The E11000 duplicate key exception contains wrong details which sometimes lead to the exception on the driver since we cannot parse it.

Consider the following queries:

// prepare collection
db.coll.createIndex( { "testValue" : 1 }, { unique : true, collation: { locale : 'sv', strength : 1, numericOrdering : true } } )
{
        "createdCollectionAutomatically" : true,
        "numIndexesBefore" : 1,
        "numIndexesAfter" : 2,
        "commitQuorum" : 1,
        "ok" : 1
}

Then, try to add two the same documents one by one:

• More simple case:

  // 1 attempt
   db.coll.insertOne({ "testValue": "def"})
  {
          "acknowledged" : true,
          "insertedId" : ObjectId("5e695401da7e6a4f1b712884")
  }
  // 2 attempt
   db.coll.insertOne({ "testValue": "def"})
  {"t":{"$date":"2020-03-11T21:11:30.702Z"},"s":"E", "c":"QUERY",   "id":0,"ctx":"js","msg":"{}","attr":{"message":"WriteError({\n\t\"index\" : 0,\n\t\"code\" : 11000,\n\t\"errmsg\" : \"E11000 duplicate key error collection: test.coll index: testValue_1 dup key: { testValue: \\\"/13\\\" }\",\n\t\"op\" : {\n\t\t\"_id\" : ObjectId(\"5e695402da7e6a4f1b712885\"),\n\t\t\"testValue\" : \"def\"\n\t}\n}) :\nWriteError({\n\t\"index\" : 0,\n\t\"code\" : 11000,\n\t\"errmsg\" : \"E11000 duplicate key error collection: test.coll index: testValue_1 dup key: { testValue: \\\"/13\\\" }\",\n\t\"op\" : {\n\t\t\"_id\" : ObjectId(\"5e695402da7e6a4f1b712885\"),\n\t\t\"testValue\" : \"def\"\n\t}\n})\nWriteError@src/mongo/shell/bulk_api.js:458:48\nmergeBatchResults@src/mongo/shell/bulk_api.js:855:49\nexecuteBatch@src/mongo/shell/bulk_api.js:919:13\nBulk/this.execute@src/mongo/shell/bulk_api.js:1163:21\nDBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:264:9\n@(shell):1:1"}}
  

  This case contains only issue with wrong exception details:

  testValue_1 dup key: { testValue: \\\"/13\\\" }
  

• Worse case:

  // 1 attempt
   db.coll.insertOne({ "testValue": "abcdefghijkl123456789"})
  {
          "acknowledged" : true,
          "insertedId" : ObjectId("5e695418da7e6a4f1b712886")
  }
  // 2 attempt
  db.coll.insertOne({ "testValue": "abcdefghijkl123456789"})
  {"t":{"$date":"2020-03-11T21:11:53.807Z"},"s":"E", "c":"QUERY",   "id":0,"ctx":"js","msg":"{}","attr":{"message":"WriteError({\n\t\"index\" : 0,\n\t\"code\" : 11000,\n\t\"errmsg\" : \"E11000 duplicate key error collection: test.coll index: testValue_1 dup key: { testValue: \\\")+-/13579;=?\\u000f�\\r9\\u000fe��\\\" }\",\n\t\"op\" : {\n\t\t\"_id\" : ObjectId(\"5e695419da7e6a4f1b712887\"),\n\t\t\"testValue\" : \"abcdefghijkl123456789\"\n\t}\n}) :\nWriteError({\n\t\"index\" : 0,\n\t\"code\" : 11000,\n\t\"errmsg\" : \"E11000 duplicate key error collection: test.coll index: testValue_1 dup key: { testValue: \\\")+-/13579;=?\\u000f�\\r9\\u000fe��\\\" }\",\n\t\"op\" : {\n\t\t\"_id\" : ObjectId(\"5e695419da7e6a4f1b712887\"),\n\t\t\"testValue\" : \"abcdefghijkl123456789\"\n\t}\n})\nWriteError@src/mongo/shell/bulk_api.js:458:48\nmergeBatchResults@src/mongo/shell/bulk_api.js:855:49\nexecuteBatch@src/mongo/shell/bulk_api.js:919:13\nBulk/this.execute@src/mongo/shell/bulk_api.js:1163:21\nDBCollection.prototype.insertOne@src/mongo/shell/crud_api.js:264:9\n@(shell):1:1"}}
  

  In this case we will see the wrong error part:

  testValue_1 dup key: { testValue: \\\")+-/13579;=?\\u000f�\\r9\\u000fe��\\\" }
  

  but the worst thing that we won't be able to parse this error on the driver side. And as a result this error will trigger a new deserialization exception which will fully hide the original one.

It looks like this ticket is related to https://jira.mongodb.org/browse/SERVER-26050