Class exhibiting the problem:

using GLib;

public class TestMutex
{
	protected int thing_to_lock_on;
	public int other_variable;

	public TestMutex()
		{
			other_variable = 0;
		}

	public void a_method()
		{
			lock(thing_to_lock_on)
			{
				other_variable = 1;
			}
		}
}

public static int main(string[] args )
{
	TestMutex tm = new TestMutex();
	tm.a_method();
	return 0;
}

Class with additional private member that does not have the problem:

using GLib;

public class TestMutex2
{
	private int var_to_fix;
	protected int thing_to_lock_on;
	public int other_variable;

	public TestMutex2()
		{
			other_variable = 0;
		}

	public void a_method()
		{
			lock(thing_to_lock_on)
			{
				other_variable = 1;
			}
			var_to_fix=1;
		}
}

public static int main(string[] args )
{
	TestMutex2 tm = new TestMutex2();
	tm.a_method();
	return 0;
}

commit c99412b26c6777230f446e28bc034b2152644aa2
Author: Jürg Billeter <j@bitron.ch>
Date:   Sat Oct 16 10:43:19 2010 +0200

    codegen: Fix lock statements in classes without private fields
    
    Fixes bug 629593.