[Chicken-hackers] Scheduler fdset assert patch

[Jonathan Chan]

Hello all,

I was in the IRC channel earlier trying to find out why my program was 
failing due to a scheduler assertion about the fdset for poll. This 
would happen consistently when I sent SIGINT.

The error looks like:

scheduler.c:51: C_fd_ready: Assertion `fd == C_fdset_set[pos].fd' failed

After looking at the scheduler code, it looks like this happened because 
the SIGINT was causing the interrupt handler to be called, which ended 
up unblocking some thread to run a signal handler, which caused a 
condition variable broadcast. In the end, this made a thread that was 
previously blocking on a file descriptor to now block on a mutex.

When create-fdset procedure iterated over the ##sys#fd-list, it skipped 
threads in the list who were not blocking on fds while it added those 
who were to the fdset using fdset-set. However, it did not remove these 
threads from ##sys#fd-list, which resulted in the scheduler throwing an 
error because it expected a fd to be in the fdset when it was not there.

I have attached a patch which modifies create-fdset to remove threads 
from ##sys#fd-list who are not blocking on an fd, rather than only skip 
them. This seems to fix my problem and has not introduced any bizarre 
behvaior in the few tests I have run, but hopefully someone who is more 
familiar with the scheduler could look it over and see if it fixes the 
issue.

Best regards,
--
  Jonathan Chan
  address@hidden

scheduler-assert.diff
Description: Text document