Force conn.Close() after MaxRuntime in handler.go (#362)

* Force conn.Close() after MaxRuntime in handler.go

* Pass the context with timeout to upload.Do and download.Do

* Remove the reference to the timeout in the warning message

* Cancel context in a defer rather than waiting for timeout

ndt7/handler/handler.go

@@ -2,6 +2,7 @@
 package handler
 
 import (
+	"context"
 	"fmt"
 	"net"
 	"net/http"
@@ -68,7 +69,17 @@ func (h Handler) runMeasurement(kind spec.SubtestKind, rw http.ResponseWriter, r
 		ndt7metrics.ClientConnections.WithLabelValues(string(kind), "websocket-error").Inc()
 		return
 	}
-	defer warnonerror.Close(conn, "runMeasurement: ignoring conn.Close result")
+	// Make sure that the connection is closed after (at most) MaxRuntime.
+	// Download and upload tests have their own timeouts, but we have observed
+	// that under particular network conditions the connection can remain open
+	// while the receiver goroutine is blocked on a read syscall, long after
+	// the client is gone. This is a workaround for that.
+	ctx, cancel := context.WithTimeout(req.Context(), spec.MaxRuntime)
+	defer cancel()
+	go func() {
+		<-ctx.Done()
+		warnonerror.Close(conn, "runMeasurement: ignoring conn.Close result")
+	}()
 	// Create measurement archival data.
 	data, err := getData(conn)
 	if err != nil {
@@ -96,11 +107,11 @@ func (h Handler) runMeasurement(kind spec.SubtestKind, rw http.ResponseWriter, r
 	var rate float64
 	if kind == spec.SubtestDownload {
 		result.Download = data
-		err = download.Do(req.Context(), conn, data)
+		err = download.Do(ctx, conn, data)
 		rate = downRate(data.ServerMeasurements)
 	} else if kind == spec.SubtestUpload {
 		result.Upload = data
-		err = upload.Do(req.Context(), conn, data)
+		err = upload.Do(ctx, conn, data)
 		rate = upRate(data.ServerMeasurements)
 	}