Fix what the last round of device fixes broke, and two dips it missed

plugins/computers/plugin.py

@@ -415,11 +415,13 @@ class ComputersPlugin(BasePlugin):
         partmarkers = self._sync_partmarker(comp, pctype, machinenumber,
                                             warnings, deviceid=deviceid)
 
-        # PC -> machine link from the reported machine number.
-        if partmarkers:
-            machinelinks = []
-        else:
+        # PC -> machine link from the reported machine number. Only a PC that
+        # is NOT a marker PC (None above) takes it: a marker PC reaches its
+        # operation through `partof` on the marker, never by claiming it.
+        if partmarkers is None:
             machinelinks = self._sync_machine_link(comp, machinenumber, warnings)
+        else:
+            machinelinks = []
 
         # Printer relationship sync (only when the payload carried printer data).
         printerlinks = self._sync_printer_links(comp.asset, payload, warnings)
@@ -444,7 +446,10 @@ class ComputersPlugin(BasePlugin):
                 'measuringtoollinkcount': len(measuringtoollinks),
                 'accessprotocols': accessprotocols,
                 'machinelinks': machinelinks,
-                'partmarkers': partmarkers,
+                # `or []`: None is the internal "not a marker PC" signal and
+                # has no business in the response, where it would read as a
+                # different state from "a marker PC that linked nothing".
+                'partmarkers': partmarkers or [],
             },
         }
 
@@ -856,10 +861,21 @@ class ComputersPlugin(BasePlugin):
         if not controls or not deviceasset or not pcasset:
             return True
 
+        # EVERY collector-owned holder, not just ones under this label. On a
+        # CMM the instrument IS the reported bay, so the incumbent's link is the
+        # machine sync's row under MACHINE_LINK_ORIGIN. Filtering on our own
+        # label found no incumbent there, handed the challenger an active link,
+        # and left two live holders of one instrument - each invisible to the
+        # other because they were labelled differently.
+        #
+        # A row made by hand carries neither label and is deliberately NOT
+        # counted: a person's link is not the collector's to archive.
+        ourlabels = (label, MACHINE_LINK_ORIGIN, MEASURINGTOOL_LINK_ORIGIN,
+                     PARTMARKER_LINK_ORIGIN)
         others = AssetRelationship.query.filter(
             AssetRelationship.targetassetid == deviceasset.assetid,
             AssetRelationship.relationshiptypeid == controls.relationshiptypeid,
-            AssetRelationship.label == label,
+            AssetRelationship.label.in_(ourlabels),
             AssetRelationship.isactive.is_(True),
             AssetRelationship.sourceassetid != pcasset.assetid,
         ).all()
@@ -923,7 +939,16 @@ class ComputersPlugin(BasePlugin):
             # Only a device that FILES UNDER an operation goes through here.
             # A measuring tool is a subordinate device too, but it does not
             # share a machine number, so it keeps the simpler path.
-            return []
+            #
+            # NONE, not []. None is the ONLY answer that means "not a marker PC,
+            # run the ordinary machine link". Every other exit below means "this
+            # IS a marker PC" - possibly one that linked nothing - and returning
+            # [] there let the caller fall through to the machine link, which
+            # for a marker PC points at the OPERATION. That is the direct claim
+            # this whole path exists to prevent: several markers serve one
+            # operation, so two marker PCs would contest a link that can only
+            # have one holder, while the warning said "not linked".
+            return None
 
         pcasset = comp.asset
         controls = RelationshipType.query.filter_by(
@@ -998,8 +1023,15 @@ class ComputersPlugin(BasePlugin):
             self._ensure_device_rows(markerasset, spec, pcasset, controls,
                                      label, warnings, active=mayhold)
         elif reuse:
-            reuse.isactive = True
+            # Re-check incumbency HERE too, not only on the named path. A
+            # challenger recorded dormant keeps its row, so the cycle after its
+            # asset-id.txt goes missing found that row as `reuse` and flipped it
+            # active with no incumbent check and no warning - two live holders,
+            # arrived at by deleting a file. _sync_machine_link never had this
+            # hole because it re-checks unconditionally.
             markerasset = db.session.get(Asset, reuse.targetassetid)
+            reuse.isactive = self._device_incumbents_yield(
+                markerasset, pcasset, label, warnings)
         else:
             coretype = AssetType.query.filter_by(
                 assettype=spec['assettype']).first()
@@ -1419,8 +1451,11 @@ class ComputersPlugin(BasePlugin):
                     tooltype.measuringtooltypeid
             targetid = adopted.assetid
         elif reuse:
-            reuse.isactive = True
+            # Same re-check as the marker path: a dormant challenger must not be
+            # promoted just because its enrollment file stopped being readable.
             toolasset = db.session.get(Asset, reuse.targetassetid)
+            reuse.isactive = self._device_incumbents_yield(
+                toolasset, pcasset, MEASURINGTOOL_LINK_ORIGIN, warnings)
             if toolasset and toolasset.measuringtool and tooltype:
                 toolasset.measuringtool.measuringtooltypeid = \
                     tooltype.measuringtooltypeid