Fix a race condition in simultaneous poetry installations updating th…

…e cache (#6186)

If one poetry installation is writing to the cache while a second
installer is attempting to read that cache file, the second installation
will fail because the in-flight cache file is invalid while it is still
being written to by the first process.

This PR resolves this issue by having Poetry write to a temporary file
in the cache directory first, and then rename the file after it's
written, which is ~atomic.

Resolves: #5142 

I'm not sure how to test this change, as the conditions which cause this
bug to appear are a little hard to reproduce.

src/poetry/installation/executor.py

@@ -28,6 +28,7 @@
 from poetry.utils._compat import decode
 from poetry.utils.authenticator import Authenticator
 from poetry.utils.env import EnvCommandError
+from poetry.utils.helpers import atomic_open
 from poetry.utils.helpers import pluralize
 from poetry.utils.helpers import remove_directory
 from poetry.utils.pip import pip_install
@@ -698,7 +699,7 @@ def _download_archive(self, operation: Install | Update, link: Link) -> Path:
         done = 0
         archive = self._chef.get_cache_directory_for_link(link) / link.filename
         archive.parent.mkdir(parents=True, exist_ok=True)
-        with archive.open("wb") as f:
+        with atomic_open(archive) as f:
             for chunk in response.iter_content(chunk_size=4096):
                 if not chunk:
                     break

src/poetry/utils/helpers.py

@@ -20,6 +20,7 @@
 
 if TYPE_CHECKING:
     from collections.abc import Callable
+    from io import BufferedWriter
 
     from poetry.core.packages.package import Package
     from requests import Session
@@ -37,6 +38,24 @@ def directory(path: Path) -> Iterator[Path]:
         os.chdir(cwd)
 
 
+@contextmanager
+def atomic_open(filename: str | os.PathLike[str]) -> Iterator[BufferedWriter]:
+    """
+    write a file to the disk in an atomic fashion
+
+    Taken from requests.utils
+    (https://github.com/psf/requests/blob/7104ad4b135daab0ed19d8e41bd469874702342b/requests/utils.py#L296)
+    """
+    tmp_descriptor, tmp_name = tempfile.mkstemp(dir=os.path.dirname(filename))
+    try:
+        with os.fdopen(tmp_descriptor, "wb") as tmp_handler:
+            yield tmp_handler
+        os.replace(tmp_name, filename)
+    except BaseException:
+        os.remove(tmp_name)
+        raise
+
+
 def _on_rm_error(func: Callable[[str], None], path: str, exc_info: Exception) -> None:
     if not os.path.exists(path):
         return