Add shim log format configuration

[austinvazquez]

Context

I have a use-case where the shim logs (shim, vminitd, and kmesg) are being wrapped at the containerd runtime level.

i.e.

{"time":"2026-06-25T19:48:55.894022-05:00","level":"INFO","msg":"time=2026-06-25T19:48:55.893-05:00 level=INFO msg=\"VM connection established\" ns=docker id=eb5effb906647cb675950ac6ecd1b041309ea72ef72d92f68ac20d20f327333c component=shim t_config=117.875µs t_boot=131.007542ms t_total=131.125542ms runtime=io.containerd.nerdbox.v1"}

From looking into this, I believe this is because containerd writes shim logs to os.Stderr using a raw io.Copy (see core/runtime/v2/shim.go) instead of re-logging the shim FIFO output. For this reason, the runtime is having to capture os.Stderr and re-emit.

This change adds shim log options to enable configuration of JSON formatted logs. This will enable a workaround to parse the logs at the containerd runtime level with more simplified parsing.

[Copilot]

Pull request overview

This PR adds shim log format auto-detection (terminal vs. pipe/FIFO) with an environment variable override so shim logs can be emitted as JSON when needed for containerd-level log wrapping/parsing.

Changes:

• Introduces a Format type plus resolveFormat and SHIM_LOG_FORMAT override to choose between text and JSON output.
• Updates SetupShimLog to select slog.NewJSONHandler vs slog.NewTextHandler based on the resolved format.
• Adds cross-platform isTerminal(io.Writer) helpers (Unix + Windows) and unit tests for the new format resolution and terminal detection.

Reviewed changes

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

File	Description
pkg/logging/shim_windows.go	Adds Windows console detection (GetConsoleMode) to support log format auto-detection.
pkg/logging/shim_unix.go	Adds Unix “terminal” detection via file mode to support log format auto-detection.
pkg/logging/logging.go	Adds format selection (JSON vs text), env override, and updates shim logger setup accordingly.
pkg/logging/logging_test.go	Adds tests for format resolution and isTerminal behavior.

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[Copilot]

Pull request overview

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

[Copilot]

Pull request overview

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