fix(session-query): avoid deep lineage recursion (round 2)

packages/session-query/session-query/src/tracing.ts

@@ -248,10 +248,26 @@ function buildDescendants(
   childrenByParent: ReadonlyMap<SessionId, readonly SessionRecord[]>,
   sessionId: SessionId,
 ): SessionLineageNode[] {
-  return (childrenByParent.get(sessionId) ?? []).map(child => ({
+  const descendants: SessionLineageNode[] = []
-    session: cloneRecord(child),
+  const stack = [{ sessionId, descendants }]
-    descendants: buildDescendants(childrenByParent, child.header.id),
+  while (stack.length > 0) {
-  }))
+    // The length guard proves a frame exists.
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    const frame = stack.pop()!
+    const nodes: SessionLineageNode[] = []
+    for (const child of childrenByParent.get(frame.sessionId) ?? []) {
+      const node = { session: cloneRecord(child), descendants: [] }
+      nodes.push(node)
+      frame.descendants.push(node)
+    }
+    for (let index = nodes.length - 1; index >= 0; index -= 1) {
+      // The loop bounds prove this indexed node exists.
+      // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+      const node = nodes[index]!
+      stack.push({ sessionId: node.session.header.id, descendants: node.descendants })
+    }
+  }
+  return descendants
 }
 
 function compareSessionsAscending(a: SessionRecord, b: SessionRecord): number {

packages/session-query/session-query/tests/tracing.spec.ts

@@ -195,6 +195,27 @@ describe('session lineage tracing', () => {
     await expect(ctx.sessionQuery.traceSession(durable.id))
       .rejects.toThrow(expectCode('SESSION_QUERY_PERSISTENCE_FAILED'))
   })
+
+  it('constructs deeply nested descendants without consuming the JavaScript call stack', async () => {
+    const ctx = await queryContext()
+    const root = ctx.sessions.create(SessionId('deep-0'), { meta: { createdAt: 0 } })
+    let parent = root
+    for (let depth = 1; depth < 3_000; depth += 1) {
+      parent = ctx.sessions.create(SessionId(`deep-${depth}`), {
+        meta: { createdAt: depth, parentSession: parent.id },
+      })
+    }
+
+    const trace = await ctx.sessionQuery.traceSession(root.id)
+    expect(trace.complete).toBe(true)
+    let node = trace.descendants[0]
+    for (let depth = 1; depth < 3_000; depth += 1) {
+      if (node === undefined) throw new Error(`lineage ended before depth ${depth}`)
+      if (depth === 2_999) expect(node.session.header.id).toBe(SessionId('deep-2999'))
+      node = node.descendants[0]
+    }
+    expect(node).toBeUndefined()
+  })
 })
 
 describe('session event tracing', () => {