Fix panic when trying to print a nil Stringer (#284)

sywhang · web-flow · commit dcbbb16e8cb3 · 2025-10-03T15:59:27.000-07:00
This fixes an issue where we're trying to print the value of a nil Stringer which causes a panic instead of failing the test, hurting the debuggability of the failure. Fixes #283.
diff --git a/gomock/string.go b/gomock/string.go
@@ -13,11 +13,24 @@ func getString(x any) string {
 		return fmt.Sprintf("%T", x)
 	}
 	if s, ok := x.(fmt.Stringer); ok {
-		return s.String()
+		// Use defer/recover to handle panics from calling String() on nil receivers
+		// This matches the behavior of fmt.Sprintf("%v", x) which handles nil Stringers safely
+		return safeString(s)
 	}
 	return fmt.Sprintf("%v", x)
 }
 
+// safeString calls String() with panic recovery to handle nil receivers
+func safeString(s fmt.Stringer) (result string) {
+	defer func() {
+		if r := recover(); r != nil {
+			// If String() panicked (e.g., nil receiver), use fmt.Sprintf instead
+			result = fmt.Sprintf("%v", s)
+		}
+	}()
+	return s.String()
+}
+
 // isGeneratedMock checks if the given type has a "isgomock" field,
 // indicating it is a generated mock.
 func isGeneratedMock(x any) bool {
diff --git a/gomock/string_test.go b/gomock/string_test.go
@@ -0,0 +1,47 @@
+package gomock
+
+import (
+	"testing"
+	"time"
+)
+
+func TestGetString(t *testing.T) {
+	now := time.Now()
+	var nilTime *time.Time
+
+	tests := []struct {
+		name  string
+		input any
+		want  string
+	}{
+		{
+			name:  "nil stringer should not panic",
+			input: nilTime,
+			want:  "<nil>",
+		},
+		{
+			name:  "non-nil stringer",
+			input: &now,
+			want:  now.String(),
+		},
+		{
+			name:  "non-stringer value",
+			input: 42,
+			want:  "42",
+		},
+		{
+			name:  "nil interface value",
+			input: nil,
+			want:  "<nil>",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := getString(tt.input)
+			if got != tt.want {
+				t.Errorf("getString() = %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
