@ddstreet here's how I reproduced: I created a VirtualBox VM with Xenial and 3 interfaces: enp0s3 and enp0s8 on an internal network, enp0s9 on a bridge to my LAN). Then I applied each of the 4 configurations below and ran "ifup -a". Try it and you'll see the same behavior.

You are correct: the proper way to bring up the bond is to bring up it's slaves. Running ifup on the bond just hangs as you have already established. This is an entirely different bug I guess, but not my main concern right now. I've had to use "bond-master" for the enp0sX interfaces and set "bond-slaves none" for the bond to get it to work. Guess we need support for setting bond-master and bond-slaves at the same time to be bale to bring up the bond both by bringing up a slave or by bringing up the bond itself.

Just to summarize: it is a duplicate of the other bug and it is fixed by your patch!

==========================================

auto lo
iface lo inet loopback

auto enp0s9
iface enp0s9 inet static
   mtu 1500
   address 192.168.1.9
   gateway 192.168.1.1
   netmask 255.255.255.0
   dns-nameservers 1.1.1.1

auto enp0s3
iface enp0s3 inet manual                                                                                                                                                                                             
   mtu 1500
   bond-master bo-adm
   bond-primary enp0s3

auto enp0s8
iface enp0s8 inet manual                                                                                                                                                                                             
   mtu 1500
   bond-master bo-adm

auto bo-adm
iface bo-adm inet static
   mtu 1500
   address 10.10.10.3
   netmask 255.255.0.0
   bond-miimon 100
   bond-mode active-backup
   bond-slaves none
   bond-downdelay 200
   bond-updelay 200

auto bo-adm.2                                                                                                                                                                                                       
iface bo-adm.2 inet static
   mtu 1500
   address 10.11.10.3
   netmask 255.255.0.0
   vlan-raw-device bo-adm

==========================================

auto lo
iface lo inet loopback

auto enp0s9
iface enp0s9 inet static
   mtu 1500
   address 192.168.1.9
   gateway 192.168.1.1
   netmask 255.255.255.0
   dns-nameservers 1.1.1.1

auto enp0s3
iface enp0s3 inet manual                                                                                                                                                                                             
   mtu 1500
   bond-master bo-adm
   bond-primary enp0s3

auto enp0s8
iface enp0s8 inet manual                                                                                                                                                                                             
   mtu 1500
   bond-master bo-adm

auto bo-adm
iface bo-adm inet manual
   mtu 1500
   bond-miimon 100
   bond-mode active-backup
   bond-slaves none
   bond-downdelay 200
   bond-updelay 200

auto bo-adm.2                                                                                                                                                                                                       
iface bo-adm.2 inet static
   mtu 1500
   address 10.11.10.3
   netmask 255.255.0.0
   vlan-raw-device bo-adm
   
 ==========================================

 auto lo
 iface lo inet loopback

 auto enp0s9
 iface enp0s9 inet static
    mtu 1500
    address 192.168.1.9
    gateway 192.168.1.1
    netmask 255.255.255.0
    dns-nameservers 1.1.1.1

 auto enp0s3
 iface enp0s3 inet manual
    mtu 1500

 auto enp0s3.2                                                                                                                                                                                                       
 iface enp0s3.2 inet static
    mtu 1500
    address 10.11.10.3
    netmask 255.255.0.0
    vlan-raw-device enp0s3
    
==========================================

auto lo
iface lo inet loopback

auto enp0s9
iface enp0s9 inet static
   mtu 1500
   address 192.168.1.9
   gateway 192.168.1.1
   netmask 255.255.255.0
   dns-nameservers 1.1.1.1

auto enp0s3
iface enp0s3 inet static
   mtu 1500
   address 10.10.10.3
   netmask 255.255.0.0
   bond-miimon 100
   bond-mode active-backup
   bond-slaves none
   bond-downdelay 200
   bond-updelay 200

auto enp0s3.2                                                                                                                                                                                                       
iface enp0s3.2 inet static
   mtu 1500
   address 10.11.10.3
   netmask 255.255.0.0
   vlan-raw-device enp0s3