Sounds great!
There more than 3000 lines in cryptohome/service.cc and many of them are duplicates.
Removing one member of the Mount family should make the code easier to maintain.

Progress update:

WIP CL (the remaining task is to add tests) is here.
https://chromium-review.googlesource.com/#/q/topic:MountExPublic

And for Chrome side:
https://chromium-review.googlesource.com/#/c/527754/

+fukino@: #2 and the above CL together should be enough for switching the Kiosk mounting to
return OLD_ENCRYPTION errors when it detected ecryptfs.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/system_api/+/e79b0c771217d6b84b85c692ae5740df8e8a2b36

commit e79b0c771217d6b84b85c692ae5740df8e8a2b36
Author: Kazuhiro Inaba <kinaba@chromium.org>
Date: Thu Jun 22 10:34:49 2017

cryptohome: Protobuf change to support public mount in MountEx.

BUG= chromium:729905 
TEST=None

Change-Id: I7ef3efdfbdfd07e1055b1a026034aa16ae47e662
Reviewed-on: https://chromium-review.googlesource.com/526839
Commit-Ready: Kazuhiro Inaba <kinaba@chromium.org>
Tested-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>

[modify] https://crrev.com/e79b0c771217d6b84b85c692ae5740df8e8a2b36/dbus/cryptohome/rpc.proto

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2e0c3f8760c89523c7a513258cd519155c21a523

commit 2e0c3f8760c89523c7a513258cd519155c21a523
Author: Kazuhiro Inaba <kinaba@chromium.org>
Date: Fri Jun 23 05:42:39 2017

cros_system_api: DEPS rolls for supporting public mount in MountEx.

It will be used from:
https://chromium-review.googlesource.com/c/527016/

BUG= 729905 
TEST=it builds.

Change-Id: I7344ed4f86705a7f46b7f6b5791f340fabb94157
Reviewed-on: https://chromium-review.googlesource.com/544453
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Queue: Kazuhiro Inaba <kinaba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481813}
[modify] https://crrev.com/2e0c3f8760c89523c7a513258cd519155c21a523/DEPS

2 out of 4 changes has landed. 2 more is needed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform2/+/d03de8e4b1ab3eab2bfbb924342b02f176b50b08

commit d03de8e4b1ab3eab2bfbb924342b02f176b50b08
Author: Kazuhiro Inaba <kinaba@chromium.org>
Date: Sat Jun 24 05:56:47 2017

cryptohome: Support public mount in MountEx.

This is meant to replace and deprecate the legacy AsyncMountPublic
method. The expectation is that MountEx with public_mount=true is
behaving as equal as AsyncMountPublic.

BUG= chromium:729905 
TEST=cros_workon_make --board=reef cryptohome --test
TEST=Manually confirmed ARC Kiosk sign-in, with exiting ARC Kiosk
 user cryptohome and without it (fresh cryptohome creation), with
 and without --direncryption enabled.
CQ-DEPEND=CL:526839

Change-Id: I5842dc738fcc6fabaa81c412d4ea7f390ff2801a
Reviewed-on: https://chromium-review.googlesource.com/527016
Commit-Ready: Kazuhiro Inaba <kinaba@chromium.org>
Tested-by: Kazuhiro Inaba <kinaba@chromium.org>
Reviewed-by: Kazuhiro Inaba <kinaba@chromium.org>

[modify] https://crrev.com/d03de8e4b1ab3eab2bfbb924342b02f176b50b08/cryptohome/service.cc
[modify] https://crrev.com/d03de8e4b1ab3eab2bfbb924342b02f176b50b08/cryptohome/service_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/11b33cca76d28e36634564175d023af0474d8ae0

commit 11b33cca76d28e36634564175d023af0474d8ae0
Author: Kazuhiro Inaba <kinaba@chromium.org>
Date: Wed Jun 28 00:53:34 2017

cryptohome: Switch the use of AsyncMountPublic() to MountEx().

The former has been used for Kiosk sing-in, but it is legacy and
doesn't support recent enhancement in MountEx() to support the
filesystem encryption migration.

As a preparation to support migration for ARK Kiosk user,
this CL first converts the uses of AsyncMountPublic to MountEx.

This change depends on the following ChromiumOS-side changes:
https://chromium-review.googlesource.com/#/q/topic:MountExPublic

BUG= 729905 
TEST=Manually confirmed ARC Kiosk sign-in, with exiting ARC Kiosk
 user cryptohome and without it (fresh cryptohome creation), with
 and without --direncryption enabled.

Change-Id: I9e02482eef3299ed497ac5e408cc0d4b3a08a951
Reviewed-on: https://chromium-review.googlesource.com/527754
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Darren Krahn <dkrahn@chromium.org>
Commit-Queue: Kazuhiro Inaba <kinaba@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482828}
[modify] https://crrev.com/11b33cca76d28e36634564175d023af0474d8ae0/chromeos/cryptohome/cryptohome_parameters.h
[modify] https://crrev.com/11b33cca76d28e36634564175d023af0474d8ae0/chromeos/cryptohome/homedir_methods.cc
[modify] https://crrev.com/11b33cca76d28e36634564175d023af0474d8ae0/chromeos/login/auth/cryptohome_authenticator.cc