Summary

This PR fixes multiple issues with mpremote when connecting to socket and RFC2217 targets, and adds comprehensive pytest-based testing infrastructure.

possibly this PR should be split into multiple PRs:

• mpremote fixes
• introduction of pytest framework for mpremote + related tests
• integration in to CI in combination with mpbridge
• code coverage reporting for python tooling, starting with mpremote

Key improvements:

1. Fixed RFC2217 REPL race conditions: pyserial's RFC2217 uses a background thread to process telnet data. The REPL loop's select() would signal ready when socket data arrived, but inWaiting() returned 0 because the background thread hadn't processed the data yet, causing delayed or missed input.

2. Fixed socket connection support: Added missing in_waiting property to SerialIntercept which was causing errors when using mpremote mount over socket:// connections.

3. Improved REPL newline handling: Fixed staircase effect when connecting to Unix port by properly configuring terminal output flags (OPOST | ONLCR) for NL → CR-NL translation.

4. Enhanced waitchar() for network connections: Improved handling of file descriptors for socket-based connections and RFC2217 (which doesn't implement fileno()).

5. Better error handling on disconnect: Device disconnection messages now properly display for RFC2217 and socket connections, even when command-line parameters are provided.

6. Optimized network performance: Improved SerialTransport performance for network connections.

Testing infrastructure:

• Added comprehensive pytest-based test suite with ~3,100 lines of tests
• Tests for interactive REPL, Unicode handling, filesystem operations, and platform-specific scenarios
• Support for running tests against multiple connection types (serial, socket, RFC2217)
• Coverage collection support for both bash and pytest tests
• Tests for edge cases including:
  • Unicode filenames and output
  • Windows console UTF-8 handling and legacy console errors
  • Filenames containing equals signs
  • REPL newline handling across platforms

Testing

Tested on the following configurations:

• Serial connections: /dev/ttyACM0, /dev/ttyUSB0
• Socket connections: socket://localhost:2218
• RFC2217 connections: rfc2217://localhost:2217
• Platforms: Linux and Windows (via WSL2 and Git Bash)

In order to enable testing on Windows - and for other tests that require some level of mocking to be created I have addedd tests using the pytest framework.

The new pytest suite includes 548+ test assertions across multiple test files covering:

• Unicode handling test_unicode.py, test_unicode_filenames.py
• Filesystem operations with edge cases test_equals_sign.py
• Windows-specific console behavior test_windows_console_utf8.py
• Test utilities validation test_utils.py
• Interactive REPL behavior test_repl_newlines.py

To run all tests:

uv venv --python 3.14
uv pip install -e tools/mpremote[test] 

cd tools/mpremote/tests
./run-mpremote-tests.sh -t <device>

$env:MPREMOTE_DEVICE = "<device>"; pytest  -s -v

Trade-offs and Alternatives

I have chosen to use the pytest framework over stdlib's unittest as:

• I have used pytest while developing and validating other mpremote PRs
• pytest tests are shorter and simpler to maintain compared to unittest
• pytest provides built-in parametrization support, allows for for testing multiple connection types (serial, socket, RFC2217) with the same test logic
• pytest's fixture system (via conftest.py) elegantly handles complex setup/teardown of device connections and test scenarios
• pytest can also run unittest-based tests, ensuring compatibility with any existing unittest code

I have not ported the existing bash based tests to pytest, but it should be possible to have the pytest test runner discover and run the existing "test_*.sh" files as part of as seperate PR

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.