Add invocation-owned Cobra sessions

internal/cobra/parse_test.go

@@ -1,6 +1,7 @@
 package cobra
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io"
@@ -179,3 +180,47 @@ func TestSelectedPathsResolveThroughBackendParents(t *testing.T) {
 		t.Fatalf("root usage failed: %v", failure)
 	}
 }
+
+func TestSessionReusesTreeForParseHelpAndUsageWithFreshState(t *testing.T) {
+	tree := &Command{Name: "app", Children: []*Command{{Name: "run", Runnable: true, Flags: []Flag{{Name: "value"}}}}}
+	var output, errors bytes.Buffer
+	session := Prepare(tree, ParseOptions{Context: context.Background(), Out: &output, Err: &errors})
+	first := session.Parse([]string{"run", "--value=old"})
+	if first.Failure != nil || !reflect.DeepEqual(first.FlagValues, [][]string{{"old"}}) {
+		t.Fatalf("first session parse = %#v", first)
+	}
+	help, failure := session.RenderHelp([]string{"run"})
+	if failure != nil || !strings.Contains(help, "run") {
+		t.Fatalf("session help = %q, %v", help, failure)
+	}
+	line, failure := session.UsageLine([]string{"run"})
+	if failure != nil || !strings.Contains(line, "run") {
+		t.Fatalf("session usage = %q, %v", line, failure)
+	}
+	if output.Len() != 0 || errors.Len() != 0 {
+		t.Fatalf("help escaped session delivery control: out=%q err=%q", output.String(), errors.String())
+	}
+
+	var secondOutput, secondErrors bytes.Buffer
+	second := Prepare(tree, ParseOptions{Context: context.Background(), Out: &secondOutput, Err: &secondErrors})
+	secondResult := second.Parse([]string{"run"})
+	if secondResult.Failure != nil || !reflect.DeepEqual(secondResult.FlagValues, [][]string{nil}) {
+		t.Fatalf("fresh session state = %#v", secondResult)
+	}
+	if output.Len() != 0 || errors.Len() != 0 || secondOutput.Len() != 0 || secondErrors.Len() != 0 {
+		t.Fatal("prepared session streams were unexpectedly used")
+	}
+}
+
+func TestMissingSessionTreeReturnsNeutralFailures(t *testing.T) {
+	session := Prepare(nil, ParseOptions{})
+	if result := session.Parse(nil); result.Failure == nil || result.Failure.Message != "command tree is nil" {
+		t.Fatalf("missing-tree parse = %#v", result)
+	}
+	if _, failure := session.RenderHelp(nil); failure == nil || failure.Message != "command tree is nil" {
+		t.Fatalf("missing-tree help failure = %#v", failure)
+	}
+	if _, failure := session.UsageLine(nil); failure == nil || failure.Message != "command tree is nil" {
+		t.Fatalf("missing-tree usage failure = %#v", failure)
+	}
+}