/etc/apt/apt.conf.d/20dbus is in aptdaemon 0.43+bzr805-0ubuntu10.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | // Notify all clients to reload the cache
APT::Update::Post-Invoke-Success { "[ ! -f /var/run/dbus/system_bus_socket ] || /usr/bin/dbus-send --system --dest=org.debian.apt --type=signal /org/debian/apt org.debian.apt.CacheChanged || true"; };
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