runutil: inform the user if exhaustion fails

pkg/runutil/runutil.go

@@ -117,7 +117,9 @@ func CloseWithLogOnErr(logger log.Logger, closer io.Closer, format string, a ...
 
 // ExhaustCloseWithLogOnErr closes the io.ReadCloser with a log message on error but exhausts the reader before.
 func ExhaustCloseWithLogOnErr(logger log.Logger, r io.ReadCloser, format string, a ...interface{}) {
-	_, _ = io.Copy(ioutil.Discard, r)
+	_, err := io.Copy(ioutil.Discard, r)
+	level.Warn(logger).Log("msg", "failed to exhaust reader, performance may be impeded", "err", err)
+
 	CloseWithLogOnErr(logger, r, format, a...)
 }
 
@@ -134,6 +136,14 @@ func CloseWithErrCapture(err *error, closer io.Closer, format string, a ...inter
 
 // ExhaustCloseWithErrCapture closes the io.ReadCloser with error capture but exhausts the reader before.
 func ExhaustCloseWithErrCapture(err *error, r io.ReadCloser, format string, a ...interface{}) {
-	_, _ = io.Copy(ioutil.Discard, r)
+	_, copyErr := io.Copy(ioutil.Discard, r)
+
 	CloseWithErrCapture(err, r, format, a...)
+
+	// Prepend the io.Copy error.
+	merr := tsdberrors.MultiError{}
+	merr.Add(copyErr)
+	merr.Add(*err)
+
+	*err = merr.Err()
 }