fix: stop expand from recursing forever on a cyclic reference

env.go

@@ -176,12 +176,24 @@ type Options struct {
 	rawEnvVars map[string]string
 }
 
-func (opts *Options) getRawEnv(s string) string {
-	val := opts.rawEnvVars[s]
-	if val == "" {
-		val = opts.Environment[s]
-	}
-	return os.Expand(val, opts.getRawEnv)
+// expand replaces ${var} or $var in val, expanding the values it refers to
+// as well. visiting holds the variables being expanded, so a reference back
+// to one of them (e.g. A=${A}) expands to an empty string instead of
+// recursing until the stack overflows.
+func (opts *Options) expand(val string, visiting map[string]bool) string {
+	return os.Expand(val, func(s string) string {
+		if visiting[s] {
+			return ""
+		}
+		visiting[s] = true
+		defer delete(visiting, s)
+
+		raw := opts.rawEnvVars[s]
+		if raw == "" {
+			raw = opts.Environment[s]
+		}
+		return opts.expand(raw, visiting)
+	})
 }
 
 func defaultOptions() Options {
@@ -601,7 +613,13 @@ func get(fieldParams FieldParams, opts Options) (val string, err error) {
 	)
 
 	if fieldParams.Expand {
-		val = os.Expand(val, opts.getRawEnv)
+		visiting := map[string]bool{}
+		// A reference to this field's own key is a cycle, unless an earlier
+		// field with the same key already cached a value to read.
+		if opts.rawEnvVars[fieldParams.Key] == "" {
+			visiting[fieldParams.Key] = true
+		}
+		val = opts.expand(val, visiting)
 	}
 
 	opts.rawEnvVars[fieldParams.Key] = val

env_test.go

@@ -1051,6 +1051,43 @@ func TestParseExpandWithDefaultOption(t *testing.T) {
 	isEqual(t, "3000:5000", cfg.NoDefault)
 }
 
+func TestParseExpandCyclicReference(t *testing.T) {
+	type config struct {
+		Self     string `env:"SELF,expand"`
+		Indirect string `env:"INDIRECT,expand"`
+		Twice    string `env:"TWICE,expand"`
+	}
+
+	t.Setenv("SELF", "a-${SELF}")
+	t.Setenv("INDIRECT", "b-${LOOP_A}")
+	t.Setenv("LOOP_A", "${LOOP_B}")
+	t.Setenv("LOOP_B", "${LOOP_A}")
+	t.Setenv("TWICE", "${ONE}${ONE}")
+	t.Setenv("ONE", "1")
+
+	cfg := config{}
+	err := Parse(&cfg)
+
+	isNoErr(t, err)
+	isEqual(t, "a-", cfg.Self)
+	isEqual(t, "b-", cfg.Indirect)
+	isEqual(t, "11", cfg.Twice)
+}
+
+func TestParseExpandReadsEarlierFieldWithSameKey(t *testing.T) {
+	type config struct {
+		RawPort string `env:"PORT" envDefault:"3000"`
+		Port    int    `env:"PORT,expand" envDefault:"${PORT}"`
+	}
+
+	var cfg config
+	err := ParseWithOptions(&cfg, Options{Environment: map[string]string{}})
+
+	isNoErr(t, err)
+	isEqual(t, "3000", cfg.RawPort)
+	isEqual(t, 3000, cfg.Port)
+}
+
 func TestParseUnsetRequireOptions(t *testing.T) {
 	type config struct {
 		Password string `env:"PASSWORD,unset,required"`