"E11000 duplicate key error" when doing replace_one with upsert (c++ driver)

[VioletGiraffe]

I'm using the C++ driver to do bulk `replace_one` operation with "_id" as the only filter. And I'm getting E11000 after some time running the program, always.

Since I'm generating the IDs myself, I thought my keys are simply colliding. But 1) they aren't, I triple check through logging all operations and analyzing the log with a purpose-written tool; and 2) even if they were, `replace_one` with filter on "_id" should simply replace the existing document, right?..

So how is this error even possible??? I'm at a loss.

bool CMongoDbInterface::registerTokens(const std::vector<std::string>& tokens)
{
    if (tokens.empty())
        return true;

    try {
        auto client = get_client();
        auto db = client->database("web");
        auto tokenCollection = db.collection("tokens");
        mongocxx::bulk_write bulk{};

        for (auto item = tokens.cbegin(), end = tokens.cend(); item != end; ++item)
        {
            const auto nextItem = std::next(item); // <- The source token list is already sorted, so duplicate items shall not pass
            if (nextItem != end && *item == *nextItem)
                continue;

            const auto tokenId = hash128bit(*item);
            logHash(item->text);// <- I'm watching all the hashes for collisions, there weren't any!
            basic::document basic_builder{};
            basic_builder.append(kvp("_id", binaryFromArray(tokenId))); // binaryFromArray constructs bsoncxx::types::b_binary from std::array
            basic_builder.append(kvp("token", bsonstring(item->text)));

            mongocxx::model::replace_one upsert_op{ stream::document{} << "_id" << binaryFromArray(tokenId) << stream::finalize, basic_builder.view() };
            upsert_op.upsert(true);

            bulk.append(upsert_op);
        }

        const auto result = tokenCollection.bulk_write(bulk);
        return (bool)result; //&& (result->upserted_count() + result->matched_count() == uniqueItems);
    }
    catch (mongocxx::bulk_write_exception& e) {
        assert_unconditional_r(std::string("MongoDB bulk_write exception: ") + e.what());
        return false;
    }
}

[Asya Kamsky]

See this section in the docs which talks about this:

https://docs.mongodb.com/manual/reference/method/db.collection.update/#use-unique-indexes

You can also see discussion about whether the server might be able to handle a subset of these cases automatically:

https://jira.mongodb.org/browse/SERVER-14322

Asya

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Asya Kamsky
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MongoDB
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