From 1bbbeded9d41d4adb8a4f25f3f8296db815a8a84 Mon Sep 17 00:00:00 2001
From: Shirong Lu <73147033+happysnaker@users.noreply.github.com>
Date: Thu, 2 Jul 2026 03:31:46 +0800
Subject: [PATCH] fix: allow empty StringToString values

---
 string_to_string.go      |  8 ++++--
 string_to_string_test.go | 55 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/string_to_string.go b/string_to_string.go
index 518e1366..37fa6db2 100644
--- a/string_to_string.go
+++ b/string_to_string.go
@@ -24,8 +24,9 @@ func newStringToStringValue(val map[string]string, p *map[string]string) *string
 // Set updates the flag value from the given string, adding additional mappings or updating existing ones.
 func (s *stringToStringValue) Set(val string) error {
 	var ss []string
-	n := strings.Count(val, "=")
-	switch n {
+	switch n := strings.Count(val, "="); {
+	case val == "":
+		ss = []string{}
 	case 0:
 		return fmt.Errorf("%s must be formatted as key=value", val)
 	case 1:
@@ -54,6 +55,9 @@ func (s *stringToStringValue) Set(val string) error {
 			delete(*s.value, k)
 		}
 	}
+	if *s.value == nil {
+		*s.value = map[string]string{}
+	}
 
 	for k, v := range out {
 		(*s.value)[k] = v
diff --git a/string_to_string_test.go b/string_to_string_test.go
index d3346d63..91d13728 100644
--- a/string_to_string_test.go
+++ b/string_to_string_test.go
@@ -398,3 +398,58 @@ func TestS2SCalledTwice(t *testing.T) {
 		}
 	}
 }
+
+func TestS2SEmptyValueClearsDefaults(t *testing.T) {
+	var s2s map[string]string
+	f := setUpS2SFlagSetWithDefault(&s2s)
+
+	if err := f.Parse([]string{"--s2s="}); err != nil {
+		t.Fatalf("expected no error; got %v", err)
+	}
+	if s2s == nil {
+		t.Fatal("expected empty map, got nil")
+	}
+	if len(s2s) != 0 {
+		t.Fatalf("expected empty map, got %v", s2s)
+	}
+
+	getS2S, err := f.GetStringToString("s2s")
+	if err != nil {
+		t.Fatalf("got an error from GetStringToString(): %v", err)
+	}
+	if getS2S == nil {
+		t.Fatal("expected empty map from GetStringToString, got nil")
+	}
+	if len(getS2S) != 0 {
+		t.Fatalf("expected empty map from GetStringToString, got %v", getS2S)
+	}
+}
+
+func TestS2SEmptyValueKeepsEmptyDefaultNonNil(t *testing.T) {
+	var s2s map[string]string
+	f := setUpS2SFlagSet(&s2s)
+
+	if err := f.Parse([]string{"--s2s="}); err != nil {
+		t.Fatalf("expected no error; got %v", err)
+	}
+	if s2s == nil {
+		t.Fatal("expected empty map, got nil")
+	}
+	if len(s2s) != 0 {
+		t.Fatalf("expected empty map, got %v", s2s)
+	}
+}
+
+func TestS2SEmptyValueCanBeFollowedByEntries(t *testing.T) {
+	var s2s map[string]string
+	f := setUpS2SFlagSetWithDefault(&s2s)
+
+	if err := f.Parse([]string{"--s2s=", "--s2s=a=1"}); err != nil {
+		t.Fatalf("expected no error; got %v", err)
+	}
+
+	expected := map[string]string{"a": "1"}
+	if !reflect.DeepEqual(s2s, expected) {
+		t.Fatalf("expected %v, got %v", expected, s2s)
+	}
+}