Serialize template subtask creation

src/constants/tasks.test.ts

@@ -0,0 +1,19 @@
+import { subtaskTemplateBatchSize } from '@/constants/tasks'
+import { runInBatches } from '@/utils/array'
+
+describe('task constants', () => {
+  it('keeps subtask template fan-out within the production Prisma pool', async () => {
+    expect(subtaskTemplateBatchSize).toBe(1)
+
+    const state = { active: 0, max: 0 }
+
+    await runInBatches([1, 2, 3], subtaskTemplateBatchSize, async () => {
+      state.active += 1
+      state.max = Math.max(state.max, state.active)
+      await new Promise((resolve) => setTimeout(resolve, 1))
+      state.active -= 1
+    })
+
+    expect(state.max).toBe(1)
+  })
+})

src/constants/tasks.ts

@@ -1,5 +1,5 @@
 export const maxSubTaskDepth = 1
 
-// Bounds how many subtasks we create concurrently when applying a template, so a
-// template with many sub-templates can't exhaust the DB connection pool.
-export const subtaskTemplateBatchSize = 5
+// Production Prisma pool limit is 2; each subtask creation performs multiple DB
+// writes, so keep template application serial within a single request.
+export const subtaskTemplateBatchSize = 1