oci: only push in parallel when forking (#42143)

lib/spack/spack/cmd/buildcache.py

@@ -7,13 +7,14 @@
 import glob
 import hashlib
 import json
+import multiprocessing
 import multiprocessing.pool
 import os
 import shutil
 import sys
 import tempfile
 import urllib.request
-from typing import Dict, List, Optional, Tuple
+from typing import Dict, List, Optional, Tuple, Union
 
 import llnl.util.tty as tty
 from llnl.string import plural
@@ -307,8 +308,30 @@ def _progress(i: int, total: int):
     return ""
 
 
-def _make_pool():
+class NoPool:
+    def map(self, func, args):
+        return [func(a) for a in args]
+
+    def starmap(self, func, args):
+        return [func(*a) for a in args]
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, *args):
+        pass
+
+
+MaybePool = Union[multiprocessing.pool.Pool, NoPool]
+
+
+def _make_pool() -> MaybePool:
+    """Can't use threading because it's unsafe, and can't use spawned processes because of globals.
+    That leaves only forking"""
+    if multiprocessing.get_start_method() == "fork":
         return multiprocessing.pool.Pool(determine_number_of_jobs(parallel=True))
+    else:
+        return NoPool()
 
 
 def push_fn(args):
@@ -591,7 +614,7 @@ def _push_oci(
     image_ref: ImageReference,
     installed_specs_with_deps: List[Spec],
     tmpdir: str,
-    pool: multiprocessing.pool.Pool,
+    pool: MaybePool,
 ) -> List[str]:
     """Push specs to an OCI registry
 
@@ -692,9 +715,7 @@ def _config_from_tag(image_ref: ImageReference, tag: str) -> Optional[dict]:
     return config if "spec" in config else None
 
 
-def _update_index_oci(
-    image_ref: ImageReference, tmpdir: str, pool: multiprocessing.pool.Pool
-) -> None:
+def _update_index_oci(image_ref: ImageReference, tmpdir: str, pool: MaybePool) -> None:
     response = spack.oci.opener.urlopen(urllib.request.Request(url=image_ref.tags_url()))
     spack.oci.opener.ensure_status(response, 200)
     tags = json.load(response)["tags"]

lib/spack/spack/test/conftest.py

@@ -1950,17 +1950,5 @@ def pytest_runtest_setup(item):
 
 @pytest.fixture(scope="function")
 def disable_parallel_buildcache_push(monkeypatch):
-    class MockPool:
-        def map(self, func, args):
-            return [func(a) for a in args]
-
-        def starmap(self, func, args):
-            return [func(*a) for a in args]
-
-        def __enter__(self):
-            return self
-
-        def __exit__(self, *args):
-            pass
-
-    monkeypatch.setattr(spack.cmd.buildcache, "_make_pool", MockPool)
+    """Disable process pools in tests."""
+    monkeypatch.setattr(spack.cmd.buildcache, "_make_pool", spack.cmd.buildcache.NoPool)