Don't swallow exceptions in transctional statements

The MySQL connection adapater swallows all StandardError exceptions,
which includes Mysql::Error and Mysql2::Error. The comment in the
exception clause claims errors thrown here indicate that transactions
aren't supported by the server but that isn't necessarily true. It's
possible the MySQL server has gone away and swallowing a failed commit
may let the application return a successful response when the data has
not been saved. Also, replication libraries like Galera require that the
application handle exceptions thrown at BEGIN/COMMIT.

I'm unable to determine what version of MySQL threw an exception for
transactional statements. I tried as far back as 3.23.49 with InnoDB
disabled but BEGIN & COMMIT statements do not throw an error. If there's
a real case for this logic to continue, we could instead push this
behavior into a configuration setting.

The exception swallowing has been there since the beginning:
db045dbb

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -314,27 +314,19 @@ def update_sql(sql, name = nil) #:nodoc:
 
       def begin_db_transaction
         execute "BEGIN"
-      rescue
-        # Transactions aren't supported
       end
 
       def begin_isolated_db_transaction(isolation)
         execute "SET TRANSACTION ISOLATION LEVEL #{transaction_isolation_levels.fetch(isolation)}"
         begin_db_transaction
-      rescue
-        # Transactions aren't supported
       end
 
       def commit_db_transaction #:nodoc:
         execute "COMMIT"
-      rescue
-        # Transactions aren't supported
       end
 
       def rollback_db_transaction #:nodoc:
         execute "ROLLBACK"
-      rescue
-        # Transactions aren't supported
       end
 
       # In the simple case, MySQL allows us to place JOINs directly into the UPDATE

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -468,8 +468,6 @@ def exec_delete(sql, name, binds)
 
       def begin_db_transaction #:nodoc:
         exec_query "BEGIN"
-      rescue Mysql::Error
-        # Transactions aren't supported
       end
 
       private

activerecord/test/cases/adapters/mysql/connection_test.rb

@@ -40,6 +40,11 @@ def test_no_automatic_reconnection_after_timeout
     @connection.update('set @@wait_timeout=1')
     sleep 2
     assert !@connection.active?
+
+    # Repair all fixture connections so other tests won't break.
+    @fixture_connections.each do |c|
+      c.verify!
+    end
   end
 
   def test_successful_reconnection_after_timeout_with_manual_reconnect
@@ -159,6 +164,27 @@ def test_mysql_set_session_variable_to_default
     end
   end
 
+  def test_mysql_begin_db_transaction_can_throw_an_exception
+    @connection.expects(:exec_query).with('BEGIN').raises('OH NOES')
+    assert_raise RuntimeError do
+      @connection.begin_db_transaction
+    end
+  end
+
+  def test_mysql_commit_db_transaction_can_throw_an_exception
+    @connection.expects(:execute).with('COMMIT').raises('OH NOES')
+    assert_raise RuntimeError do
+      @connection.commit_db_transaction
+    end
+  end
+
+  def test_mysql_rollback_db_transaction_can_throw_an_exception
+    @connection.expects(:execute).with('ROLLBACK').raises('OH NOES')
+    assert_raise RuntimeError do
+      @connection.rollback_db_transaction
+    end
+  end
+
   private
 
   def run_without_connection

activerecord/test/cases/adapters/mysql2/connection_test.rb

@@ -18,6 +18,11 @@ def test_no_automatic_reconnection_after_timeout
     @connection.update('set @@wait_timeout=1')
     sleep 2
     assert !@connection.active?
+
+    # Repair all fixture connections so other tests won't break.
+    @fixture_connections.each do |c|
+      c.verify!
+    end
   end
 
   def test_successful_reconnection_after_timeout_with_manual_reconnect
@@ -84,6 +89,27 @@ def test_logs_name_rename_column_sql
     @connection.execute "DROP TABLE `bar_baz`"
   end
 
+  def test_mysql_begin_db_transaction_can_throw_an_exception
+    @connection.expects(:execute).with('BEGIN').raises('OH NOES')
+    assert_raise RuntimeError do
+      @connection.begin_db_transaction
+    end
+  end
+
+  def test_mysql_commit_db_transaction_can_throw_an_exception
+    @connection.expects(:execute).with('COMMIT').raises('OH NOES')
+    assert_raise RuntimeError do
+      @connection.commit_db_transaction
+    end
+  end
+
+  def test_mysql_rollback_db_transaction_can_throw_an_exception
+    @connection.expects(:execute).with('ROLLBACK').raises('OH NOES')
+    assert_raise RuntimeError do
+      @connection.rollback_db_transaction
+    end
+  end
+
   private
 
   def run_without_connection