Require Config to be created by ParseConfig

config.go

@@ -26,7 +26,8 @@ import (
 type AfterConnectFunc func(ctx context.Context, pgconn *PgConn) error
 type ValidateConnectFunc func(ctx context.Context, pgconn *PgConn) error
 
-// Config is the settings used to establish a connection to a PostgreSQL server.
+// Config is the settings used to establish a connection to a PostgreSQL server. It must be created by ParseConfig and
+// then it can be modified. A manually initialized Config will cause ConnectConfig to panic.
 type Config struct {
 	Host              string // host (e.g. localhost) or path to unix domain socket directory (e.g. /private/tmp)
 	Port              uint16
@@ -55,6 +56,8 @@ type Config struct {
 
 	// OnNotification is a callback function called when a notification from the LISTEN/NOTIFY system is received.
 	OnNotification NotificationHandler
+
+	createdByParseConfig bool // Used to enforce created by ParseConfig rule.
 }
 
 // FallbackConfig is additional settings to attempt a connection with when the primary Config fails to establish a
@@ -157,10 +160,12 @@ func ParseConfig(connString string) (*Config, error) {
 	}
 
 	config := &Config{
-		Database:      settings["database"],
-		User:          settings["user"],
-		Password:      settings["password"],
-		RuntimeParams: make(map[string]string),
+		createdByParseConfig: true,
+		Database:             settings["database"],
+		User:                 settings["user"],
+		Password:             settings["password"],
+		RuntimeParams:        make(map[string]string),
+		BuildFrontendFunc:    makeDefaultBuildFrontendFunc(),
 	}
 
 	if connectTimeout, present := settings["connect_timeout"]; present {

pgconn.go

@@ -99,25 +99,18 @@ func Connect(ctx context.Context, connString string) (*PgConn, error) {
 	return ConnectConfig(ctx, config)
 }
 
-// Connect establishes a connection to a PostgreSQL server using config. ctx can be used to cancel a connect attempt.
+// Connect establishes a connection to a PostgreSQL server using config. config must have been constructed with
+// ParseConfig. ctx can be used to cancel a connect attempt.
 //
 // If config.Fallbacks are present they will sequentially be tried in case of error establishing network connection. An
 // authentication error will terminate the chain of attempts (like libpq:
 // https://www.postgresql.org/docs/11/libpq-connect.html#LIBPQ-MULTIPLE-HOSTS) and be returned as the error. Otherwise,
 // if all attempts fail the last error is returned.
 func ConnectConfig(ctx context.Context, config *Config) (pgConn *PgConn, err error) {
-	// For convenience set a few defaults if not already set. This makes it simpler to directly construct a config.
-	if config.Port == 0 {
-		config.Port = 5432
-	}
-	if config.DialFunc == nil {
-		config.DialFunc = makeDefaultDialer().DialContext
-	}
-	if config.BuildFrontendFunc == nil {
-		config.BuildFrontendFunc = makeDefaultBuildFrontendFunc()
-	}
-	if config.RuntimeParams == nil {
-		config.RuntimeParams = make(map[string]string)
+	// Default values are set in ParseConfig. Enforce initial creation by ParseConfig rather than setting defaults from
+	// zero values.
+	if !config.createdByParseConfig {
+		panic("config must be created by ParseConfig")
 	}
 
 	// Simplify usage by treating primary config and fallbacks the same.

pgconn_test.go

@@ -263,6 +263,14 @@ func TestConnectWithAfterConnect(t *testing.T) {
 	assert.Equal(t, []byte("foobar"), results[0].Rows[0][0])
 }
 
+func TestConnectConfigRequiresConfigFromParseConfig(t *testing.T) {
+	t.Parallel()
+
+	config := &pgconn.Config{}
+
+	require.PanicsWithValue(t, "config must be created by ParseConfig", func() { pgconn.ConnectConfig(context.Background(), config) })
+}
+
 func TestConnPrepareSyntaxError(t *testing.T) {
 	t.Parallel()