Secure Send documented, and the machine form stops re-creating a controlling-PC link that already exists

plugins/machines/frontend/views/controllingPc.js

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+// What saving the machine form should do about its controlling-PC link.
+//
+// Pulled out of MachineForm.vue because the component decided this inline and
+// got it wrong in a way nothing could catch: it treated "I know the id of the
+// existing link" as "there IS an existing link". The id was read from
+// `rel.assetrelationshipid`, a key the API has never emitted - the column is
+// `relationshipid` and to_dict serialises column names - so it was undefined on
+// every load, and the two questions gave opposite answers on a machine that
+// plainly had a controlling PC.
+//
+// Both failures followed from that single conflation:
+//   - Saving ANY change, a map position included, re-POSTed the link that was
+//     already there. The server answered 409 "this relationship already
+//     exists", correctly, and the form surfaced it as a failed save.
+//   - Clearing the controlling PC did nothing at all, because "no selection and
+//     no known id" reads as "nothing to do".
+//
+// So THE LOADED PC IS THE AUTHORITY on whether a link exists, and the
+// relationship id is only ever used to delete a row. If the id goes missing
+// again the worst case is a link that cannot be removed, never a duplicate
+// create.
+
+/**
+ * Decide what to do with the controlling-PC link on save.
+ *
+ * @param {object} state
+ * @param {number|string|null} state.selected - PC id chosen in the form now.
+ * @param {number|string|null} state.loaded - PC id the link named on load.
+ * @param {number|null} state.existingRelationshipId - row id to delete, if known.
+ * @returns {{action: 'none'|'unlink'|'link'|'relink', deleteId: number|null}}
+ */
+export function planControllingPcChange({ selected, loaded, existingRelationshipId } = {}) {
+  const now = normalize(selected)
+  const before = normalize(loaded)
+  const deleteId = existingRelationshipId ?? null
+
+  // Unchanged is not a change. Somebody moving a marker on the map is not
+  // asking to be re-linked to the same PC, and delete-then-create on an
+  // untouched link opens a window where the machine has no controlling PC -
+  // which resolve_asset_position walks across, so a failure between the two
+  // calls would lose the machine's inherited position along with the link.
+  if (now === before) {
+    return { action: 'none', deleteId: null }
+  }
+
+  if (now === null) {
+    // Cleared. Nothing to delete means nothing to do, rather than an error:
+    // the link is already absent, which is the state being asked for.
+    return deleteId === null
+      ? { action: 'none', deleteId: null }
+      : { action: 'unlink', deleteId }
+  }
+
+  if (before === null) {
+    return { action: 'link', deleteId: null }
+  }
+
+  // Swapped to a different PC. The old row goes first: 'controls' is singular
+  // per source, not per target, so leaving it would leave the machine with two.
+  return { action: 'relink', deleteId }
+}
+
+// Ids arrive as numbers from the API and strings from a <select>, and '' means
+// "nothing chosen". Comparing those raw is how an unchanged PC looks changed.
+function normalize(value) {
+  if (value === null || value === undefined || value === '') return null
+  return String(value)
+}