This may sound obvious, but I only just thought of it.
Suppose you make a change to Metacity which causes it to segfault on startup. What you’d ordinarily do is to load it into gdb and have a look at what’s going on in the backtrace with the bt command. But you can’t do that, because it will keep Metacity suspended and so no new window manager will be spawned, and that means that you’ll be running without a window manager. You can get around the problem by sshing into your computer from elsewhere, or by running Xnest or similar, or by using a virtual machine. But here’s a much, much simpler way.
Firstly, create a file called test.gdb containing the text
Then simply give the command
tthurman@haematite:metacity$ gdb src/metacity --batch -x test.gdb
[Thread debugging using libthread_db enabled]
[New Thread 0xb7131720 (LWP 16959)]
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7131720 (LWP 16959)]
0xb7943a6e in g_error_free () from /usr/lib/libglib-2.0.so.0
#0 0xb7943a6e in g_error_free () from /usr/lib/libglib-2.0.so.0
#1 0xb7943adc in g_clear_error () from /usr/lib/libglib-2.0.so.0
#2 0x080a70ba in meta_frame_style_draw (style=0x8119c78, widget=0x8122090, drawable=0x80f4e20, x_offset=0, y_offset=0, clip=0x0, fgeom=0xbfc67384, client_width=1365, client_height=718, title_layout=0x80e1850, text_height=17, button_states=0xbfc67784, mini_icon=0x8123418, icon=0x8123398) at ui/theme.c:4553
#21 0xb79561e7 in g_main_loop_run () from /usr/lib/libglib-2.0.so.0
#22 0x08070992 in main (argc=1, argv=0xbfc68514) at core/main.c:479
Easy as that.