Merge pull request #1605 from mwhudson/explicit-install-media

do not let size: largest in storage autoinstall config match the inst…

documentation/autoinstall-reference.md

@@ -345,6 +345,8 @@ A match spec supports the following keys:
  * `ssd: yes|no`: matches a disk that is or is not an SSD (vs a rotating drive)
  * `size: largest|smallest`: take the largest or smallest disk rather than an arbitrary one if there are multiple matches (support for `smallest` added in version 20.06.1)
 
+A special sort of key is `install-media: yes`, which will take the disk the installer was loaded from (the `ssd` and `size` selectors will never return this disk). If installing to the install media, care obviously needs to be take to not overwrite the installer itself!
+
 So for example, to match an arbitrary disk it is simply:
 
      - type: disk

subiquity/models/filesystem.py

@@ -1185,6 +1185,12 @@ class FilesystemModel(object):
             is_ssd = disk.info_for_display()['rotational'] == 'false'
             return is_ssd == match['ssd']
 
+        def match_install_media(disk):
+            return disk in self._exclusions
+
+        if match.get('install-media', False):
+            matchers.append(match_install_media)
+
         if 'serial' in match:
             matchers.append(match_serial)
         if 'model' in match:
@@ -1209,6 +1215,8 @@ class FilesystemModel(object):
                     break
             else:
                 candidates.append(candidate)
+        if 'size' in match or 'ssd' in match:
+            candidates = [c for c in candidates if c not in self._exclusions]
         if match.get('size') == 'smallest':
             candidates.sort(key=lambda d: d.size)
         if match.get('size') == 'largest':