nixos/nextcloud: update upgrade-path for nextcloud19

Enhance the heuristics to make sure that a user doesn't accidentally
upgrade across two major versions of Nextcloud (e.g. from v17 to v19).

The original idea/discussion has been documented in the nixpkgs manual[1].

This includes the following changes:

* `nextcloud19` will be selected automatically when having a stateVersion
  greater or equal than 20.09. For existing setups, the package has to
  be selected manually to avoid accidental upgrades.

* When using `nextcloud18` or older, a warning will be thrown which recommends
  upgrading to `nextcloud19`.

* Added a brief paragraph about `nextcloud19` in the NixOS 19.09 release
  notes.

* Restart `phpfpm` if the Nextcloud-package (`cfg.package`) changes[2].

[1] https://nixos.org/nixos/manual/index.html#module-services-nextcloud-maintainer-info
[2] https://github.com/NixOS/nixpkgs/pull/89427#issuecomment-638885727

nixos/doc/manual/release-notes/rl-2009.xml

@@ -642,6 +642,16 @@ systemd.services.nginx.serviceConfig.ReadWritePaths = [ "/var/www" ];
       <package>netbeans</package> was upgraded to 12.0 and now defaults to OpenJDK 11. This might cause problems if your projects depend on packages that were removed in Java 11.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     <package>nextcloud</package> has been updated to <link xlink:href="https://nextcloud.com/blog/nextcloud-hub-brings-productivity-to-home-office/">v19</link>.
+    </para>
+    <para>
+     If you have an existing installation, please make sure that you're on
+     <package>nextcloud18</package> before upgrading to <package>nextcloud19</package>
+     since Nextcloud doesn't support upgrades across multiple major versions.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/services/web-apps/nextcloud.nix

@@ -336,7 +336,16 @@ in {
              server, and wait until the upgrade to 17 is finished.
 
           Then, set `services.nextcloud.package` to `pkgs.nextcloud18` to upgrade to
-          Nextcloud version 18.
+          Nextcloud version 18. Please note that Nextcloud 19 is already out and it's
+          recommended to upgrade to nextcloud19 after that.
+        '')
+        ++ (optional (versionOlder cfg.package.version "19") ''
+          A legacy Nextcloud install (from before NixOS 20.09/unstable) may be installed.
+
+          If/After nextcloud18 is installed successfully, you can safely upgrade to
+          nextcloud19. If not, please upgrade to nextcloud18 first since Nextcloud doesn't
+          support upgrades that skip multiple versions (i.e. an upgrade from 17 to 19 isn't
+          possible, but an upgrade from 18 to 19).
         '');
 
       services.nextcloud.package = with pkgs;
@@ -348,7 +357,8 @@ in {
               `pkgs.nextcloud`.
             ''
           else if versionOlder stateVersion "20.03" then nextcloud17
-          else nextcloud18
+          else if versionOlder stateVersion "20.09" then nextcloud18
+          else nextcloud19
         );
     }
 
@@ -360,6 +370,11 @@ in {
       };
 
       systemd.services = {
+        # When upgrading the Nextcloud package, Nextcloud can report errors such as
+        # "The files of the app [all apps in /var/lib/nextcloud/apps] were not replaced correctly"
+        # Restarting phpfpm on Nextcloud package update fixes these issues (but this is a workaround).
+        phpfpm-nextcloud.restartTriggers = [ cfg.package ];
+
         nextcloud-setup = let
           c = cfg.config;
           writePhpArrary = a: "[${concatMapStringsSep "," (val: ''"${toString val}"'') a}]";

nixos/modules/services/web-apps/nextcloud.xml

@@ -161,5 +161,11 @@
   };
 }</programlisting>
   </para>
+
+  <para>
+   Ideally we should make sure that it's possible to jump two NixOS versions forward:
+   i.e. the warnings and the logic in the module should guard a user to upgrade from a
+   Nextcloud on e.g. 19.09 to a Nextcloud on 20.09.
+  </para>
  </section>
 </chapter>