http: Join worker threads before deleting work queue

This prevents a potential race condition if control flow ends up in
`ShutdownHTTPServer` before the thread gets to `queue->Run()`,
deleting the work queue while workers are still going to use it.

Meant to fix bitcoin#12362.

Signed-off-by: Wladimir J. van der Laan <laanwj@gmail.com>

src/httpserver.cpp

@@ -466,6 +466,7 @@ bool UpdateHTTPServerLogging(bool enable) {
 
 std::thread threadHTTP;
 std::future<bool> threadResult;
+static std::vector<std::thread> g_thread_http_workers;
 
 bool StartHTTPServer()
 {
@@ -477,8 +478,7 @@ bool StartHTTPServer()
     threadHTTP = std::thread(std::move(task), eventBase, eventHTTP);
 
     for (int i = 0; i < rpcThreads; i++) {
-        std::thread rpc_worker(HTTPWorkQueueRun, workQueue);
-        rpc_worker.detach();
+        g_thread_http_workers.emplace_back(HTTPWorkQueueRun, workQueue);
     }
     return true;
 }
@@ -502,6 +502,10 @@ void StopHTTPServer()
     if (workQueue) {
         LogPrint(BCLog::HTTP, "Waiting for HTTP worker threads to exit\n");
         workQueue->WaitExit();
+        for (auto& thread: g_thread_http_workers) {
+            thread.join();
+        }
+        g_thread_http_workers.clear();
         delete workQueue;
     }
     MilliSleep(500); // Avoid race condition while the last HTTP-thread is exiting