gsd-media-key's sync leaves keys ungrabbed after `dconf update`

There appears to be a problem in keys_sync_continue' sync functionality. Running dconf update causes gsd-media-keys to ungrab and re-grab a bunch of accelerators, but when the dust settles, some are left un-grabbed, and so do not work.

Oddly, some accelerators are un-grabbed and re-grabbed multiple times. While this is not a problem per se, it's an indication that something has gone awry in gsd-media-keys-manager.c's invariants.

Example log of accelerator grab lifetimes attached, pieced together from dbus-monitor logs and running imports.ui.main.shellDBusService._grabbedAccelerators.forEach((k, v) => global.log(k + ' = ' + v)) in lg to get the initial and final grabs. dconf update runs at t=30.

For example, consider the grabs for XF86AudioLowerVolume:

$ grep LowerVolume grab-lifetimes-by-id.txt 
1080  "<Ctrl>XF86AudioLowerVolume"       0        inf
1083  "<Alt>XF86AudioLowerVolume"        0        inf
1085  "<Shift>XF86AudioLowerVolume"      0        inf
1108  "XF86AudioLowerVolume"             0        30.5547
1181  "XF86AudioLowerVolume"             30.5563  30.6864
1227  "XF86AudioLowerVolume"             30.6869  30.7239

The grabs with a modifier are left alone, but the grab with no modifier key is released and re-grabbed twice before being release and left released, causing the XF86AudioLowerVolume key to stop working (but still work when a modifier key is pressed).

dbus-monitor logs for the no-modifier XF86AudioLowerVolume keygrab:

...(98 entries omitted)...

method call time=1581456844.654341 sender=:1.293 -> destination=:1.10 serial=154 path=/org/gnome/Shell; interface=org.gnome.Shell; member=UngrabAccelerator
   uint32 1108
method call time=1581456844.654345 sender=:1.293 -> destination=:1.10 serial=155 path=/org/gnome/Shell; interface=org.gnome.Shell; member=GrabAccelerator
   string "XF86AudioLowerVolume"
   uint32 4294967295
   uint32 0

... (326 entries omitted)...

method return time=1581456845.554723 sender=:1.10 -> destination=:1.293 serial=7402 reply_serial=154
   boolean true

... (2 entries omitted)...

method return time=1581456845.556270 sender=:1.10 -> destination=:1.293 serial=7403 reply_serial=155
   uint32 1181

... (1 entry omitted)...

method call time=1581456845.556593 sender=:1.293 -> destination=:1.10 serial=252 path=/org/gnome/Shell; interface=org.gnome.Shell; member=UngrabAccelerator
   uint32 1181
method call time=1581456845.556612 sender=:1.293 -> destination=:1.10 serial=253 path=/org/gnome/Shell; interface=org.gnome.Shell; member=GrabAccelerator
   string "XF86AudioLowerVolume"
   uint32 4294967295
   uint32 0

... (371 entries omitted)...

method return time=1581456845.686425 sender=:1.10 -> destination=:1.293 serial=7607 reply_serial=252
   boolean true
method return time=1581456845.686872 sender=:1.10 -> destination=:1.293 serial=7608 reply_serial=253
   uint32 1227
method call time=1581456845.686892 sender=:1.293 -> destination=:1.10 serial=316 path=/org/gnome/Shell; interface=org.gnome.Shell; member=UngrabAccelerator
   uint32 1227

... (65 entries omitted)...

method return time=1581456845.723860 sender=:1.10 -> destination=:1.293 serial=7672 reply_serial=316
   boolean true

... (68 entries omitted)...

Version: 3.32.2

successful-grab-lifetimes-by-accelerator.txt

successful-grab-lifetimes-by-id.txt

gsd-media-keys-G_MESSAGES_DEBUG-all-during-dconf-update.txt