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  1. Node.js Driver
  2. NODE-2483

Regression using hasNext on cursor with limit

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    Details

    • Type: Bug
    • Status: Closed
    • Priority: Major - P3
    • Resolution: Fixed
    • Affects Version/s: 3.5.4
    • Fix Version/s: 3.5.5
    • Component/s: None
    • Labels:
      None
    • MongoDB Version:
      Not Applicable

      Description

      From user Peter Broadhurst:

      We've had to pin our version to 3.5.3 as this code change changed behaviour.

      Simple sample recreate of the change in behavior:

      async function run() {
        const MongoClient = require('mongodb').MongoClient;
        const client =  await MongoClient.connect(process.env.MONGODB_URL);
        const db = client.db('354regr');
        const collection = db.collection('numbers');
       
        try {
          await collection.deleteMany({});
          for (let i = 0; i < 10; i++) await collection.insertOne({i});
          let cursor = await collection.find({});
          cursor.limit(5);
          while (await cursor.hasNext()) {
            const doc = await cursor.next();
            console.log(`Doc: ${JSON.stringify(doc)}`);
          }
        }
        catch(err) {
          console.error(err.stack);
        }
       
        client.close();
      }
      run()
      

      Output with mongodb@3.5.3 (or mongodb@3.5.2 and previous) - see the limit of 5 records returned as expected:

      Doc: {"_id":"5e6012c0bab232836f58f35b","i":0}
      Doc: {"_id":"5e6012c0bab232836f58f35c","i":1}
      Doc: {"_id":"5e6012c0bab232836f58f35d","i":2}
      Doc: {"_id":"5e6012c0bab232836f58f35e","i":3}
      Doc: {"_id":"5e6012c0bab232836f58f35f","i":4}
      

      Output with mongodb@3.5.4:

      Doc: {"_id":"5e601344f32806839a905a99","i":0}
      Doc: {"_id":"5e601344f32806839a905a9a","i":1}
      Doc: null
      MongoError: Cursor is closed
          at Function.create (/private/tmp/node_modules/mongodb/lib/core/error.js:43:12)
          at Cursor.hasNext (/private/tmp/node_modules/mongodb/lib/cursor.js:197:24)
          at run (/private/tmp/t/index.js:12:25)
          at process._tickCallback (internal/process/next_tick.js:68:7)
      

      See that each time hasNext is called, it seems to be consuming one of the limit entries, resulting in a null doc being returned on the third call to next() and the cursor being closed.

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              Assignee:
              matt.broadstone Matt Broadstone
              Reporter:
              matt.broadstone Matt Broadstone
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                Updated:
                Resolved: