fix(log): route production logs directly to the systemd journal

[JanZachmann]

Summary

• Replace env_logger with systemd-journal-logger for production builds; keep env_logger for the mock feature only.
• MESSAGE bodies lose the inline "{target}: " prefix; the target now lives in the structured TARGET= journal field.
• Move logger setup from main.rs into src/logging.rs.

Reason

base_test.sh in omnect-os/ci/tests/lib_test.sh runs journalctl -b -q -p err and checks every PRIORITY≤3 line against a per-platform whitelist. It has been tripping on ods debug messages like:

omnect-device-service : omnect_device_service::twin::network: <7>omnect_device_service::twin::network: signal received, (re)arming debounce
base_test.sh: found syslog errors not in whitelist

Mechanism

env_logger encodes severity as an in-band "<N>" text prefix at the start of each line. journald's SyslogLevelPrefix parser only accepts "<N>" at byte 0 — if anything precedes it, parsing fails silently and the line falls back to the stream's default priority, which on our unit config is ERROR. So a debug!() call lands at PRIORITY=3 and gets flagged.

We didn't identify the exact byte-perturbation source. The class of failure is inherent to in-band textual prefixes: it cannot be ruled out as long as severity travels through the MESSAGE byte stream.

Why systemd-journal-logger

systemd-journal-logger sends records via the native sd-journal socket protocol. PRIORITY becomes a structured field alongside MESSAGE, SYSLOG_IDENTIFIER, and TARGET. It is not text, not parsed from a stream, and cannot be corrupted by concurrent writes.

Advantages

• Correct severity, always. No textual prefix to lose. journalctl -p err only returns actual errors.
• Structured fields for free. TARGET=, CODE_FILE=, CODE_LINE=, CODE_MODULE= become queryable (journalctl TARGET=omnect_device_service::twin::network).
• Filter semantics unchanged. RUST_LOG and the same warn,azure_iot_sdk=debug,... defaults are preserved via env_filter (env_logger's filter half, published as a standalone crate).
• Mock path untouched. cargo test --features mock and cargo run --features mock still log to stdout through env_logger with the <N>{args} format, so test harness output stays readable.
• Backend selection is compile-time. Production binaries never link env_logger; mock builds never link the journal backend. The mock feature now implies dep:env_logger.

[Copilot]

Pull request overview

Routes production logging directly to journald using systemd-journal-logger, while keeping an env_logger-based stdout logger for the mock feature, and centralizes logger setup into a dedicated module.

Changes:

• Move logger initialization out of main.rs into src/logging.rs.
• Use journald-native logging for non-mock builds; keep env_logger for mock.
• Update dependencies/features and bump crate version to 0.42.2.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/main.rs	Switches startup to call logging::init() and handles init failure via stderr.
src/logging.rs	Introduces feature-gated logger initialization for mock (stdout) vs production (journald).
src/lib.rs	Exposes the new logging module.
Cargo.toml	Adds env_filter + systemd-journal-logger, makes env_logger optional, wires mock feature.
Cargo.lock	Locks new dependency additions and version bump.

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