Prevent flags from being set again by defaultv0.3.0

Flag values that are safe to be set again should implement the new
MutableFlagValue interface.

2 files changed, 22 insertions, 0 deletions

diff --git a/flag.go b/flag.go
index 6fe41f3..4ef45d1 100644
--- a/flag.go
+++ b/flag.go
@@ -82,6 +82,12 @@ func InitFlagSet(fs *flag.FlagSet, env []string, cfg map[string]string, args []s
 			return
 		}
 
+		if f.DefValue != f.Value.String() {
+			if _, ok := f.Value.(MutableFlagValue); !ok {
+				return
+			}
+		}
+
 		var next string
 		if val, found := environ[strings.ToUpper(strings.ReplaceAll(f.Name, "-", "_"))]; found {
 			next = val
@@ -102,6 +108,13 @@ func InitFlagSet(fs *flag.FlagSet, env []string, cfg map[string]string, args []s
 	return err
 }
 
+// MutableFlagValue is used to signal whether it is safe to set a flag
+// to another value if it has already been set before, i.e. if its
+// current value (as a string) is the same as its default value.
+type MutableFlagValue interface {
+	MutableFlagValue()
+}
+
 // ParseString returns the string passed with no error set.
 func ParseString(s string) (string, error) {
 	return s, nil
diff --git a/flag_test.go b/flag_test.go
index 26568d6..1f36b63 100644
--- a/flag_test.go
+++ b/flag_test.go
@@ -110,4 +110,13 @@ func TestInitFlagSet(s *testing.T) {
 			t.AssertEqual(tc.expStr, *fm)
 		})
 	}
+
+	t.Run("NoMutableFlagValue", func(t *core.T) {
+		fs := flag.NewFlagSet("", flag.PanicOnError)
+		fi := fs.Int("int", 0, "")
+		t.AssertErrorIs(nil, core.InitFlagSet(fs, nil, nil, []string{"-int=42"}))
+		t.AssertEqual(42, *fi)
+		t.AssertErrorIs(nil, core.InitFlagSet(fs, nil, nil, []string{"-int=21"}))
+		t.AssertEqual(42, *fi)
+	})
 }