fix: implement driver.DriverContext interface (#365)

Change OpenConnector return type from *Connector to driver.Connector
so that Driver properly satisfies the driver.DriverContext interface
as recommended by database/sql/driver documentation.

Add compile-time interface assertion to prevent future regressions.

Update internal test to use type assertion for concrete Connector
access.

Fixes #236

Author: dlevy-msft-sql

bad_server_test.go

@@ -13,9 +13,10 @@ import (
 func testConnectionBad(t *testing.T, connStr string) (err error) {
 	conn, err := sql.Open("mssql", connStr)
 	if err != nil {
-		// should not fail here
-		t.Fatal("Open connection failed:", err.Error())
-		return
+		// With DriverContext, sql.Open calls OpenConnector which parses the
+		// DSN eagerly. Some protocol dialers (e.g. shared memory) reject
+		// invalid hosts during parsing, so Open itself may fail.
+		return err
 	}
 	defer conn.Close()
 	row := conn.QueryRow("select 1")

mssql.go

@@ -65,7 +65,7 @@ type Driver struct {
 }
 
 // OpenConnector opens a new connector. Useful to dial with a context.
-func (d *Driver) OpenConnector(dsn string) (*Connector, error) {
+func (d *Driver) OpenConnector(dsn string) (driver.Connector, error) {
 	params, err := msdsn.Parse(dsn)
 	if err != nil {
 		return nil, err
@@ -1196,6 +1196,7 @@ func (r *Result) RowsAffected() (int64, error) {
 	return r.rowsAffected, nil
 }
 
+var _ driver.DriverContext = &Driver{}
 var _ driver.Pinger = &Conn{}
 
 // Ping is used to check if the remote server is available and satisfies the Pinger interface.

queries_go110_test.go

@@ -19,10 +19,11 @@ func TestSessionInitSQL(t *testing.T) {
 	tl := testLogger{t: t}
 	defer tl.StopLogging()
 	d := &Driver{logger: optionalLogger{loggerAdapter{&tl}}}
-	connector, err := d.OpenConnector(makeConnStr(t).String())
+	driverConnector, err := d.OpenConnector(makeConnStr(t).String())
 	if err != nil {
 		t.Fatal("unable to open connector", err)
 	}
+	connector := driverConnector.(*Connector)
 
 	// Do not use these settings in your application
 	// unless you know what they do.