Created attachment 245651 [details] [review]
possible patch

in g_source_add_child_source() we assert that the child_source has no context. If the source is blocked, we try to block the new child source as well (before attaching it to the context). in block_source we call g_main_context_remove_poll_unlocked() with the child_source context (which is NULL) and we get a segfault.

I believe two things need to be done:

1) when blocking a source without a context, don't call g_main_context_remove_poll_unlocked() with the NULL context. 

2) when calling g_source_attach with a blocked source, don't call 
g_main_context_add_poll_unlocked() this is similar to how g_source_add_poll() and others deals with blocked sources.