Created attachment 231344 [details] [review]
printers: Avoid possible crash

It can happen that "data->ppd_file_name" is unlinked and freed in install_missing_executables_cb() and again in get_missing_executables_cb().
You can reproduce this by adding new printer in Printers panel when runnnig gnome-control-center in valgrind and looking for errors.

==28050== 2 errors in context 6 of 6:
==28050== Invalid read of size 8
==28050==    at 0x13A90B3A: get_missing_executables_cb (pp-new-printer.c:1164)
==28050==    by 0x320B06DFF6: g_simple_async_result_complete (gsimpleasyncresult.c:775)
==28050==    by 0x320B0BE901: g_dbus_connection_call_done (gdbusconnection.c:5339)
==28050==    by 0x320B06DFF6: g_simple_async_result_complete (gsimpleasyncresult.c:775)
==28050==    by 0x320B06E0F8: complete_in_idle_cb (gsimpleasyncresult.c:787)
==28050==    by 0x3205C47A74: g_main_context_dispatch (gmain.c:2715)
==28050==    by 0x3205C47DA7: g_main_context_iterate.isra.24 (gmain.c:3290)
==28050==    by 0x3205C47E63: g_main_context_iteration (gmain.c:3351)
==28050==    by 0x320B09A7EB: g_application_run (gapplication.c:1624)
==28050==    by 0x408244: main (control-center.c:259)
==28050==  Address 0x1bedb548 is 24 bytes inside a block of size 48 free'd
==28050==    at 0x4A077A6: free (vg_replace_malloc.c:446)
==28050==    by 0x3205C4D7BE: g_free (gmem.c:252)
==28050==    by 0x13A90338: install_missing_executables_cb (pp-new-printer.c:940)
==28050==    by 0x13A90AEA: get_missing_executables_cb (pp-new-printer.c:1158)
==28050==    by 0x320B06DFF6: g_simple_async_result_complete (gsimpleasyncresult.c:775)
==28050==    by 0x320B0BE901: g_dbus_connection_call_done (gdbusconnection.c:5339)
==28050==    by 0x320B06DFF6: g_simple_async_result_complete (gsimpleasyncresult.c:775)
==28050==    by 0x320B06E0F8: complete_in_idle_cb (gsimpleasyncresult.c:787)
==28050==    by 0x3205C47A74: g_main_context_dispatch (gmain.c:2715)
==28050==    by 0x3205C47DA7: g_main_context_iterate.isra.24 (gmain.c:3290)
==28050==    by 0x3205C47E63: g_main_context_iteration (gmain.c:3351)
==28050==    by 0x320B09A7EB: g_application_run (gapplication.c:1624)
==28050==    by 0x408244: main (control-center.c:259)

The problem is present since 3.6. Attached patch fixes it for me.

Marek