Error while connecting to AWS DocumentDB: Command isMaster not supported on $external database.

[krinart]

Versions/Environment

1. What version of Rust are you using? - rustc 1.88.0
2. What operating system are you using? - macos 15.1.1
3. What versions of the driver and its dependencies are you using? (Run
   cargo pkgid mongodb & cargo pkgid bson) - mongodb@3.2.3, bson@2.15.0
4. What version of MongoDB are you using? (Check with the MongoDB shell using db.version()) - DocumentDB 5.0.0
5. What is your MongoDB topology (standalone, replica set, sharded cluster, serverless)? - replicate set

Describe the bug

Following code:

use std::path::PathBuf;
use mongodb::Client;
use mongodb::options::{AuthMechanism, ClientOptions, Credential, Tls, TlsOptions};

#[tokio::main]
async fn main() {
    let uri = "mongodb://localhost:27017/tpch?directConnection=true";

    let mut opts = ClientOptions::parse(uri).await.unwrap();

    opts.credential = Some(
        Credential::builder()
            .username("<access_key>".to_string())
            .password("<secret_access_key>".to_string())
            .source("$external".to_string())
            .mechanism(AuthMechanism::MongoDbAws)
            .build()
    );

    let ca_path = PathBuf::from("/Users/krinart/global-bundle.pem");
    opts.tls = Some(Tls::Enabled(
        TlsOptions::builder()
            .ca_file_path(Some(ca_path))
            .allow_invalid_hostnames(true)
            .build(),
    ));

    let client = Client::with_options(opts).unwrap();

    let db = client.database("tpch");
    println!("Collections: {:?}", db.list_collection_names().await.unwrap());
}

Returns:

Error { kind: Command(CommandError { code: 73, code_name: "", message: "Command isMaster not supported on $external database.", topology_version: None }), labels: {}, wire_version: None, source: None }

For the context this command works:

AWS_ACCESS_KEY_ID=<> AWS_SECRET_ACCESS_KEY=<> mongosh \
  --host localhost \
  --port 27017 \
  --tls \
  --tlsCAFile /Users/krinart/global-bundle.pem \
  --tlsAllowInvalidHostnames \
  --authenticationDatabase '$external' \
  --authenticationMechanism MONGODB-AWS \
  tpch

[krinart]

I was able to make it work by commenting out this line:

mongo-rust-driver/src/cmap/establish/handshake.rs

Line 421 in b918cd6

command.target_db = cred.resolved_source().to_string();

Any suggestion on what would be the right way forward? I am happy to implement a fix

[krinart]

I also found that when running this command locally:

mongosh "mongodb://localhost:27017/tpch?directConnection=true&authSource=%24external&authMechanism=MONGODB-AWS"

Two following messages will be sent:

Message 1 (handshake):

    Document length: 299
    Elements
        Element: ismaster
            Type: Int32 (0x10)
            Value: 1
        Element: helloOk
            Type: Boolean (0x08)
            Value: True
        Element: client
            ...

Message 2 (auth):

    Document length: 132
    Elements
        Element: saslStart
            Type: Int32 (0x10)
            Value: 1
        Element: mechanism
            Type: String (0x02)
            Length: 12
            Value: MONGODB-AWS
        Element: payload
            Type: Binary (0x05)
            Length: 52
            Value: ...
        Element: $db
            Type: String (0x02)
            Length: 10
            Value: $external

As you can see during hadshake there's no $db attribute, it only appears during authorization.

This makes me think at least for aws we should not populate command.target_db during handshake

[kevinAlbs]

Thanks for reporting @krinart. I expect this is a discrepancy with the spec. The spec uses the admin database in the hello / isMaster regardless of auth.

We do not support or test DocumentDB. But since this appears to be a spec discrepancy, we are open to accepting a PR to match the spec if you would be interested.

[krinart]

Thanks @kevinAlbs. I've created a PR #1463

Please let me know if there's anything else needed.