Summary

Fixes #6646

The Google Batch API does not expose the actual zone where a task executes — it only provides the configured region (e.g. europe-west2). This means trace records report the region rather than the specific zone (e.g. europe-west2-a), which is important for cost analysis and debugging placement decisions.

This PR implements the workaround suggested by @pditommaso: querying the GCP instance metadata service from within the task to capture the real zone.

Changes

• TaskRun.groovy — Added CMD_ZONE = '.command.zone' constant for the zone metadata file
• GoogleBatchScriptLauncher.groovy — Inject a curl call to the GCP metadata endpoint (http://metadata.google.internal/computeMetadata/v1/instance/zone) in headerScript(), writing the result to .command.zone in the task work directory. The call is silent and non-fatal (2>/dev/null || true)
• GoogleBatchTaskHandler.groovy — On task completion, read .command.zone, parse the zone name from the metadata format (projects/<id>/zones/<zone>), and update the CloudMachineInfo with the actual zone. Uses a volatile boolean zoneUpdated flag to ensure the file is read at most once (avoiding repeated remote I/O on gcsfuse)

Design decisions

• Zone capture in headerScript() — This runs at the top of .command.run, which executes for both regular tasks and array task children (each child runs its own .command.run). The parent array launcher (.command.sh) does not capture zone since each child writes its own .command.zone
• Graceful degradation — If the metadata service is unreachable (e.g. non-GCP environments, Fusion-enabled tasks that bypass GoogleBatchScriptLauncher), the trace record falls back to the configured region as before
• Read-once semantics — The zoneUpdated flag is set in a finally block, so even if reading fails, we don't retry on every getMachineInfo() call (which is invoked frequently by the polling monitor, Tower observer, and error handlers)

Test plan

• GoogleBatchScriptLauncherTest — 2 new tests: zone capture present in header script; zone capture absent from array launch command
• GoogleBatchTaskHandlerTest — 6 new tests:
  • Zone updated from file on completion
  • Original zone preserved when file is missing
  • Null machineInfo returns null
  • Zone not updated when task is not completed
  • Malformed file content handled gracefully
  • Zone file read only once across multiple getMachineInfo() calls
• All 76 tests pass (7 launcher + 69 handler), 0 failures

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