Harden runtime and add portable releases

[slepp]

Summary

• harden upstream connection lifecycle, client protocol framing, logging, cleanup, signals, and port validation
• add regression coverage for delayed handshakes, fragmented commands, reconnect shutdown, port validation, and standby restarts
• modernize the Makefile and Docker build for static amd64, arm64, and ARMv7 binaries
• add GitHub Actions CI, tagged GitHub releases, and multi-architecture Docker Hub publishing
• replace obsolete GitLab references and refresh quick-start, LAN safety, build, Docker, and release documentation

Validation

• make test
• static Alpine/musl amd64 artifact build
• static scratch Docker image execution
• Valgrind shutdown and stats endpoint leak checks

[Copilot]

Pull request overview

This PR hardens rtlmux’s runtime behavior (upstream connection lifecycle, protocol framing, logging safety, cleanup, signal handling, and port validation) and adds portable release artifacts (multi-arch static binaries, Docker images, and GitHub Actions CI/release automation), with new integration/regression tests to lock in the expected behavior.

Changes:

• Added Python integration tests covering delayed handshakes, fragmented commands, clean reconnect shutdown, port validation, and delayed restarts.
• Hardened core runtime: safer logging formatting, more robust upstream/client lifecycle handling, improved cleanup paths, and stricter config validation.
• Modernized build and release pipeline: refreshed Makefile, multi-stage Docker builds (static + scratch), removed GitLab CI config, and added GitHub Actions workflows for CI/releases/Docker publishing.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/integration.py	New integration regression suite for handshake framing, command fragmentation, port validation, reconnect shutdown, and restart behavior.
slog.c	Replaces unsafe sprintf/vsprintf with bounded snprintf/vsnprintf.
rtlmux.h	Uses sig_atomic_t for exit flag and fixed-width types for protocol command framing.
rtlmux.c	Refactors upstream connect/reconnect, client readiness handling, safer logging callback formatting, improved allocation/error checks, and cleanup flow.
main.c	Improves signal handler setup and thread lifecycle error handling; switches exit flag type.
config.c	Adds validation for port ranges and non-empty host; converts validated values to uint16_t.
Makefile	Modernizes flags, dependency tracking (.d), adds static, test, install, and generate targets.
Dockerfile	Multi-stage build for static multi-arch artifacts and scratch runtime image.
README.md	Updates quick-start, LAN safety note, build, Docker, and release documentation; replaces obsolete CI references.
.github/workflows/ci.yml	Adds GitHub Actions CI to build, run tests, and verify static artifact build.
.github/workflows/release.yml	Adds tag-driven multi-arch binary builds, GitHub releases, and conditional Docker Hub publishing.
.gitlab-ci.yml	Removes obsolete GitLab CI configuration.
.gitignore	Adds *.d dependency file ignores.
.dockerignore	Excludes git metadata and build outputs from Docker build context.

Suppressed comments (3)

tests/integration.py:195

• This helper thread is non-daemon, and addCleanup(thread.join, 2) only waits briefly. If upstream.accept() blocks due to a test failure, the thread can keep the Python process alive and hang CI.

        thread = threading.Thread(target=serve)
        thread.start()
        self.addCleanup(thread.join, 2)

tests/integration.py:139

• This helper thread is non-daemon, and addCleanup(thread.join, 2) only waits briefly. If upstream.accept() blocks due to a test failure, the thread can keep the Python process alive and hang CI.

        thread = threading.Thread(target=serve)
        thread.start()
        self.addCleanup(thread.join, 2)

tests/integration.py:227

• This helper thread is non-daemon, and addCleanup(thread.join, 4) only waits briefly. If upstream.accept() blocks due to a test failure, the thread can keep the Python process alive and hang CI.

        thread = threading.Thread(target=serve)
        thread.start()
        self.addCleanup(thread.join, 4)

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