Parse native Go Dags and show each Dag's own source

[jason810496]

• related: TS SDK: embed one source region per native Dag file #73723, Show each native Dag's own source in the Code tab #74249, Parse native Java Dags and show their source #74036
• related: ADR-0015 (airflow-core/adr/lang-sdk/0015-per-dag-source-in-bundle-artifact.md)
• next: Go SDK: embed each native Dag's source file in the bundle #74342

Why

ExecutableCoordinator had no Dag importer, so Airflow could not parse native Go Dags, and an executable bundle embedded a single source file for all of its Dags.

How

• An executable bundle now embeds one source file per native Dag plus the entrypoint, back to back in the existing source region. The AFBNDL01 trailer is unchanged (footer_ver = 1), and the manifest indexes the files:

  entrypoint_path: "example/bundle/main.go"
  dag_source_paths:
    orders: "example/bundle/main.go"
    reports: "example/bundle/dags/reports.go"
  sources:
    - {path: "example/bundle/main.go", offset: 0, length: 1532, sha256: "..."}
    - {path: "example/bundle/dags/reports.go", offset: 1532, length: 811, sha256: "..."}

• ExecutableDagImporter claims a file by its AFBNDL01 trailer, not its name, so a Go binary with no extension is parsed.

• get_source_code(definition, dag_id=None) follows Show each native Dag's own source in the Code tab #74249: a Dag's own file, the entrypoint for an unmapped Dag, and a notice when the bundle embeds no source.

• As in Parse native Java Dags and show their source #74036, safe_mode does not apply and every bundle is parsed, including bundles that only register task handlers.

What

• executable/_bundle_reader.py: read_bundle_source, read_bundle_entrypoint_source and read_bundle_language. Each region's bounds and sha256 are checked.

• ExecutableCoordinator: opens and verifies a bundle once for its metadata and sources. It runs [<bundle>] with the bundle's supervisor schema version both to parse the bundle and to run a task of a native Go Dag, so that task runs the binary its Dag was parsed from.

• ExecutableDagImporter, registered in COORDINATOR_DAG_IMPORTERS.

• executable-bundle-spec.rst and airflow-metadata.schema.json: source is replaced by entrypoint_path, dag_source_paths and sources.

• go.rst: how native Go Dags are parsed and run. A binary that only registers task handlers is parsed too and finds no Dags, so the Quick start shows how to keep task-handler binaries out of parsing with .airflowignore, as java.rst does:

  # task_handler_bundle_name names a bundle that holds only handler binaries
  echo '*' > /opt/airflow/go-task-handlers/.airflowignore   # glob, the default
  echo '.' > /opt/airflow/go-task-handlers/.airflowignore   # regexp
  
  # handler binaries in the Dag bundle itself, kept in bin/
  echo 'bin/*' >> /opt/airflow/dags/.airflowignore           # glob
  echo '^bin/' >> /opt/airflow/dags/.airflowignore           # regexp

Not in scope

• A mixed-language task still finds its bundle through the dag_ids in the bundle metadata. Launching it without that inventory is left to a follow-up.

---

Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5.5 following the guidelines

[henry3260]

Just want to help unblock the technical side here, not a formal review.

[jason810496]

Thanks for the review.