feat(cgo): normalize Errno for negative return codes

Co-Authored-By: Virgil <virgil@lethean.io>

call_test.go

@@ -181,6 +181,12 @@ func TestErrnoMapping(t *testing.T) {
 	} else if !errors.Is(err, syscall.Errno(2)) {
 		t.Fatalf("expected error type %v, got %T %v", syscall.Errno(2), err, err)
 	}
+
+	if err := Errno(-2); err == nil {
+		t.Fatal("expected non-nil error for negative errno")
+	} else if !errors.Is(err, syscall.Errno(2)) {
+		t.Fatalf("expected positive errno mapping from -2, got %T %v", err, err)
+	}
 }
 
 func TestWithErrnoReturnsBothResultAndError(t *testing.T) {

string_conversion.go

@@ -847,7 +847,13 @@ func Errno(resultCode C.int) error {
 	if resultCode == 0 {
 		return nil
 	}
-	return syscall.Errno(resultCode)
+
+	recordedCode := resultCode
+	if recordedCode < 0 {
+		recordedCode = -recordedCode
+	}
+
+	return syscall.Errno(recordedCode)
 }
 
 // WithErrno runs a function that returns C.int and maps the result to Go error.