[Bug 1849918] [NEW] Release upgrade disco->eoan fails when Ubuntu mirrors are specified in mirrors:// format

[Philipp Reichmuth]

Public bug reported:

When updating Ubuntu 19.04 (disco) to 19.10 (eoan), ubuntu-release-
upgrader throws the following error about missing package information
and a missing ubuntu-minimal-package (in German):

```
Ungültige Paketinformationen 

Das benötigte Paket »ubuntu-minimal« wurde nach der Aktualisierung 
Ihrer Paketinformationen nicht gefunden. Dies kann daran liegen, dass 
Sie keine offiziellen Spiegel-Server in Ihren Paketquellen 
eingetragen haben oder ein Spiegel-Server überlastet ist. 
Ihre derzeit eingerichteten Paketquellen finden Sie in 
/etc/apt/sources.list. Im Falle eines überlasteten Spiegel-Servers 
können Sie es später erneut versuchen. 
```

In my `/etc/apt/sources.list`, the Ubuntu mirrors are specified through
`mirrors.txt` and `mirrors://` URLs, as in

```
deb mirror://mirrors.ubuntu.com/mirrors.txt/ disco main restricted
...
```

When I change that manually to a regular HTTP URL for some fixed mirror
(e.g. `deb http://de.ubuntu.com/ubuntu/ disco main restricted`), it
works and the upgrade process proceeds normally.

It looks like ubuntu-release-upgrader does not recognize the
`mirrors://` URL format when the package sources from disco to eoan. As
a result during the upgrade process the sources list ends up almost
empty (only InRelease packages), and consequently the ubuntu-minimal
package is not found.

ProblemType: Bug
DistroRelease: Ubuntu 19.04
Package: ubuntu-release-upgrader-core 1:19.04.16.7
ProcVersionSignature: Ubuntu 5.0.0-31.33-generic 5.0.21
Uname: Linux 5.0.0-31-generic x86_64
ApportVersion: 2.20.10-0ubuntu27.1
Architecture: amd64
CrashDB: ubuntu
Date: Sat Oct 26 10:52:49 2019
InstallationDate: Installed on 2018-12-26 (304 days ago)
InstallationMedia: Kubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to disco on 2019-10-26 (0 days ago)
VarLogDistupgradeAptlog:
 Log time: 2019-10-26 10:52:28.812605
 Log time: 2019-10-26 10:52:35.715386
 Log time: 2019-10-26 10:52:49.543592
 Log time: 2019-10-26 10:52:51.766789

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug disco dist-upgrade

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https://bugs.launchpad.net/bugs/1849918

Title:
  Release upgrade disco->eoan fails when Ubuntu mirrors are specified in
  mirrors:// format

Status in ubuntu-release-upgrader package in Ubuntu:
  New

Bug description:
  When updating Ubuntu 19.04 (disco) to 19.10 (eoan), ubuntu-release-
  upgrader throws the following error about missing package information
  and a missing ubuntu-minimal-package (in German):

  ```
  Ungültige Paketinformationen 

  Das benötigte Paket »ubuntu-minimal« wurde nach der Aktualisierung 
  Ihrer Paketinformationen nicht gefunden. Dies kann daran liegen, dass 
  Sie keine offiziellen Spiegel-Server in Ihren Paketquellen 
  eingetragen haben oder ein Spiegel-Server überlastet ist. 
  Ihre derzeit eingerichteten Paketquellen finden Sie in 
  /etc/apt/sources.list. Im Falle eines überlasteten Spiegel-Servers 
  können Sie es später erneut versuchen. 
  ```

  In my `/etc/apt/sources.list`, the Ubuntu mirrors are specified
  through `mirrors.txt` and `mirrors://` URLs, as in

  ```
  deb mirror://mirrors.ubuntu.com/mirrors.txt/ disco main restricted
  ...
  ```

  When I change that manually to a regular HTTP URL for some fixed
  mirror (e.g. `deb http://de.ubuntu.com/ubuntu/ disco main
  restricted`), it works and the upgrade process proceeds normally.

  It looks like ubuntu-release-upgrader does not recognize the
  `mirrors://` URL format when the package sources from disco to eoan.
  As a result during the upgrade process the sources list ends up almost
  empty (only InRelease packages), and consequently the ubuntu-minimal
  package is not found.

  ProblemType: Bug
  DistroRelease: Ubuntu 19.04
  Package: ubuntu-release-upgrader-core 1:19.04.16.7
  ProcVersionSignature: Ubuntu 5.0.0-31.33-generic 5.0.21
  Uname: Linux 5.0.0-31-generic x86_64
  ApportVersion: 2.20.10-0ubuntu27.1
  Architecture: amd64
  CrashDB: ubuntu
  Date: Sat Oct 26 10:52:49 2019
  InstallationDate: Installed on 2018-12-26 (304 days ago)
  InstallationMedia: Kubuntu 18.10 "Cosmic Cuttlefish" - Release amd64 (20181017.2)
  PackageArchitecture: all
  SourcePackage: ubuntu-release-upgrader
  UpgradeStatus: Upgraded to disco on 2019-10-26 (0 days ago)
  VarLogDistupgradeAptlog:
   Log time: 2019-10-26 10:52:28.812605
   Log time: 2019-10-26 10:52:35.715386
   Log time: 2019-10-26 10:52:49.543592
   Log time: 2019-10-26 10:52:51.766789

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