Mock TDS server hangs autocommit-off clients: TransactionManager (0x0E) requests are unhandled

[saurabh500]

Summary

The mock TDS server (mssql-mock-tds / mssql-mock-tds-py) hangs any client that connects with autocommit disabled. After a successful FedAuth login, such clients send a TransactionManager (0x0E) request to begin a transaction. The mock does not recognize packet type 0x0E, so it silently drops the request and never replies — the client then blocks indefinitely waiting for the transaction response.

This makes the mock unusable for driver integration tests on Linux, where connect/login timeouts are not always honored against a half-responsive server, so the client hangs forever instead of erroring out.

Impact

We are adding mock-TDS FedAuth regression tests to mssql-python (regressions like microsoft/mssql-python#596). The tests hang on Linux CI (hit the 1-hour worker timeout). They only "pass" on Windows because there the ODBC login timeout is honored, so the still-open connection is eventually torn down.

Both mssql-python and pyodbc reproduce this — because Python DB-API drivers default to autocommit = OFF, so the ODBC Driver 18 begins a transaction immediately after login and issues a TransactionManager request.

Root cause

mssql-mock-tds/src/protocol.rs — the PacketType enum / TryFrom<u8> does not include 0x0E (TransactionManager). In server.rs::process_packet, PacketHeader::parse therefore fails, process_packet returns Ok(None), and no response is written. The read loop then blocks on the next read_buf, and the client blocks waiting for the transaction reply.

Evidence (mock trace log, TDS 7.4 / optional TLS, ODBC Driver 18 on Linux)

INFO  ...server: TLS handshake successful for 127.0.0.1:56527
DEBUG ...server: Handling Login7 from 127.0.0.1:56527
DEBUG ...protocol: Parsed access token from Login7: 36 bytes
DEBUG ...server: Stored access token (36 bytes) from 127.0.0.1:56527 for verification
DEBUG ...server: Sending 118 encrypted bytes response          # LoginAck + FeatureExtAck + Done
DEBUG ...server: Received 34 encrypted bytes from 127.0.0.1:56527 (TDS-wrapped TLS)
WARN  ...server: Failed to parse packet header: Invalid packet type: 14   # 14 == 0x0E TransactionManager
# ... no response sent; client blocks until it is force-closed 14s later
DEBUG ...server: TLS connection closed by client 127.0.0.1:56527

The FedAuth token is parsed correctly; the connection simply never completes because the TransactionManager request goes unanswered.

Requested change

1. Handle TransactionManager (0x0E) requests. Add TransactionManager = 0x0E to PacketType and its TryFrom<u8>, and add a process_packet arm that:

   • Parses the RequestType (u16 LE) that follows the leading ALL_HEADERS block (u32 LE total length + that many header bytes).
   • Replies with an appropriate token stream so the client can proceed, e.g.:
     • TM_BEGIN_XACT (5) → ENVCHANGE type 8 (Begin Transaction) carrying an 8-byte transaction descriptor + DONE.
     • TM_COMMIT_XACT (7) → ENVCHANGE type 9 (Commit) + DONE.
     • TM_ROLLBACK_XACT (8) → ENVCHANGE type 10 (Rollback) + DONE.
     • anything else → DONE.
   • Wrap the tokens in a TabularResult (0x04) packet, like the Login7/SqlBatch paths do.

   This lets an autocommit-off connection complete its BEGIN TRAN handshake, so connect() returns successfully on all platforms and the connection closes cleanly.

2. (Optional, nice-to-have) Record received token as soon as it is parsed. Today ConnectionStore is only populated after the per-connection read loop exits (i.e. after the client disconnects). Storing the token into the shared store at Login7 time would let tests observe has_received_token() even while the connection is still open, and makes the store resilient to clients that keep the connection alive.

Repro

• Mock: TDS 7.4 optional-TLS mode (--tls-mode optional --cert ... --key ..., or PyMockTdsServer(port=0, tls=True)).
• Client: mssql-python (or pyodbc) with ODBC Driver 18 on Linux, connecting with an access token via SQL_COPT_SS_ACCESS_TOKEN and default autocommit (OFF). The connect call never returns.