[core] Create the PlatformIO HTTP cache dir before the prefetch resolve threads run (#20579)

tests/unit_tests/test_platformio_prefetch.py

@@ -561,6 +561,19 @@ def test_registry_lock_deadline_skips_registration(tmp_path: Path) -> None:
     manager.set_download_utime.assert_not_called()
 
 
+def test_prefetch_creates_http_cache_dir_first(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+) -> None:
+    """Eight resolve threads each build a registry client whose cache
+    init has no exist_ok; the dir must exist before any of them run."""
+    from platformio.project import helpers
+
+    monkeypatch.setattr(helpers, "get_project_cache_dir", lambda: str(tmp_path))
+    with patch.object(pf, "_project_platform_and_config", return_value=(None, None)):
+        pf._prefetch(tmp_path, "testenv")
+    assert (tmp_path / "http").is_dir()
+
+
 def test_main_interrupt_exits_quietly(tmp_path: Path) -> None:
     """Ctrl-C reaches the child via the shared process group; it must exit
     without a traceback."""

tests/unit_tests/test_platformio_runner.py

@@ -4,6 +4,7 @@ from __future__ import annotations
 
 from collections.abc import Callable
 import io
+from pathlib import Path
 import sys
 from types import ModuleType
 from unittest.mock import Mock
@@ -110,6 +111,20 @@ def test_main_applies_registry_private_packages_patch(
     assert order == ["patch", "pio"]
 
 
+def test_ensure_http_cache_dir_is_idempotent(
+    monkeypatch: pytest.MonkeyPatch, tmp_path: Path
+) -> None:
+    """The dir exists before PlatformIO's non-exist_ok makedirs, and a
+    second caller (another thread or build) does not trip over it."""
+    from platformio.project import helpers
+
+    monkeypatch.setattr(helpers, "get_project_cache_dir", lambda: str(tmp_path / "c"))
+
+    runner.ensure_http_cache_dir()
+    assert (tmp_path / "c" / "http").is_dir()
+    runner.ensure_http_cache_dir()
+
+
 # Snapshot PlatformIO's own probe at import, before any test can patch it
 _PIO_PROBE = RegistryClient.__dict__["allowed_private_packages"]