Fix sfx submissions reporting a timeout when one encounters an error (#…

…684)

* Only send one result if an sfx flush error occurs

The submitDatapoints function would send the error across the channel
and then immediately send nil also, which would cut a Flush function's
waiting period short. Oops.

* Thank me

CHANGELOG.md

@@ -1,5 +1,7 @@
 # 12.0.0, in progress
 
+## Bugfixes
+* The signalfx client no longer reports a timeout when submission to the datapoint API endpoint encounters an error. Thanks, [antifuchs](https://github.com/antifuchs)!
 
 # 11.0.0, 2019-01-22
 

sinks/signalfx/signalfx.go

@@ -51,9 +51,8 @@ func submitDatapoints(ctx context.Context, cl *trace.Client, client dpsink.Sink,
 	if err != nil {
 		span.Error(err)
 		span.Add(ssf.Count("flush.error_total", 1, map[string]string{"cause": "io", "sink": "signalfx"}))
-		errs <- err
 	}
-	errs <- nil
+	errs <- err
 }
 
 func (c *collection) submit(ctx context.Context, cl *trace.Client, maxPerFlush int) error {