Preserve joined rendering error evidence

reporter_error_test.go

@@ -0,0 +1,60 @@
+package commandkit
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"testing"
+)
+
+func TestReporterPreservesJoinedRendererEvidence(t *testing.T) {
+	for _, kind := range []string{"output", "detail"} {
+		for _, test := range []struct {
+			name      string
+			combine   func(error, error) error
+			rendering bool
+		}{
+			{"joined", func(render, write error) error { return errors.Join(render, write) }, true},
+			{"wrapped join", func(render, write error) error {
+				return fmt.Errorf("rendering context: %w", errors.Join(write, render))
+			}, true},
+			{"plain write", func(_, write error) error { return write }, false},
+			{"wrapped write", func(_, write error) error { return fmt.Errorf("writing context: %w", write) }, false},
+			{"joined write only", func(_, write error) error { return errors.Join(write, fmt.Errorf("same writer: %w", write)) }, false},
+		} {
+			t.Run(kind+"/"+test.name, func(t *testing.T) {
+				renderErr := errors.New("independent rendering failure")
+				// The destination error can itself be a joined value. Returning that
+				// complete error still propagates a single destination failure.
+				writeErr := errors.Join(errors.New("sink failure"), errors.New("sink detail"))
+				destination := &reporterTestWriter{err: writeErr}
+				reporter := newReporter(Streams{Out: destination, Err: destination})
+				reporter.setVisibility(modeVerbose, false, false)
+				render := func(w io.Writer) error {
+					_, err := io.WriteString(w, "report")
+					return test.combine(renderErr, err)
+				}
+				var err error
+				if kind == "output" {
+					err = reporter.RenderOutput(render)
+				} else {
+					err = reporter.Warn("", render)
+				}
+				if !errors.Is(err, writeErr) || errors.Is(err, renderErr) != test.rendering {
+					t.Fatalf("returned error lost evidence: %v", err)
+				}
+				got := reporter.finalize(Success(), nil)
+				result := got.Presentation.Output
+				if kind == "detail" {
+					result = got.Presentation.Diagnostics
+				}
+				if result.State != DeliveryIncomplete || !errors.Is(result.WriteError, writeErr) || errors.Is(result.RenderError, renderErr) != test.rendering {
+					t.Fatalf("presentation=%#v", result)
+				}
+				if !test.rendering && result.RenderError != nil {
+					t.Fatalf("write propagation became rendering evidence: %v", result.RenderError)
+				}
+			})
+		}
+	}
+}