computers: never file a part marker under its own PC

A PC first seen before the machine-number fix was created with the machine
number as its OWN asset number, and the fix deliberately does not overwrite an
existing PC's asset number. So resolving the reported number can return the
reporting PC itself, and the marker was then filed partof its own PC - which
reads, on the machine page, as the PC being the operation.

The machine-link path already guarded this case; the marker path did not. It
now refuses and says why, naming the repair: rename the PC asset to its
hostname, or create the operation asset.

plugins/computers/plugin.py

@@ -798,19 +798,26 @@ class ComputersPlugin(BasePlugin):
                 rel.isactive = False
 
         operation = self._link_marker_to_operation(
-            markerasset, machinenumber, warnings)
+            markerasset, machinenumber, pcasset, warnings)
 
         return [{'assetid': markerasset.assetid,
                  'assetnumber': markerasset.assetnumber,
                  'operationassetid': operation}]
 
-    def _link_marker_to_operation(self, markerasset, machinenumber, warnings):
+    def _link_marker_to_operation(self, markerasset, machinenumber, pcasset,
+                                  warnings):
         """Make a marker `partof` the operation whose number its PC reports.
 
         Unlike the PC-to-machine link this does NOT contest: an operation can
         hold any number of markers, which is the whole point. Moving a marker to
         another operation archives the old membership rather than deleting it,
         so where a marker used to live stays answerable.
+
+        Refuses to file a marker under the reporting PC. A PC first seen before
+        the machine-number fix was created with the machine number as its OWN
+        asset number, and that is deliberately never overwritten, so looking up
+        the number can return the PC itself. Filing the marker partof its own PC
+        would read, on the machine page, as the PC being the operation.
         """
         from shopdb.api import AssetRelationship, RelationshipType, Asset
 
@@ -834,6 +841,13 @@ class ComputersPlugin(BasePlugin):
             return None
         if operation.assetid == markerasset.assetid:
             return None
+        if pcasset is not None and operation.assetid == pcasset.assetid:
+            warnings.append(
+                'machine number {!r} is this PC\'s own asset number; marker '
+                'not filed under an operation. Rename the PC asset to its '
+                'hostname, or create the operation asset.'.format(
+                    machinenumber))
+            return None
 
         links = AssetRelationship.query.filter(
             AssetRelationship.sourceassetid == markerasset.assetid,