sys-apps/dbus 1.10.18, which is a current stable, may block boot waiting for entropy due to dev-libs/expat being more strict from version 2.2.1 (which is a current stable) about quality of random used to salt hashing algorithms. In case of systemd (where most os systemd daemons including netword and logind require dbus) and virtual machines (where entropy might be very low) it means that machine may fail to boot at all.
Upstream bug and patch:
Steps to Reproduce:
Depends on entropy available at boot time and is really critical in case of low entropy virtual machines and systemd.
dbus >=1.10.24 has a fix, according to:
Expat >=2.2.3 has a fix in itself, too. The related pull request: