test: add config and priority fuzz tests

fuzz_config_test.go

@@ -0,0 +1,98 @@
+package viper
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func runConfigFuzz(tb testing.TB, data []byte, configType string) {
+	if len(data) > 1<<20 {
+		return
+	}
+
+	v := New()
+	v.SetConfigType(configType)
+
+	if err := v.ReadConfig(bytes.NewReader(data)); err != nil {
+		return
+	}
+
+	allKeys := v.AllKeys()
+	for _, key := range allKeys {
+		// Exercise various getters to maximize code coverage
+		_ = v.Get(key)
+		_ = v.GetString(key)
+		_ = v.GetInt(key)
+		_ = v.GetBool(key)
+		_ = v.GetStringSlice(key)
+		_ = v.IsSet(key)
+	}
+	v.AllSettings()
+	_ = v.Unmarshal(&map[string]any{})
+}
+
+func FuzzConfigYAML(f *testing.F) {
+	f.Add([]byte(`key: value`))
+	f.Add([]byte(`nested: { key: value }`))
+	f.Add([]byte(`key: null`))
+	f.Add([]byte(strings.Repeat("a: b\n", 100)))
+	f.Add([]byte(``))
+	f.Add([]byte(`---`))
+	f.Add([]byte(`~`))
+
+	f.Fuzz(func(t *testing.T, data []byte) {
+		runConfigFuzz(t, data, "yaml")
+	})
+}
+
+func FuzzConfigJSON(f *testing.F) {
+	f.Add([]byte(`{"key": "value"}`))
+	f.Add([]byte(`{}`))
+	f.Fuzz(func(t *testing.T, data []byte) {
+		runConfigFuzz(t, data, "json")
+	})
+}
+
+func FuzzConfigTOML(f *testing.F) {
+	f.Add([]byte(`key = "value"`))
+	f.Add([]byte(``))
+	f.Fuzz(func(t *testing.T, data []byte) {
+		runConfigFuzz(t, data, "toml")
+	})
+}
+
+func FuzzConfigPriority(f *testing.F) {
+	f.Add([]byte(`target_key: "file_val"`), "env_val", "def_val")
+
+	f.Fuzz(func(t *testing.T, data []byte, envVal string, defaultVal string) {
+		if strings.ContainsAny(envVal, "=\n\r\x00") {
+			return
+		}
+
+		const envKey = "VIPER_FUZZ_ENV_KEY"
+		t.Setenv(envKey, envVal)
+
+		v := New()
+		v.SetConfigType("yaml")
+		v.SetDefault("target_key", defaultVal)
+
+		if err := v.BindEnv("target_key", envKey); err != nil {
+			t.Fatal(err)
+		}
+
+		_ = v.ReadConfig(bytes.NewReader(data))
+
+		// Verify priority: Environment variables should override config and defaults
+		got := v.GetString("target_key")
+		if envVal != "" {
+			if got != envVal {
+				t.Errorf("Priority error: Env should win. got %q, want %q", got, envVal)
+			}
+		}
+
+		_ = v.GetInt("target_key")
+		_ = v.GetBool("target_key")
+		_ = v.GetStringSlice("target_key")
+	})
+}