Add support for database/sql Connector and OpenDB (#749)

* Add support for database/sql Connector and OpenDB

What
===
Add support for `database/sql/driver` `Connector` and `database/sql`
`OpenDB` that were added in go1.10.

Why
===
To allow users to open a connection without using the registered drivers
interface, and instead explicitly getting a `Connector` from the this
driver and passing that connector to `database/sql` `OpenDB`.

Notes
===
- The new types and functions are only built for go1.10 or newer because
the `database/sql/driver` `Connector` interface and `database/sql`
`OpenDB` function are only available in go1.10 or later.
- The `NewConnector` has support for context, but the context isn't used
at this point. Use of it could be added if desired, but that could occur
separately to this change.

connector.go

@@ -0,0 +1,43 @@
+// +build go1.10
+
+package pq
+
+import (
+	"context"
+	"database/sql/driver"
+)
+
+// Connector represents a fixed configuration for the pq driver with a given
+// name. Connector satisfies the database/sql/driver Connector interface and
+// can be used to create any number of DB Conn's via the database/sql OpenDB
+// function.
+//
+// See https://golang.org/pkg/database/sql/driver/#Connector.
+// See https://golang.org/pkg/database/sql/#OpenDB.
+type connector struct {
+	name string
+}
+
+// Connect returns a connection to the database using the fixed configuration
+// of this Connector. Context is not used.
+func (c *connector) Connect(_ context.Context) (driver.Conn, error) {
+	return (&Driver{}).Open(c.name)
+}
+
+// Driver returnst the underlying driver of this Connector.
+func (c *connector) Driver() driver.Driver {
+	return &Driver{}
+}
+
+var _ driver.Connector = &connector{}
+
+// NewConnector returns a connector for the pq driver in a fixed configuration
+// with the given name. The returned connector can be used to create any number
+// of equivalent Conn's. The returned connector is intended to be used with
+// database/sql.OpenDB.
+//
+// See https://golang.org/pkg/database/sql/driver/#Connector.
+// See https://golang.org/pkg/database/sql/#OpenDB.
+func NewConnector(name string) (driver.Connector, error) {
+	return &connector{name: name}, nil
+}

connector_example_test.go

@@ -0,0 +1,33 @@
+// +build go1.10
+
+package pq_test
+
+import (
+	"database/sql"
+	"fmt"
+
+	"github.com/lib/pq"
+)
+
+func ExampleNewConnector() {
+	name := ""
+	connector, err := pq.NewConnector(name)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	db := sql.OpenDB(connector)
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer db.Close()
+
+	// Use the DB
+	txn, err := db.Begin()
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	txn.Rollback()
+}

connector_test.go

@@ -0,0 +1,67 @@
+// +build go1.10
+
+package pq
+
+import (
+	"context"
+	"database/sql"
+	"database/sql/driver"
+	"testing"
+)
+
+func TestNewConnector_WorksWithOpenDB(t *testing.T) {
+	name := ""
+	c, err := NewConnector(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db := sql.OpenDB(c)
+	defer db.Close()
+	// database/sql might not call our Open at all unless we do something with
+	// the connection
+	txn, err := db.Begin()
+	if err != nil {
+		t.Fatal(err)
+	}
+	txn.Rollback()
+}
+
+func TestNewConnector_Connect(t *testing.T) {
+	name := ""
+	c, err := NewConnector(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db, err := c.Connect(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	// database/sql might not call our Open at all unless we do something with
+	// the connection
+	txn, err := db.(driver.ConnBeginTx).BeginTx(context.Background(), driver.TxOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	txn.Rollback()
+}
+
+func TestNewConnector_Driver(t *testing.T) {
+	name := ""
+	c, err := NewConnector(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	db, err := c.Driver().Open(name)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer db.Close()
+	// database/sql might not call our Open at all unless we do something with
+	// the connection
+	txn, err := db.(driver.ConnBeginTx).BeginTx(context.Background(), driver.TxOptions{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	txn.Rollback()
+}