sql: fix missing mutex unlock in an error case

Fixes #2542

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5483054

src/pkg/exp/sql/sql.go

@@ -134,6 +134,7 @@ func (db *DB) maxIdleConns() int {
 func (db *DB) conn() (driver.Conn, error) {
 	db.mu.Lock()
 	if db.closed {
+		db.mu.Unlock()
 		return nil, errors.New("sql: database is closed")
 	}
 	if n := len(db.freeConn); n > 0 {

src/pkg/exp/sql/sql_test.go

@@ -228,3 +228,16 @@ func TestTxStmt(t *testing.T) {
 		t.Fatalf("Commit = %v", err)
 	}
 }
+
+// Tests fix for issue 2542, that we release a lock when querying on
+// a closed connection.
+func TestIssue2542Deadlock(t *testing.T) {
+	db := newTestDB(t, "people")
+	closeDB(t, db)
+	for i := 0; i < 2; i++ {
+		_, err := db.Query("SELECT|people|age,name|")
+		if err == nil {
+			t.Fatalf("expected error")
+		}
+	}
+}