discourse.plugins: Make the updater able to package plugins

Let the update.py script handle the initial, repetitive task of
packaging new plugins. With this in place, the plugin only needs to be
added to the list in `update-plugins` and most of the work will be
done automatically when the script is run. Metadata still needs to be
filled in manually and some packages may of course require additional
work/patching.

(cherry picked from commit f8096460bd15d4f13a01cfddf0a30798921fdb42)

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

@@ -284,11 +284,22 @@ services.discourse = {
       Ruby dependencies are listed in its
       <filename>plugin.rb</filename> file as function calls to
       <literal>gem</literal>. To construct the corresponding
-      <filename>Gemfile</filename>, run <command>bundle
+      <filename>Gemfile</filename> manually, run <command>bundle
       init</command>, then add the <literal>gem</literal> lines to it
       verbatim.
     </para>
 
+    <para>
+      Much of the packaging can be done automatically by the
+      <filename>nixpkgs/pkgs/servers/web-apps/discourse/update.py</filename>
+      script - just add the plugin to the <literal>plugins</literal>
+      list in the <function>update_plugins</function> function and run
+      the script:
+      <programlisting language="bash">
+./update.py update-plugins
+</programlisting>.
+    </para>
+
     <para>
       Some plugins provide <link
       linkend="module-services-discourse-site-settings">site

pkgs/servers/web-apps/discourse/update.py

@@ -12,6 +12,7 @@ import os
 import stat
 import json
 import requests
+import textwrap
 from distutils.version import LooseVersion
 from pathlib import Path
 from typing import Iterable
@@ -77,7 +78,11 @@ def _call_nix_update(pkg, version):
 
 def _nix_eval(expr: str):
     nixpkgs_path = Path(__file__).parent / '../../../../'
-    return json.loads(subprocess.check_output(['nix', 'eval', '--json', f'(with import {nixpkgs_path} {{}}; {expr})'], text=True))
+    try:
+        output = subprocess.check_output(['nix', 'eval', '--json', f'(with import {nixpkgs_path} {{}}; {expr})'], text=True)
+    except subprocess.CalledProcessError:
+        return None
+    return json.loads(output)
 
 
 def _get_current_package_version(pkg: str):
@@ -206,13 +211,59 @@ def update_plugins():
         repo_name = plugin.get('repo_name') or name
 
         repo = DiscourseRepo(owner=owner, repo=repo_name)
+
+        filename = _nix_eval(f'builtins.unsafeGetAttrPos "src" discourse.plugins.{name}')
+        if filename is None:
+            filename = Path(__file__).parent / 'plugins' / name / 'default.nix'
+            filename.parent.mkdir()
+
+            has_ruby_deps = False
+            for line in repo.get_file('plugin.rb', repo.latest_commit_sha).splitlines():
+                if 'gem ' in line:
+                    has_ruby_deps = True
+                    break
+
+            with open(filename, 'w') as f:
+                f.write(textwrap.dedent(f"""
+                         {{ lib, mkDiscoursePlugin, fetchFromGitHub }}:
+
+                         mkDiscoursePlugin {{
+                           name = "{name}";"""[1:] + ("""
+                           bundlerEnvArgs.gemdir = ./.;""" if has_ruby_deps else "") + f"""
+                           src = {fetcher} {{
+                             owner = "{owner}";
+                             repo = "{repo_name}";
+                             rev = "replace-with-git-rev";
+                             sha256 = "replace-with-sha256";
+                           }};
+                           meta = with lib; {{
+                             homepage = "";
+                             maintainers = with maintainers; [ ];
+                             license = licenses.mit; # change to the correct license!
+                             description = "";
+                           }};
+                         }}"""))
+
+            all_plugins_filename = Path(__file__).parent / 'plugins' / 'all-plugins.nix'
+            with open(all_plugins_filename, 'r+') as f:
+                content = f.read()
+                pos = -1
+                while content[pos] != '}':
+                    pos -= 1
+                content = content[:pos] + f'  {name} = callPackage ./{name} {{}};' + os.linesep + content[pos:]
+                f.seek(0)
+                f.write(content)
+                f.truncate()
+
+        else:
+            filename = filename['file']
+
         prev_commit_sha = _nix_eval(f'discourse.plugins.{name}.src.rev')
 
         if prev_commit_sha == repo.latest_commit_sha:
             click.echo(f'Plugin {name} is already at the latest revision')
             continue
 
-        filename = _nix_eval(f'builtins.unsafeGetAttrPos "src" discourse.plugins.{name}')['file']
         prev_hash = _nix_eval(f'discourse.plugins.{name}.src.outputHash')
         new_hash = subprocess.check_output([
             'nix-universal-prefetch', fetcher,