Expose db attribute of MYSQL client struct

Adds an accessor method that returns the current value of the `MSYQL`
client struct's `db` attribute.

The `MYSQL` client struct includes a field `char *db`.  When the
`session_track_schema` setting is enabled, this field will be updated
using information from server-provided "OK" packets, keeping it in sync
as the client switches between databases.

Author: richardlarocque

ext/mysql2/client.c

@@ -1259,6 +1259,26 @@ static VALUE rb_mysql_client_encoding(VALUE self) {
   return wrapper->encoding;
 }
 
+/* call-seq:
+ *    client.db
+ *
+ * Returns the currently selected db.
+ *
+ * The result may be stale if `session_track_schema` is disabled.  Read
+ * https://dev.mysql.com/doc/refman/5.7/en/session-state-tracking.html for more
+ * information.
+ */
+static VALUE rb_mysql_client_db(VALUE self) {
+  GET_CLIENT(self);
+
+  char *db = wrapper->client->db;
+  if (!db) {
+    return Qnil;
+  }
+
+  return rb_str_new_cstr(wrapper->client->db);
+}
+
 /* call-seq:
  *    client.automatic_close?
  *
@@ -1500,6 +1520,7 @@ void init_mysql2_client() {
   rb_define_method(cMysql2Client, "ssl_cipher", rb_mysql_get_ssl_cipher, 0);
   rb_define_method(cMysql2Client, "encoding", rb_mysql_client_encoding, 0);
   rb_define_method(cMysql2Client, "session_track", rb_mysql_client_session_track, 1);
+  rb_define_method(cMysql2Client, "db", rb_mysql_client_db, 0);
 
   rb_define_private_method(cMysql2Client, "connect_timeout=", set_connect_timeout, 1);
   rb_define_private_method(cMysql2Client, "read_timeout=", set_read_timeout, 1);

spec/mysql2/client_spec.rb

@@ -1144,6 +1144,49 @@ def run_gc
     end
   end
 
+  context 'db' do
+    before(:example) do
+      2.times do |i|
+        @client.query("CREATE DATABASE test_db#{i}")
+      end
+    end
+
+    after(:example) do
+      2.times do |i|
+        @client.query("DROP DATABASE test_db#{i}")
+      end
+    end
+
+    it "should be `nil` when no database is selected" do
+      client = new_client(database: nil)
+      expect(client.db).to eq(nil)
+    end
+
+    it "should reflect the initially connected database" do
+      client = new_client(database: 'test_db0')
+      expect(client.db).to eq('test_db0')
+    end
+
+    context "when session tracking is on" do
+      it "should change to reflect currently selected database" do
+        client = new_client(database: 'test_db0')
+        client.query('SET session_track_schema=on')
+        expect { client.query('USE test_db1') }.to change {
+          client.db
+        }.from('test_db0').to('test_db1')
+      end
+    end
+
+    context "when session tracking is off" do
+      it "does not change when a new database is selected" do
+        client = new_client(database: 'test_db0')
+        client.query('SET session_track_schema=off')
+        expect(client.db).to eq('test_db0')
+        expect { client.query('USE test_db1') }.not_to change { client.db }
+      end
+    end
+  end
+
   it "#thread_id should return a boolean" do
     expect(@client.ping).to eql(true)
     @client.close