Merge pull request 'Add $PORT env var overrides for MCP agent port assignment' (#1) from port-env-support into master

config/config.go

@@ -144,6 +144,8 @@ func Load[T any](path string, envPrefix string) (*T, error) {
 		applyEnvToStruct(reflect.ValueOf(&cfg).Elem(), envPrefix)
 	}
 
+	applyPortEnv(&cfg)
+
 	applyBaseDefaults(&cfg)
 
 	if err := validateBase(&cfg); err != nil {
@@ -239,6 +241,70 @@ func findBase(cfg any) *Base {
 	return nil
 }
 
+// applyPortEnv overrides ServerConfig.ListenAddr and ServerConfig.GRPCAddr
+// from $PORT and $PORT_GRPC respectively. These environment variables are
+// set by the MCP agent to assign authoritative port bindings, so they take
+// precedence over both TOML values and generic env overrides.
+func applyPortEnv(cfg any) {
+	sc := findServerConfig(cfg)
+	if sc == nil {
+		return
+	}
+
+	if port, ok := os.LookupEnv("PORT"); ok {
+		sc.ListenAddr = ":" + port
+	}
+	if port, ok := os.LookupEnv("PORT_GRPC"); ok {
+		sc.GRPCAddr = ":" + port
+	}
+}
+
+// findServerConfig returns a pointer to the ServerConfig in the config
+// struct. It first checks for an embedded Base (which contains Server),
+// then walks the struct tree via reflection to find any ServerConfig field
+// directly (e.g., the Metacrypt pattern where ServerConfig is embedded
+// without Base).
+func findServerConfig(cfg any) *ServerConfig {
+	if base := findBase(cfg); base != nil {
+		return &base.Server
+	}
+
+	return findServerConfigReflect(reflect.ValueOf(cfg))
+}
+
+// findServerConfigReflect walks the struct tree to find a ServerConfig field.
+func findServerConfigReflect(v reflect.Value) *ServerConfig {
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+	if v.Kind() != reflect.Struct {
+		return nil
+	}
+
+	scType := reflect.TypeOf(ServerConfig{})
+	t := v.Type()
+	for i := range t.NumField() {
+		field := t.Field(i)
+		fv := v.Field(i)
+
+		if field.Type == scType {
+			sc, ok := fv.Addr().Interface().(*ServerConfig)
+			if ok {
+				return sc
+			}
+		}
+
+		// Recurse into embedded or nested structs.
+		if fv.Kind() == reflect.Struct && field.Type != scType {
+			if sc := findServerConfigReflect(fv); sc != nil {
+				return sc
+			}
+		}
+	}
+
+	return nil
+}
+
 // applyEnvToStruct recursively walks a struct and overrides field values
 // from environment variables. The env variable name is built from the
 // prefix and the toml tag: PREFIX_SECTION_FIELD (uppercased).

config/config_test.go

@@ -401,3 +401,109 @@ func TestEmptyEnvPrefix(t *testing.T) {
 		t.Fatalf("ListenAddr = %q, want %q", cfg.Server.ListenAddr, ":8443")
 	}
 }
+
+// directServerConfig embeds ServerConfig without Base (Metacrypt pattern).
+type directServerConfig struct {
+	Server ServerConfig `toml:"server"`
+	Extra  string       `toml:"extra"`
+}
+
+func TestPortEnvOverridesListenAddr(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+	t.Setenv("PORT", "9999")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Fatalf("ListenAddr = %q, want %q", cfg.Server.ListenAddr, ":9999")
+	}
+}
+
+func TestPortGRPCEnvOverridesGRPCAddr(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+	t.Setenv("PORT_GRPC", "9998")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.GRPCAddr != ":9998" {
+		t.Fatalf("GRPCAddr = %q, want %q", cfg.Server.GRPCAddr, ":9998")
+	}
+}
+
+func TestPortEnvOverridesTOMLValue(t *testing.T) {
+	// fullTOML sets listen_addr = ":8443" and grpc_addr = ":9443".
+	path := writeTOML(t, fullTOML)
+	t.Setenv("PORT", "9999")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Fatalf("ListenAddr = %q, want %q ($PORT should override TOML)", cfg.Server.ListenAddr, ":9999")
+	}
+}
+
+func TestPortEnvOverridesGenericEnv(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+	t.Setenv("TEST_SERVER_LISTEN_ADDR", ":7777")
+	t.Setenv("PORT", "9999")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Fatalf("ListenAddr = %q, want %q ($PORT should override generic env)", cfg.Server.ListenAddr, ":9999")
+	}
+}
+
+func TestNoPortEnvNoChange(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	// minimalTOML sets listen_addr = ":8443", no $PORT set.
+	if cfg.Server.ListenAddr != ":8443" {
+		t.Fatalf("ListenAddr = %q, want %q (TOML value preserved without $PORT)", cfg.Server.ListenAddr, ":8443")
+	}
+}
+
+func TestPortEnvDirectServerConfig(t *testing.T) {
+	// Test the Metacrypt pattern: ServerConfig embedded without Base.
+	toml := `
+[server]
+listen_addr = ":8443"
+tls_cert = "/tmp/cert.pem"
+tls_key = "/tmp/key.pem"
+grpc_addr = ":9443"
+
+extra = "value"
+`
+	path := writeTOML(t, toml)
+	t.Setenv("PORT", "5555")
+	t.Setenv("PORT_GRPC", "5556")
+
+	cfg, err := Load[directServerConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":5555" {
+		t.Fatalf("ListenAddr = %q, want %q ($PORT on direct ServerConfig)", cfg.Server.ListenAddr, ":5555")
+	}
+	if cfg.Server.GRPCAddr != ":5556" {
+		t.Fatalf("GRPCAddr = %q, want %q ($PORT_GRPC on direct ServerConfig)", cfg.Server.GRPCAddr, ":5556")
+	}
+}