fix(questions): make addQuestionToSession atomic and bounded

[ionfwsrijan]

Problem

addQuestionToSession performed two writes that were not atomic:

const createdQuestions = await Question.insertMany(questions.map(...)); // write 1
session.questions.push(...createdQuestions.map((q) => q._id));          // memory
await session.save();                                                   // write 2

1. If session.save() failed (or the connection dropped between the
   writes), the Question documents were already inserted but never linked —
   permanently orphaned rows no API path surfaces.
2. Two concurrent addQuestionToSession calls each loaded the session,
   pushed their own IDs, and save()d; the later save() overwrote the
   earlier push, silently dropping one batch's linkage.
3. questions had no upper bound per request.

Fix

backend/controllers/questionController.js — addQuestionToSession now
runs entirely inside a mongoose.startSession() transaction (mirroring
createSession / deleteSession), and the linkage is an atomic $push:

• The session ownership check, Question.insertMany, and the session update
  commit or roll back together — a failed session update can no longer leave
  orphaned questions.
• Linkage uses Session.updateOne({ _id }, { $push: { questions: { $each: ids } } })
  instead of read-modify-write + save(), so concurrent adds both persist
  without losing a batch.
• Input/ownership errors map to 400/404/403 explicitly.

backend/Input_validators/ValidateQuestions.js — the questions array is
now capped at max(50) per request.

Files changed

• backend/controllers/questionController.js - transactional
  addQuestionToSession with atomic $push linkage.
• backend/Input_validators/ValidateQuestions.js - questions capped at 50.
• backend/tests/questionController.ownership.unit.test.js - rewrote with a
  Module._load shim (the previous vi.mock could not intercept the CJS
  controller's require() calls; it was already failing 5/5 on
  origin/main).

Testing

npx vitest run tests/questionController.ownership.unit.test.js - 6/6 pass:

• 403 cross-user write: no insert, no linkage update.
• 201 happy path: insertMany once; linkage via $push: { questions: { $each } }
  (no session.save()).
• Linkage update failure -> 500; both writes run inside one withTransaction,
  so no orphaned questions are possible.
• 404 when the session does not exist.
• 400 for invalid/missing input.
• questionBank.unit.test.js still passes (6/6).

Related

Closes #1453

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📥 Commits

Reviewing files that changed from the base of the PR and between 8acb5b8 and 979b3e6.

📒 Files selected for processing (3)

• backend/Input_validators/ValidateQuestions.js
• backend/controllers/questionController.js
• backend/tests/questionController.ownership.unit.test.js

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