rpc: work-around an upstream libevent bug

A rare race condition may trigger while awaiting the body of a message, see
upsteam commit 5ff8eb26371c4dc56f384b2de35bea2d87814779 for details.

This may fix some reported rpc hangs/crashes.

src/httpserver.cpp

@@ -24,6 +24,7 @@
 
 #include <event2/thread.h>
 #include <event2/buffer.h>
+#include <event2/bufferevent.h>
 #include <event2/util.h>
 #include <event2/keyvalq_struct.h>
 
@@ -239,6 +240,16 @@ static std::string RequestMethodString(HTTPRequest::RequestMethod m)
 /** HTTP request callback */
 static void http_request_cb(struct evhttp_request* req, void* arg)
 {
+    // Disable reading to work around a libevent bug, fixed in 2.2.0.
+    if (event_get_version_number() < 0x02020001) {
+        evhttp_connection* conn = evhttp_request_get_connection(req);
+        if (conn) {
+            bufferevent* bev = evhttp_connection_get_bufferevent(conn);
+            if (bev) {
+                bufferevent_disable(bev, EV_READ);
+            }
+        }
+    }
     std::unique_ptr<HTTPRequest> hreq(new HTTPRequest(req));
 
     LogPrint(BCLog::HTTP, "Received a %s request for %s from %s\n",
@@ -601,8 +612,21 @@ void HTTPRequest::WriteReply(int nStatus, const std::string& strReply)
     struct evbuffer* evb = evhttp_request_get_output_buffer(req);
     assert(evb);
     evbuffer_add(evb, strReply.data(), strReply.size());
-    HTTPEvent* ev = new HTTPEvent(eventBase, true,
-        std::bind(evhttp_send_reply, req, nStatus, (const char*)nullptr, (struct evbuffer *)nullptr));
+    auto req_copy = req;
+    HTTPEvent* ev = new HTTPEvent(eventBase, true, [req_copy, nStatus]{
+        evhttp_send_reply(req_copy, nStatus, nullptr, nullptr);
+        // Re-enable reading from the socket. This is the second part of the libevent
+        // workaround above.
+        if (event_get_version_number() < 0x02020001) {
+            evhttp_connection* conn = evhttp_request_get_connection(req_copy);
+            if (conn) {
+                bufferevent* bev = evhttp_connection_get_bufferevent(conn);
+                if (bev) {
+                    bufferevent_enable(bev, EV_READ | EV_WRITE);
+                }
+            }
+        }
+    });
     ev->trigger(nullptr);
     replySent = true;
     req = nullptr; // transferred back to main thread