[BUG] Check what curl_slist_append and curl_multi_init return

CHANGELOG.md

@@ -71,6 +71,20 @@ Increment the:
   deprecated C headers (`stdint.h`, `stddef.h`, `stdlib.h`, `string.h`,
   `stdio.h`, `ctype.h`, `limits.h`, `assert.h`) with their C++ equivalents
   ([#4349](https://github.com/open-telemetry/opentelemetry-cpp/pull/4349))
+* [BUG] Check what curl_easy_init, curl_slist_append and curl_multi_init return
+  instead of treating a failed allocation as success
+  [#4404](https://github.com/open-telemetry/opentelemetry-cpp/issues/4404)
+* [BUG] Stop the curl IO thread spinning, flooding the log, and refusing to be
+  destroyed, when the multi handle cannot be created
+  [#4404](https://github.com/open-telemetry/opentelemetry-cpp/issues/4404)
+* [BUG] Keep the curl IO thread from retiring while a request it accepted is
+  still waiting to be scheduled, and release the easy handles and header lists
+  left queued when the client is destroyed
+  [#4404](https://github.com/open-telemetry/opentelemetry-cpp/issues/4404)
+* [BUG] Take an operation that was cancelled or torn down out of the curl retry
+  queue, and check what curl_multi_remove_handle and curl_multi_add_handle
+  return when a retry is scheduled
+  [#4404](https://github.com/open-telemetry/opentelemetry-cpp/issues/4404)
 
 * [CONFIGURATION] Add SDK component builder interfaces to the registry
   [#4358](https://github.com/open-telemetry/opentelemetry-cpp/issues/4358)

ext/include/opentelemetry/ext/http/client/curl/http_client_curl.h

@@ -364,7 +364,22 @@ class HttpClient : public opentelemetry::ext::http::client::HttpClient
   bool doAbortSessions();
   bool doRemoveSessions();
   bool doRetrySessions(bool report_all);
-  void resetMultiHandle();
+  // Returns true if the background thread still owes somebody an answer. Drops what it finds
+  // that nothing can be owed for, so that a queue which is merely not empty does not read as
+  // work. Call it on the background thread only: it prunes pending_to_retry_sessions_, which
+  // has no lock because that thread is the only one that touches it.
+  bool hasActionableWork();
+  // Cleans up the multi handle if there is one, and leaves none behind either way. Call it
+  // holding multi_handle_m_. It answers with what curl_multi_cleanup said rather than reporting
+  // it, because reporting reaches a log handler the application supplies, and one that comes back
+  // into this client would do it while the caller still holds that mutex.
+  CURLMcode ReleaseMultiHandle();
+  // Returns true if the client has a multi handle afterwards.
+  bool resetMultiHandle();
+
+  // Declared before multi_handle_ on purpose: members are initialised in declaration
+  // order, and curl_multi_init() may not run before curl_global_init().
+  nostd::shared_ptr<HttpCurlGlobalInitializer> curl_global_initializer_;
 
   std::mutex multi_handle_m_;
   CURLM *multi_handle_;
@@ -387,8 +402,10 @@ class HttpClient : public opentelemetry::ext::http::client::HttpClient
 
   std::chrono::milliseconds background_thread_wait_for_;
   std::atomic<bool> is_shutdown_{false};
-
-  nostd::shared_ptr<HttpCurlGlobalInitializer> curl_global_initializer_;
+  // Raised by every producer. curl_multi_wakeup is how the background thread is woken out of
+  // curl_multi_poll and it needs a multi handle, so the wait taken when there is none watches
+  // this instead.
+  std::atomic<uint64_t> wakeup_generation_{0};
 };
 
 }  // namespace curl

ext/include/opentelemetry/ext/http/client/curl/http_operation_curl.h

@@ -325,7 +325,8 @@ class HttpOperation
 
   char curl_error_message_[CURL_ERROR_SIZE]{};
   HttpCurlEasyResource curl_resource_;
-  CURLcode last_curl_result_{CURLE_OK};  // Curl result OR HTTP status code if successful
+  CURLcode last_curl_result_{CURLE_OK};     // Curl result OR HTTP status code if successful
+  CURLcode construction_result_{CURLE_OK};  // Non-OK if setup failed; Send() refuses on it
 
   opentelemetry::ext::http::client::EventHandler *event_handle_{nullptr};