Add a way to force federationsender to retry sending transactions (#1077)

* Add a way to force federationsender to retry sending transactions

And use it in P2P mode when we pick up new nodes.

* Linting

* Use atomic bool to stop us blocking on the channel

federationsender/api/api.go

@@ -42,6 +42,12 @@ type FederationSenderInternalAPI interface {
 		request *PerformLeaveRequest,
 		response *PerformLeaveResponse,
 	) error
+	// Notifies the federation sender that these servers may be online and to retry sending messages.
+	PerformServersAlive(
+		ctx context.Context,
+		request *PerformServersAliveRequest,
+		response *PerformServersAliveResponse,
+	) error
 }
 
 // NewFederationSenderInternalAPIHTTP creates a FederationSenderInternalAPI implemented by talking to a HTTP POST API.

federationsender/api/perform.go

@@ -18,6 +18,9 @@ const (
 
 	// FederationSenderPerformLeaveRequestPath is the HTTP path for the PerformLeaveRequest API.
 	FederationSenderPerformLeaveRequestPath = "/federationsender/performLeaveRequest"
+
+	// FederationSenderPerformServersAlivePath is the HTTP path for the PerformServersAlive API.
+	FederationSenderPerformServersAlivePath = "/federationsender/performServersAlive"
 )
 
 type PerformDirectoryLookupRequest struct {
@@ -88,3 +91,22 @@ func (h *httpFederationSenderInternalAPI) PerformLeave(
 	apiURL := h.federationSenderURL + FederationSenderPerformLeaveRequestPath
 	return internalHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
 }
+
+type PerformServersAliveRequest struct {
+	Servers []gomatrixserverlib.ServerName
+}
+
+type PerformServersAliveResponse struct {
+}
+
+func (h *httpFederationSenderInternalAPI) PerformServersAlive(
+	ctx context.Context,
+	request *PerformServersAliveRequest,
+	response *PerformServersAliveResponse,
+) error {
+	span, ctx := opentracing.StartSpanFromContext(ctx, "PerformServersAlive")
+	defer span.Finish()
+
+	apiURL := h.federationSenderURL + FederationSenderPerformServersAlivePath
+	return internalHTTP.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+}