Make HTTP server shutdown more graceful

Shutting down the HTTP server currently breaks off all current requests.
This can create a race condition with RPC `stop` command, where the calling
process never receives confirmation.

This change removes the listening sockets on shutdown so that no new
requests can come in, but no longer breaks off requests in progress.

Meant to fix bitcoin/zcash#6717.

Zcash: cherry-picked from commit 5e0c221

src/httpserver.cpp

@@ -155,6 +155,8 @@ static std::vector<CSubNet> rpc_allow_subnets;
 static WorkQueue<HTTPClosure>* workQueue = 0;
 //! Handlers for (sub)paths
 std::vector<HTTPPathHandler> pathHandlers;
+//! Bound listening sockets
+std::vector<evhttp_bound_socket *> boundSockets;
 
 /** Check if a network address is allowed to access the HTTP server */
 static bool ClientAllowed(const CNetAddr& netaddr)
@@ -264,6 +266,13 @@ static void http_request_cb(struct evhttp_request* req, void* arg)
     }
 }
 
+/** Callback to reject HTTP requests after shutdown. */
+static void http_reject_request_cb(struct evhttp_request* req, void*)
+{
+    LogPrint("http", "Rejecting request while shutting down\n");
+    evhttp_send_error(req, HTTP_SERVUNAVAIL, NULL);
+}
+
 /** Event dispatcher thread */
 static void ThreadHTTP(struct event_base* base, struct evhttp* http)
 {
@@ -278,7 +287,6 @@ static void ThreadHTTP(struct event_base* base, struct evhttp* http)
 static bool HTTPBindAddresses(struct evhttp* http)
 {
     int defaultPort = GetArg("-rpcport", BaseParams().RPCPort());
-    int nBound = 0;
     std::vector<std::pair<std::string, uint16_t> > endpoints;
 
     // Determine what addresses to bind to
@@ -304,13 +312,14 @@ static bool HTTPBindAddresses(struct evhttp* http)
     // Bind addresses
     for (std::vector<std::pair<std::string, uint16_t> >::iterator i = endpoints.begin(); i != endpoints.end(); ++i) {
         LogPrint("http", "Binding RPC on address %s port %i\n", i->first, i->second);
-        if (evhttp_bind_socket(http, i->first.empty() ? NULL : i->first.c_str(), i->second) == 0) {
-            nBound += 1;
+        evhttp_bound_socket *bind_handle = evhttp_bind_socket_with_handle(http, i->first.empty() ? NULL : i->first.c_str(), i->second);
+        if (bind_handle) {
+            boundSockets.push_back(bind_handle);
         } else {
             LogPrintf("Binding RPC on address %s port %i failed.\n", i->first, i->second);
         }
     }
-    return nBound > 0;
+    return !boundSockets.empty();
 }
 
 /** Simple wrapper to set thread name and run work queue */
@@ -414,8 +423,14 @@ bool StartHTTPServer(boost::thread_group& threadGroup)
 void InterruptHTTPServer()
 {
     LogPrint("http", "Interrupting HTTP server\n");
-    if (eventBase)
-        event_base_loopbreak(eventBase);
+    if (eventHTTP) {
+        // Unlisten sockets
+        BOOST_FOREACH (evhttp_bound_socket *socket, boundSockets) {
+            evhttp_del_accept_socket(eventHTTP, socket);
+        }
+        // Reject requests on current connections
+        evhttp_set_gencb(eventHTTP, http_reject_request_cb, NULL);
+    }
     if (workQueue)
         workQueue->Interrupt();
 }

src/rpcserver.cpp

@@ -246,7 +246,8 @@ UniValue stop(const UniValue& params, bool fHelp)
         throw runtime_error(
             "stop\n"
             "\nStop Zcash server.");
-    // Shutdown will take long enough that the response should get back
+    // Event loop will exit after current HTTP requests have been handled, so
+    // this reply will get back to the client.
     StartShutdown();
     return "Zcash server stopping";
 }