Pipeline: prefer to report non-pipe errors to the caller

If a later stage of the pipeline fails, it is often the case that
earlier stages fail with some sort of pipe-related error (because
their `stdout` pipe gets closed). Those pipe errors are usually not
very helpful in figuring out the underlying problem. So if there are
any non-pipe errors, report the earliest one of those. If there are
only pipe errors, report the latest one of those.

Author: mhagger

internal/pipe/pipeline.go

@@ -185,12 +185,17 @@ func (p *Pipeline) Wait() error {
 	for i := len(p.stages) - 1; i >= 0; i-- {
 		s := p.stages[i]
 		err := s.Wait()
-		if err != nil {
+
+		// We want to report the error that is most informative. We
+		// take that to be the error from the earliest pipeline stage
+		// that failed of a non-pipe error. If that didn't happen,
+		// take the error from the last pipeline stage that failed due
+		// to a pipe error.
+		if err != nil && (earliestStageErr == nil || !IsPipeError(err)) {
 			// Overwrite any existing values here so that we end up
 			// retaining the last error that we see; i.e., the error
 			// that happened earliest in the pipeline.
-			earliestStageErr = err
-			earliestFailedStage = s
+			earliestFailedStage, earliestStageErr = s, err
 		}
 	}