cmd/lint: show JSON errors

Signed-off-by: Charlie Egan <charlie@styra.com>

cmd/lint.go

@@ -1,6 +1,7 @@
 package cmd
 
 import (
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -100,8 +101,10 @@ func init() {
 
 			rep, err := lint(args, params)
 			if err != nil {
-				log.SetOutput(os.Stderr)
-				log.Println(err)
+				if !errors.Is(err, &alreadyReportedError{}) {
+					log.SetOutput(os.Stderr)
+					log.Println(err)
+				}
 
 				return exit(1)
 			}
@@ -313,7 +316,7 @@ func lint(args []string, params *lintCommandParams) (report.Report, error) {
 
 	result, err := regal.Lint(ctx)
 	if err != nil {
-		return report.Report{}, fmt.Errorf("error(s) encountered while linting: %w", err)
+		return report.Report{}, reportErrors(params.format, outputWriter, err)
 	}
 
 	rep, err := getReporter(params.format, outputWriter)
@@ -360,3 +363,34 @@ func getWriterForOutputFile(filename string) (io.Writer, error) {
 
 	return f, nil
 }
+
+// alreadyReportedError is an error type that is used to signal that an error has already been reported
+// and should not shown again, however it should still trigger the error exit code.
+type alreadyReportedError struct{}
+
+func (*alreadyReportedError) Error() string {
+	return "error already reported"
+}
+
+func reportErrors(format string, outputWriter io.Writer, err error) error {
+	// currently, JSON and SARIF will get the same generic JSON error format
+	if format == formatJSON || format == formatSarif {
+		enc := json.NewEncoder(outputWriter)
+		enc.SetIndent("", "  ")
+
+		if err = enc.Encode(map[string]interface{}{
+			"errors": []string{err.Error()},
+		}); err != nil {
+			return fmt.Errorf("failed to encode error in %s: %w", format, err)
+		}
+
+		return &alreadyReportedError{}
+	}
+
+	_, err = outputWriter.Write([]byte(fmt.Sprintf("error(s) encountered while linting: %s\n", err)))
+	if err != nil {
+		return fmt.Errorf("failed to write error to output: %w", err)
+	}
+
+	return &alreadyReportedError{}
+}