for more context, see http://b/114266161

For posterity (we should add this documentation to network_state_handler.h):

The Shill prioritization code is here in Service::Compare:

https://cs.corp.google.com/chromeos_public/src/platform2/shill/service.cc?q=Service::Compare+file:%5Esrc/platform2/shill/+package:%5Echromeos_public$&l=1016

Interpreting that, I believe the order is determined by the following, in order:
(+hendrich@ to verify my interpretation)

Connection State:
 Online
 Connected
 Portal (I am assuming all Portal networks are connected)
 Connecting
 (Other states, i.e. not connected)
 Failed
Connectable (i.e. configured)
Dependency (I think that VPN depends on the underlying network?)
Technology (vpn, ethernet, wifi, wimax, cellular)
Priority
Managed
AutoConnect
Security
Profile
 User profile
 Device profile
Has Ever Connected
SignalStrength
Serial Number (I'm not actually sure what that is).

i.e. SignalStrength determines order for networks where everything above that is the same.


In NetworkStateHandler, we make the following adjustments to the order in NetworkStateHandler::SortNetworkList:

* All Cellular network are listed between "Active" (Online/Connected/Portal) networks and other networks. I believe this is incorrect and the root of this issue; we just want to list any inactive Cellular networks before WiFi networks and rely on Shill to order the active networks correctly. i.e. we can remove this entirely and rely on the next order change.
* Non WiFi (i.e. WiMAX) networks are listed before WiFi networks (since there is generally only 1, maybe 2, Cellular or WiMAX network).
* Hidden (shill::kVisibleProperty = false) networks are listed last (and generally not shown in the UI). I'm not sure that we need to include them at all.

Yes, that network sorting order is correct.

Serial Number is basically the last criteria, where we don't actually care anymore (we might as well use rand() here, but that would give different orderings on consecutive rankings).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bd3bdcc78039cf25f3fccadb5b4bfdf4a5ec2f37

commit bd3bdcc78039cf25f3fccadb5b4bfdf4a5ec2f37
Author: Toni Barzic <tbarzic@chromium.org>
Date: Wed Jan 02 21:37:01 2019

Add tests for network_icon::GetDefaultNetworkImageAndLabel

BUG= 916669 

Change-Id: I53239fc461494bb7af6a35edeecdc2083bde27c3
Reviewed-on: https://chromium-review.googlesource.com/c/1383492
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619502}
[modify] https://crrev.com/bd3bdcc78039cf25f3fccadb5b4bfdf4a5ec2f37/ash/system/network/network_icon_unittest.cc
[modify] https://crrev.com/bd3bdcc78039cf25f3fccadb5b4bfdf4a5ec2f37/chromeos/network/network_state_test.cc
[modify] https://crrev.com/bd3bdcc78039cf25f3fccadb5b4bfdf4a5ec2f37/chromeos/network/network_state_test.h

Note that fix for this landed as:
https://chromium-review.googlesource.com/c/chromium/src/+/1388922