云函数是给两个记录使用事务保证同时更新成功，代码如下：

// 云函数入口文件
const cloud = require('wx-server-sdk')


cloud.init()
const db = cloud.database({
  // 该参数从 wx-server-sdk 1.7.0 开始支持，默认为 true，指定 false 后可使得 doc.get 在找不到记录时不抛出异常
  throwOnNotFound: false,
})
const _ = db.command


// 云函数入口函数
exports.main = async (event, context) => {
  let docId1 = event.docId1
  let openid1 = event.openid1
  let docId2 = event.docId2
  let openid2 = event.openid2
  console.log('docId1 = ' + docId1 + ', openid1 = ' + openid1)
  console.log('docId2 = ' + docId2 + ', openid2 = ' + openid2)
  try {
    const result = await db.runTransaction(async transaction => {
      const doc1 = await transaction.collection('matchpool').doc(docId1).get()
      const doc2 = await transaction.collection('matchpool').doc(docId2).get()


      if (doc1.data && doc2.data) {
        const updateDoc1Res = await transaction.collection('matchpool').doc(docId1).update({
          data: {
            oppositeDocid: docId2,
            oppositeOpenid: openid2
          }
        })


        const updateDoc2Res = await transaction.collection('matchpool').doc(docId2).update({
          data: {
            oppositeDocid: docId1,
            oppositeOpenid: openid1
          }
        })


        console.log('transaction succeeded', result)


        // 会作为 runTransaction resolve 的结果返回
        return {
          doc1: doc1,
          doc2: doc2
        }
      } else {
        // 会作为 runTransaction reject 的结果出去
        await transaction.rollback(-100)
      }
    })


    return {
      success: true,
      doc1: result.doc1,
      doc2: result.doc2
    }
  } catch (e) {
    console.error('transaction error', e)
    return {
      success: false,
      error: e
    }
  }
}