GLib critical "'old_val > 0' failed" in imap-engine-minimal-folder.vala
The stack from the worker thread's perspective:
(gdb) bt
#0 _g_log_abort (breakpoint=1) at ../../../glib/gmessages.c:554
#1 0x00007ffff768a3b9 in g_logv (log_domain=0x7ffff77a0300 "GLib-GObject", log_level=G_LOG_LEVEL_CRITICAL, format=<optimised out>, args=args@entry=0x7fff42ffc560)
at ../../../glib/gmessages.c:1373
#2 0x00007ffff768a583 in g_log
(log_domain=log_domain@entry=0x7ffff77a0300 "GLib-GObject", log_level=log_level@entry=G_LOG_LEVEL_CRITICAL, format=format@entry=0x7ffff76db893 "%s: assertion '%s' failed")
at ../../../glib/gmessages.c:1415
#3 0x00007ffff768ad7d in g_return_if_fail_warning
(log_domain=log_domain@entry=0x7ffff77a0300 "GLib-GObject", pretty_function=pretty_function@entry=0x7ffff77a39d8 <__func__.15627> "g_object_ref", expression=expression@entry=0x7ffff77a277b "old_val > 0") at ../../../glib/gmessages.c:2771
#4 0x00007ffff777399a in g_object_ref (_object=0x555557d099f0) at ../../../gobject/gobject.c:3368
#5 g_object_ref (_object=_object@entry=0x555557d099f0) at ../../../gobject/gobject.c:3360
#6 0x00007ffff7141eca in _g_object_ref0 (self=0x555557d099f0) at hashmap.c:2335
#7 gee_hash_map_real_get_keys (base=0x555557eb4d90 [GeeHashMap]) at hashmap.c:2335
#8 0x00007ffff7d88be7 in ___lambda109_ (_data93_=0x5555557a8340, error=0x7fff42ffc850) at ../src/engine/imap-engine/imap-engine-minimal-folder.vala:595
#9 0x00007ffff7d89031 in ____lambda109__geary_nonblocking_concurrent_concurrent_callback (cancellable=0x7fff3ca935a0 [GCancellable], self=0x5555557a8340, error=0x7fff42ffc850)
at ../src/engine/imap-engine/imap-engine-minimal-folder.vala:594
#10 0x00007ffff7c4a0e3 in geary_nonblocking_concurrent_concurrent_operation_execute (self=0x7fff3c0d12d0 [GearyNonblockingConcurrentConcurrentOperation])
at ../src/engine/nonblocking/nonblocking-concurrent.vala:59
#11 0x00007ffff7c49914 in geary_nonblocking_concurrent_on_work_ready (self=0x555556695850 [GearyNonblockingConcurrent], op=0x7fff3c0d12d0 [GearyNonblockingConcurrentConcurrentOperation])
at ../src/engine/nonblocking/nonblocking-concurrent.vala:140
#12 0x00007ffff7c48ebd in _geary_nonblocking_concurrent_on_work_ready_gfunc (data=0x7fff3c0d12d0, self=0x555556695850) at ../src/engine/nonblocking/nonblocking-concurrent.vala:110
#13 0x00007ffff76ad1d4 in g_thread_pool_thread_proxy (data=<optimised out>) at ../../../glib/gthreadpool.c:354
#14 0x00007ffff76ac931 in g_thread_proxy (data=0x5555575f0d80) at ../../../glib/gthread.c:807
#15 0x00007ffff1638609 in start_thread (arg=<optimised out>) at pthread_create.c:477
#16 0x00007ffff7381153 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95