Review comments

federationapi/routing/send.go

@@ -148,7 +148,7 @@ type inputWorker struct {
 	input   *sendFIFOQueue
 }
 
-var txnIDs sync.Map       // transaction ID -> chan util.JSONResponse
+var inFlightTxnsPerOrigin sync.Map // transaction ID -> chan util.JSONResponse
 var inputWorkers sync.Map          // room ID -> *inputWorker
 
 // Send implements /_matrix/federation/v1/send/{txnID}
@@ -169,14 +169,21 @@ func Send(
 	// txn ID to us. If they have and the txnIDs map contains an entry,
 	// the transaction is still being worked on. The new client can wait
 	// for it to complete rather than creating more work.
-	index := string(request.Origin()) + string(txnID)
-	if v, ok := txnIDs.Load(index); ok {
+	index := string(request.Origin()) + "\000" + string(txnID)
+	ch, ok := inFlightTxnsPerOrigin.LoadOrStore(index, make(chan util.JSONResponse, 1))
+	if ok {
 		// This origin already submitted this txn ID to us, and the work
 		// is still taking place, so we'll just wait for it to finish.
+		ctx, cancel := context.WithTimeout(httpReq.Context(), time.Minute*5)
+		defer cancel()
 		select {
-		case <-httpReq.Context().Done():
+		case <-ctx.Done():
+			// If the caller gives up then return straight away. We don't
+			// want to attempt to process what they sent us any further.
 			return util.JSONResponse{Code: http.StatusRequestTimeout}
-		case res := <-v.(chan util.JSONResponse):
+		case res := <-ch.(chan util.JSONResponse):
+			// The original task just finished processing so let's return
+			// the result of it.
 			if res.Code == 0 {
 				return util.JSONResponse{Code: http.StatusAccepted}
 			}
@@ -185,10 +192,8 @@ func Send(
 	}
 	// Otherwise, store that we're currently working on this txn from
 	// this origin. When we're done processing, close the channel.
-	ch := make(chan util.JSONResponse, 1)
-	txnIDs.Store(index, ch)
-	defer close(ch)
-	defer txnIDs.Delete(index)
+	defer close(ch.(chan util.JSONResponse))
+	defer inFlightTxnsPerOrigin.Delete(index)
 
 	t := txnReq{
 		rsAPI:      rsAPI,