feat(dashboardapi): diff-skip regen via flows:started predicate (#36)

Subscribes to Node-RED's flows:started runtime event, caches the {diff}
payload on the dashboardAPI source, and short-circuits the child.register
handler when none of {dashboardAPI id, child id, grandchild ids} appears in
diff.added/changed/removed/rewired. Predicate verified by S1 spike (#32).

- src/nodeClass.js: _attachLifecycleHook subscribes, _attachCloseHandler
  cleans up. No-op when RED.events isn't available (unit-test friendly).
- src/specificClass.js: subtreeChanged() predicate + subtreeIdsFor() helper.
- src/commands/handlers.js: registerChild consults predicate before composing;
  logs {event:'regen-skipped', outcome:'no-diff'} when skipping.
- test/basic/slice36-diff-predicate.basic.test.js: 8 cases — null/empty diff,
  affected/unaffected ids, tab-id over-triggering avoidance, grandchild
  inclusion, no-grandchild case.

Cold start (no cached diff yet) always regenerates — safe default. Edge case
documented in #32: when a brand-new child is wired to a registered parent,
the new child's id is in diff.added but not yet in registeredChildren when
flows:started fires. Mitigation (b) per spike findings: one-deploy race
accepted for R&D — next deploy picks up the new registration.

Closes #36

src/commands/handlers.js

@@ -24,12 +24,33 @@ function resolveChildNode(childId, ctx) {
 
 // On child.register: build the dashboard graph (root + direct children) and
 // emit one Grafana upsert HTTP request per dashboard on Port 0.
+//
+// Diff-skip behavior (PRD F-1, S1 spike #32): if the latest flows:started
+// payload's `diff` indicates that NEITHER the dashboardAPI itself NOR this
+// child NOR its grandchildren changed, skip composition and log no-diff. The
+// first call after startup (no cached diff yet) regenerates unconditionally.
 function registerChild(source, msg, ctx) {
   const childSource = resolveChildSource(msg.payload, ctx);
   if (!childSource?.config) {
     throw new Error('Missing or invalid child node');
   }
 
+  const subtreeIds = source.subtreeIdsFor(ctx.node?.id, childSource);
+  const changed = source.subtreeChanged(source.lastFlowsStartedDiff, subtreeIds);
+  if (!changed) {
+    if (source.logger?.info) {
+      source.logger.info({
+        event: 'regen-skipped',
+        outcome: 'no-diff',
+        trigger: 'child.register',
+        dashboardApiId: ctx.node?.id,
+        childId: childSource?.config?.general?.id,
+        subtreeSize: subtreeIds.size,
+      });
+    }
+    return;
+  }
+
   const dashboards = source.generateDashboardsForGraph(childSource, {
     includeChildren: Boolean(msg.includeChildren ?? true),
   });

src/nodeClass.js

@@ -26,6 +26,29 @@ class nodeClass {
 
     this._attachInputHandler();
     this._attachCloseHandler();
+    this._attachLifecycleHook();
+  }
+
+  // Subscribe to Node-RED's `flows:started` event to cache the deploy diff so
+  // the child.register handler can decide whether *this* dashboardAPI's
+  // subtree was affected. Predicate documented in Gitea issue #32 spike.
+  _attachLifecycleHook() {
+    if (!this.RED?.events?.on) return;
+    this._flowsStartedListener = (payload) => {
+      const diff = payload?.diff || null;
+      this.source.lastFlowsStartedDiff = diff;
+      this.source.lastFlowsStartedAt = Date.now();
+      if (this.source?.logger?.debug) {
+        const summary = diff
+          ? Object.fromEntries(
+              ['added', 'changed', 'removed', 'rewired', 'linked', 'flowChanged']
+                .map((k) => [k, (diff[k] || []).length])
+            )
+          : null;
+        this.source.logger.debug({ event: 'flows:started', type: payload?.type, diff: summary });
+      }
+    };
+    this.RED.events.on('flows:started', this._flowsStartedListener);
   }
 
   _buildConfig(uiConfig) {
@@ -78,6 +101,10 @@ class nodeClass {
 
   _attachCloseHandler() {
     this.node.on('close', (done) => {
+      if (this._flowsStartedListener && this.RED?.events?.off) {
+        this.RED.events.off('flows:started', this._flowsStartedListener);
+        this._flowsStartedListener = null;
+      }
       if (typeof done === 'function') done();
     });
   }

src/specificClass.js

@@ -168,6 +168,34 @@ class DashboardApi {
     return out;
   }
 
+  // Predicate from Gitea issue #32 spike (S1 findings). Given the diff payload
+  // from Node-RED's flows:started event and a set of node ids that constitute
+  // "my subtree", decides whether the subtree changed on this deploy.
+  // `null` diff (first deploy / startup) → always regen (safe default).
+  subtreeChanged(diff, subtreeIds) {
+    if (!diff) return true;
+    const mine = new Set(subtreeIds);
+    for (const field of ['added', 'changed', 'removed', 'rewired']) {
+      const arr = diff[field] || [];
+      if (arr.some((id) => mine.has(id))) return true;
+    }
+    return false;
+  }
+
+  // Collect ids that constitute "this dashboardAPI + this child + its grandchildren"
+  // for the diff predicate. Pulls grandchildren via the existing extractChildren walk.
+  subtreeIdsFor(dashboardApiNodeId, childSource) {
+    const ids = new Set();
+    if (dashboardApiNodeId) ids.add(dashboardApiNodeId);
+    const childId = childSource?.config?.general?.id;
+    if (childId) ids.add(childId);
+    for (const { childSource: gc } of this.extractChildren(childSource)) {
+      const gcId = gc?.config?.general?.id;
+      if (gcId) ids.add(gcId);
+    }
+    return ids;
+  }
+
   generateDashboardsForGraph(rootSource, { includeChildren = true } = {}) {
     if (!rootSource?.config) {
       this.logger.warn('generateDashboardsForGraph skipped: root source missing config');