Gfilui

I'm trying to upgrade ubuntu, and I'm getting the following error from do-release-upgrade:

Calculating the changes

Could not determine the upgrade 

An unresolvable problem occurred while calculating the upgrade. 

This can be caused by: 
* Upgrading to a pre-release version of Ubuntu 
* Running the current pre-release version of Ubuntu 
* Unofficial software packages not provided by Ubuntu 

If none of this applies, then please report this bug using the 
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.

There are tons of questions on here about this message, but all of them are about specific issues, and in most cases the answers are just guesses ("try removing X").

Most answers are based on looking at grep Broken /var/log/dist-upgrade/apt.log, but as others have pointed out this lists many packages which the upgrader did actually find a solution for:

Investigating (0) perl-base [ amd64 ] < 5.18.2-2ubuntu1.3 -> 5.22.1-9ubuntu0.2 > ( perl )
Broken perl-base:amd64 Breaks on perl-modules [ amd64 ] < 5.18.2-2ubuntu1.3 > ( perl ) (< 5.22.1~)
 Considering perl-modules:amd64 7 as a solution to perl-base:amd64 5277
 Added perl-modules:amd64 to the remove list
 Fixing perl-base:amd64 via remove of perl-modules:amd64
 MarkDelete perl-modules [ amd64 ] < 5.18.2-2ubuntu1.3 > ( perl ) FU=0

So, my question is, how do I read this file? I want to know how work out for myself specifically which package is causing is the issue.

Is there any documentation that explains this log file?

1. What does Investigating (0), Investigating (1), etc mean? In my log file this goes up to (9). What does this mean?


2. When it says Broken X Breaks on Y, which package is currently installed and which is being upgraded?


3. When it says Considering X as a solution to Y, why does it then say Adding X to the remove list? If X is the solution shouldn't it be removing Y?


4. If it says Holding Back X rather than change Y, how do I find out why?


5. I realise this is probably working through a chain of dependencies until it either finds a solution or gives up. In which case, should I be looking at the first package in the list, the first package with an error, or the last package in the list?

The answer appears to be "look in /var/log/dist-upgrade/main.log":

2018-01-29 11:10:40,663 DEBUG blacklist expr '^postgresql-.*[0-9].[0-9].*' matches 'postgresql-9.3-postgis-2.1'
2018-01-29 11:10:40,663 DEBUG The package 'postgresql-9.3-postgis-2.1' is marked for removal but it's in the removal blacklist
2018-01-29 11:10:40,732 ERROR Dist-upgrade failed: 'The package 'postgresql-9.3-postgis-2.1' is marked for removal but it is in the removal blacklist.'

After removing postgis, the upgrade was able to continue.

I wish do-release-upgrade would just report these errors on its main output.

It would seem that all the Broken messages in apt.log was just apt working through the dependencies, but it did in fact find a solution to them all.