -52,7 +52,13 @@ cmake --build . || exit 1
 
 if [[ "$1" != "skiptest" ]]; then
   echo -e "${BLUE}==== test ====${NC}"
-  GTEST_COLOR=1 ASAN_OPTIONS=detect_container_overflow=0 ctest --verbose
+
+  if [[ "$(uname -s)" == "Darwin" ]]; then
+    export ASAN_OPTIONS="detect_container_overflow=0"
+    echo "== export ASAN_OPTIONS=$ASAN_OPTIONS"
+  fi
+
+  GTEST_COLOR=1 ctest --verbose
 fi
 
 popd || exit 1

spectator/http_client.cc

@@ -78,7 +78,12 @@ class CurlHandle {
 
   auto handle() const noexcept -> CURL* { return handle_; }
 
-  auto perform() -> CURLcode { return curl_easy_perform(handle()); }
+  auto perform() -> CURLcode {
+    if (response_.size() > 0) {
+      response_.clear();
+    }
+    return curl_easy_perform(handle());
+  }
 
   auto set_opt(CURLoption option, const void* param) -> CURLcode {
     return curl_easy_setopt(handle(), option, param);
@@ -199,7 +204,7 @@ auto HttpClient::perform(const char* method, const std::string& url,
                          int attempt_number) const -> HttpResponse {
   LogEntry entry{registry_, method, url};
 
-  CurlHandle curl;
+  thread_local CurlHandle curl;
   auto total_timeout = config_.connect_timeout + config_.read_timeout;
   curl.set_timeout(total_timeout);
   curl.set_connect_timeout(config_.connect_timeout);