fix: emit JSON errors for --json commands

cmd/specter/commands/root.go

@@ -1,7 +1,9 @@
 package commands
 
 import (
+	"encoding/json"
 	"fmt"
+	"io"
 
 	"github.com/ghostwright/specter/internal/tui"
 	"github.com/ghostwright/specter/pkg/version"
@@ -42,3 +44,14 @@ func init() {
 func Execute() error {
 	return rootCmd.Execute()
 }
+
+func WriteError(w io.Writer, err error) {
+	if jsonOutput {
+		_ = json.NewEncoder(w).Encode(map[string]string{
+			"error": err.Error(),
+		})
+		return
+	}
+
+	fmt.Fprintln(w, err)
+}

cmd/specter/commands/root_test.go

@@ -0,0 +1,63 @@
+package commands
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"testing"
+
+	"github.com/spf13/cobra"
+)
+
+func TestRootCommandErrorOutputJSON(t *testing.T) {
+	err := executeFailingRootCommand(t, "--json", "fail")
+
+	var buf bytes.Buffer
+	WriteError(&buf, err)
+
+	var got map[string]string
+	if err := json.Unmarshal(buf.Bytes(), &got); err != nil {
+		t.Fatalf("error output is not valid JSON: %v", err)
+	}
+	if got["error"] != "boom" {
+		t.Fatalf("error = %q, want %q", got["error"], "boom")
+	}
+}
+
+func TestRootCommandErrorOutputPlainText(t *testing.T) {
+	err := executeFailingRootCommand(t, "fail")
+
+	var buf bytes.Buffer
+	WriteError(&buf, err)
+
+	if got, want := buf.String(), "boom\n"; got != want {
+		t.Fatalf("error output = %q, want %q", got, want)
+	}
+}
+
+func executeFailingRootCommand(t *testing.T, args ...string) error {
+	t.Helper()
+
+	oldJSONOutput := jsonOutput
+	failCmd := &cobra.Command{
+		Use: "fail",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			return errors.New("boom")
+		},
+	}
+
+	rootCmd.AddCommand(failCmd)
+	rootCmd.SetArgs(args)
+	t.Cleanup(func() {
+		rootCmd.RemoveCommand(failCmd)
+		rootCmd.SetArgs(nil)
+		jsonOutput = oldJSONOutput
+	})
+
+	err := Execute()
+	if err == nil {
+		t.Fatal("Execute() error = nil, want error")
+	}
+
+	return err
+}

cmd/specter/main.go

@@ -15,7 +15,7 @@ func main() {
 		runDashboard()
 	} else {
 		if err := commands.Execute(); err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			commands.WriteError(os.Stderr, err)
 			os.Exit(1)
 		}
 	}