cups-browsed uses GMainLoop and global variables, how to introduce locks against race conditions?

[Till Kamppeter]

Hi,

cups-browsed is a daemon which automatically creates local print queues 
when it discovers remote printers on the network.

For this it has to observe different sources for appropriate events:

- Bonjour: For IPP network printers and for remote CUPS queues of CUPS 
1.6.x or newer
- Legacy CUPS broadcasts: For remote CUPS queues of CUPS 1.5.x or older
- D-Bus notifications: For load-balancing and maintenance of the local 
queues.

In addition, timeouts are used to repeat failed attempts of creating and 
taking down queues, to wait until all jobs are printed before 
auto-shutdown, for expiring legacy CUPS queues, ...

All this is managed via a GLib GMainLoop and the list of discovered 
printers is held in a global variable.

Now there come error reports from time to time, reporting crashes of 
cups-browsed and as I did not find any problem with memory allocation 
and freeing for the data structure I am assuming that the problem is 
missing locking.

Probably (I am not sure how it works exactly) events like appearing or 
disappearing printers, legacy CUPS broadcasts, ... make the main loop 
starting threads to treat the events appropriately, as for example 
adding and removing printers in the list of discovered printers.

Here probably race conditions come up when events happen close to each 
other and the printer list gets manipulated by various threads at the 
same time.

The solution would be to acquire a lock when starting to manipulate the 
printer list and releasing the lock when done.

Now my qestion is, which functions I have to use for acquiring and 
releasing locks when using GLib and GMainLoop? Probably it is not 
correct to use the locks of pthread. Also it is probably best to use 
Read/Write locks where only writing is exclusive but reading is allowed 
to more than one thread at a time.

Any kind of help is welcome.

    Till