feat(router): hot config reloading

[endigma]

Motivation and Context

As a user who deploys the router with a dynamic config from the filesystem, I'd like it such that when the file changes on disk, there is the option to have the router automatically reload itself when it detects this change.

Changes

• Better responsibility boundary between cmd/router/main.go and router/pkg/config/config.go
  • config.LoadConfig no longer loads dotenv into the environment or decides the config path under any circumstance
  • These responsibilities fall to main.Main now
• The log level is now atomic and mutable at runtime if you hold a reference
• The router is now capable of executing a full reload when either the config file changes or SIGHUP signal is sent to the process
• Moved randomDuration utility out of controlplane package into internal/timex for reuse
• Added configuration for watching the config file and triggering a reload when a change is detected (example below)

Config responsibility change

As a side effect of drawing a clearer boundary between the responsibilities of main.go and config.go for determining the config file to read, direct calls to LoadConfig will no longer check CONFIG_FILE environment variable. This is intentional, as this responsibility now falls to the caller.

LoadConfig continues to default to config.DefaultConfigPath when the provided path is "".

Limitations

The config path loading and the file watcher are outside the reload loop, so:

• Configuration hot-reloading will not detect changes to the config path
• Configuration for the file watching behaviour itself will not be updated during reloads

Configuration Example

watch_config:
  enabled: true
  interval: "10s"

  startup_delay:
    enabled: false
    maximum: "10s"

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/cosmo-docs.

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/wundergraph/cosmo/router:sha-665fa6436a60b33e7252eb76891d029ebc54a9c6

[Copilot]

Pull Request Overview

This PR introduces hot configuration reloading for the router by adding file watching and SIGHUP trigger support, while also refining the config loading responsibilities between main.go and config.go.

• Updated configuration to include hot-reload settings
• Modified LoadConfig signature and updated tests accordingly
• Enhanced shutdown error handling with a new concurrent error joiner and integrated config watcher in the main command

Reviewed Changes

Copilot reviewed 21 out of 26 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
router/pkg/config/fixtures/full.yaml	Added watch_config section for enabling hot config reloading
router/pkg/config/config_test.go	Updated tests to use the new LoadConfig signature and added tests for watch configuration via environment variables
router/core/router.go	Introduced a concurrent error joiner and updated router shutdown logic with improvements
router/internal/timex/duration_test.go	Added tests for RandomDuration function; contains an error in the loop iteration
Other files	Minor adjustments (e.g., quoting in YAML/compose files, dotenv usage, test env helpers) to support the overall changes

Files not reviewed (5)

• docker/nats/Dockerfile: Language not supported
• docker/nats/build-push.sh: Language not supported
• docker/nats/nats-server.conf: Language not supported
• router-tests/go.mod: Language not supported
• router/pkg/config/config.schema.json: Language not supported

[StarpTech]

LGTM