Harden Go CLI subprocess behavior

internal/cli/run.go

@@ -26,7 +26,7 @@ var processRunner = func(inv processInvocation, stdout io.Writer) error {
 	if err != nil {
 		return err
 	}
-	if err := coreio.ValidateWellFormedJSON(transcriptBytes); err != nil {
+	if err := coreio.ValidateWellFormedJSON(inv.TranscriptPath, transcriptBytes); err != nil {
 		return err
 	}
 	if _, err := coreio.ReadRequiredFile(inv.GlossaryPath, "glossary"); err != nil {

internal/cli/run_test.go

@@ -156,7 +156,7 @@ func TestRunProcessMalformedTranscriptJSON(t *testing.T) {
 	if stdout.Len() != 0 {
 		t.Fatalf("expected empty stdout, got %q", stdout.String())
 	}
-	if !strings.Contains(stderr.String(), "transcript file is not valid JSON") {
+	if !strings.Contains(stderr.String(), "is not valid JSON") {
 		t.Fatalf("expected malformed transcript error, got %q", stderr.String())
 	}
 }

internal/core/io/files.go

@@ -14,9 +14,9 @@ func ReadRequiredFile(path string, label string) ([]byte, error) {
 	return contents, nil
 }
 
-func ValidateWellFormedJSON(raw []byte) error {
+func ValidateWellFormedJSON(path string, raw []byte) error {
 	if !json.Valid(raw) {
-		return fmt.Errorf("transcript file is not valid JSON")
+		return fmt.Errorf("transcript file %q is not valid JSON", path)
 	}
 	return nil
 }