Created attachment 206288 [details]
Test program: runs fine if CUPS is started, always hangs if CUPS is stopped

As the gtk_enumerate_printers() documentation says, the destroy argument is a function called when data is no longer needed. However, if

- CUPS is supported by GTK,
- and it is listed as the last on the list of the printing backends,
- and CUPS is not installed, not running, or not configured properly,

then the destroy notification callback is never called.

This happens because gtk_enumerate_printers() calls list_printers_init() and if list_printers_init() fails then it just returns. list_printers_init() may fail if the print backend reports status GTK_PRINT_BACKEND_STATUS_UNAVAILABLE. If this happens, list_printers_init() removes the backend from the printer_list->backends list. If this backend is the only item in this list then printer_list->backends becomes NULL. In all other places if printer_list->backends becomes NULL (or is never initialized) then free_printer_list(printer_list) is called which calls the destroy notification callback. However, in this case this callback is not called and the function just quits.

As a consequence:
- if the callback is used to free the allocated memory then a memory leak occurs,
- if the callback is used to notify the caller that all printers have been enumerated and the caller waits for the callback then caller hangs.

I attach a simple test program which runs fine if CUPS is running but always hangs if CUPS is turned off. Here is a copy of the test session:

=========================================================================
$ ./printer_hangs 
This program hangs if CUPS subsystem is not running correctly
Enumerating your printers:
Printer name: Print to File
destroy_notify called
destroy_notify already called, end of the program
$ su
Password: 
# /etc/init.d/cups stop
Stopping cups:                                             [  OK  ]
# exit
$ ./printer_hangs 
This program hangs if CUPS subsystem is not running correctly
Enumerating your printers:
Printer name: Print to File
destroy_notify not yet called, waiting
^C
$ su
Password: 
# /etc/init.d/cups start
Starting cups:                                             [  OK  ]
# exit
$ ./printer_hangs 
This program hangs if CUPS subsystem is not running correctly
Enumerating your printers:
Printer name: Print to File
destroy_notify called
destroy_notify already called, end of the program
$ 
=========================================================================

I have tested this on 3 different machines and the result is always the same. I have also checked the latest sources and they are same as 2.x series so I believe the behavior in 3.x series is the same.

You can compile the test program with:

gcc -o printer_hangs `pkg-config --cflags --libs gtk+-2.0 gtk+-unix-print-2.0` printer_hangs.c