Comment 1 for bug 1727699

Hi Tom,
on the version originally release in Xenial I see:

3.1.0-3:
#submission inet n - y - - smtpd

3.1.0-3ubuntu0.1
#submission inet n - y - - smtpd

After the upgrade it still is the same, and it is a comment in both.
There is another similar line related, which is:

3.1.0-3:
smtp inet n - y - - smtpd

3.1.0-3ubuntu0.1
smtp inet n - y - - smtpd

The switch form "-" to "y" means swicth from built-in default to an explicit yes, the column is for using a chroot. The doc you are referring is exactly documenting that:
  Chroot (default: Postfix >= 3.0: n, Postfix <3.0: y)
That does not mean "set y / n in that file" but instead it means if you have set "-" then depending on the version this is what you get.

Since on my upgrade I didn't see the change I checked for a potential Trusty->Xenial upgrade.
There I found the config change (as upstream changed defaults from 2->3).

On trusty it had:
2.11.0-1ubuntu1:
smtp inet n - - - - smtpd
[...]
#submission inet n - - - - smtpd

So on 2.x it had "-" which was implying the default (which was yes), but on 3.x it is "y" (as the default is no now) to have no change in behavior upon upgrade.

Since "submission" is a comment anyway I looked at what happens on upgrade.
Note: This is no normal conffile, it is handled by the *inst scripts.

So I took a trusty system, modified the conf to have the line uncommented.
Then I upgraded.
There the file is as it was before and the admin has to fix it u on upgrade.
But I think this was not forgotten, instead it is just too complex to convert the old config to a surely working new one.

So yes going from Trusty to Xenial if you had "-" set (which was default yes) it now changes its meaning to default "no". The defaults of a new install are good, and I found the following from the changelog:
postfix (3.1.0-1) unstable; urgency=medium

  [LaMont Jones]

  * Explicitly chroot services that we want chrooted in master.cf on fresh
    installs.
  * Convert defaults as needed for 3.0+ on upgrade to minimize compatibility
    warnings.

Not sure if there is better handling that I overlook.
Subscribing LaMont for an opinion.