[BUG] GkeTpuCluster queries metadata.google.internal for MEGASCALE_SLICE_ID even when TPU_SKIP_MDS_QUERY is set

从 jax/_src/clusters/cloud_tpu_cluster.py 开始，阅读 get_tpu_env_value() 及其调用方，尤其是 GkeTpuCluster.get_process_id()。在 TPU_SKIP_MDS_QUERY=true 且无法访问元数据的情况下重现；完成的标准是不会查询元数据服务器，并且现有的回退值可以让初始化继续进行。

Description

Summary

In jax/_src/clusters/cloud_tpu_cluster.py, TPU_SKIP_MDS_QUERY is checked in GceTpuCluster.is_env_present() (line 186) to bypass querying the GCE Metadata Server. However, get_tpu_env_value() (line 70) completely ignores TPU_SKIP_MDS_QUERY:

def get_tpu_env_value(key) -> str | None:
  # First try to get the value from the environment.
  value = os.environ.get(key, None)
  if value is None:
    # If not found, try to get it from the metadata.
    value = get_tpu_env_value_from_metadata(key)
  return value

When running in Kubernetes with GkeTpuCluster, calling GkeTpuCluster.get_process_id() queries MEGASCALE_SLICE_ID via get_tpu_env_value('MEGASCALE_SLICE_ID').

Even when TPU_SKIP_MDS_QUERY=true is explicitly set in the container (e.g. by Kubernetes TPU device plugins, or on bare-metal/testbed clusters), get_tpu_env_value() still unconditionally calls get_tpu_env_value_from_metadata(), which tries to reach http://metadata.google.internal/computeMetadata/v1/instance/attributes/tpu-env.

When the metadata server is unreachable or unresolvable, JAX hangs for 6 retries with 60-second timeouts and then crashes with requests.exceptions.ConnectionError.

Notice that the caller functions in BaseTpuCluster already have built-in defaults if get_tpu_env_value() returns None:

• _get_slice_id(): if not slice_id: return 0
• _get_num_slices(): if not num_slices: return 1
• get_coordinator_address(): if not coordinator_address: coordinator_address = cls._get_worker_list_in_slice()[0]

Therefore, simply checking TPU_SKIP_MDS_QUERY in get_tpu_env_value() allows all existing fallbacks to work cleanly without contacting the metadata server.

Steps to Reproduce

Run JAX in a Kubernetes TPU container where TPU_SKIP_MDS_QUERY=true and TPU_WORKER_HOSTNAMES are set, but without access to metadata.google.internal:

import jax
# GkeTpuCluster is selected, but crashes trying to reach metadata.google.internal
jax.distributed.initialize()

Stack Trace

Traceback (most recent call last):
  File "jax/_src/clusters/cloud_tpu_cluster.py", line 146, in get_process_id
    slice_id = cls._get_slice_id()
  File "jax/_src/clusters/cloud_tpu_cluster.py", line 162, in _get_slice_id
    slice_id = get_tpu_env_value('MEGASCALE_SLICE_ID')
  File "jax/_src/clusters/cloud_tpu_cluster.py", line 75, in get_tpu_env_value
    value = get_tpu_env_value_from_metadata(key)
  File "jax/_src/clusters/cloud_tpu_cluster.py", line 45, in get_metadata
    api_resp = requests.get(
requests.exceptions.ConnectionError: HTTPConnectionPool(host='metadata.google.internal', port=80): Max retries exceeded with url: /computeMetadata/v1/instance/attributes/tpu-env (Caused by NameResolutionError("HTTPConnection(host='metadata.google.internal', port=80): Failed to resolve 'metadata.google.internal'"))

Proposed Fix

In jax/_src/clusters/cloud_tpu_cluster.py:

def get_tpu_env_value(key) -> str | None:
  # First try to get the value from the environment.
  value = os.environ.get(key, None)
  if value is None and os.environ.get("TPU_SKIP_MDS_QUERY") is None:
    # If not found, try to get it from the metadata.
    value = get_tpu_env_value_from_metadata(key)
  return value

System info (python version, jaxlib version, accelerator, etc.)

• OS: Linux
• Accelerator: Google Cloud TPU
• JAX version: main / 0.4.x+
• Environment: Kubernetes / Bare-metal TPU cluster with TPU_SKIP_MDS_QUERY=true