Fix on windows.

src/outpack/filestore.py

@@ -2,11 +2,11 @@
 import os.path
 import shutil
 import stat
-import tempfile
 from contextlib import contextmanager
 from pathlib import Path
 
 from outpack.hash import Hash, hash_parse, hash_validate_file
+from outpack.util import openable_temporary_file
 
 
 class FileStore:
@@ -85,16 +85,11 @@ def onerror(func, path, _exc_info):
 
     @contextmanager
     def tmp(self):
-        # On a newer version of tempfile we could use `delete_on_close = False`
-        # see
-        # https://github.com/mrc-ide/outpack-py/pull/33#discussion_r1500522877
         path = self._path / "tmp"
         path.mkdir(exist_ok=True)
-        f = tempfile.NamedTemporaryFile(dir=path, delete=False)
-        try:
+
+        with openable_temporary_file(dir=path) as f:
+            # Close the file (but don't delete it). We are only interested in
+            # using the path.
+            f.close()
             yield f.name
-        finally:
-            try:
-                os.unlink(f.name)
-            except OSError:
-                pass

src/outpack/util.py

@@ -1,10 +1,12 @@
 import datetime
 import os
 import runpy
+import tempfile
 import time
 from contextlib import contextmanager
 from itertools import filterfalse, tee
 from pathlib import Path
+from typing import Optional
 
 
 def find_file_descend(filename, path):
@@ -141,3 +143,23 @@ def partition(pred, iterable):
     # partition(is_odd, range(10)) --> 1 3 5 7 9 and 0 2 4 6 8
     t1, t2 = tee(iterable)
     return list(filter(pred, t1)), list(filterfalse(pred, t2))
+
+
+@contextmanager
+def openable_temporary_file(*, mode: str = "w+b", dir: Optional[str] = None):
+    # On Windows, a NamedTemporaryFile with `delete=True` cannot be reopened,
+    # which makes its name pretty useless. On Python 3.12, a new
+    # delete_on_close flag is solves this issue, but we can't depend on that
+    # yet. This block mimicks that feature.
+    #
+    # https://bugs.python.org/issue14243
+    # https://github.com/mrc-ide/outpack-py/pull/33#discussion_r1500522877
+    f = tempfile.NamedTemporaryFile(mode=mode, dir=dir, delete=False)
+    try:
+        yield f
+    finally:
+        f.close()
+        try:
+            os.unlink(f.name)
+        except OSError:
+            pass

tests/helpers/__init__.py

@@ -2,7 +2,6 @@
 import random
 import shutil
 import string
-import tempfile
 import textwrap
 from contextlib import contextmanager
 from pathlib import Path
@@ -16,6 +15,7 @@
 from outpack.packet import Packet
 from outpack.root import root_open
 from outpack.schema import outpack_schema_version
+from outpack.util import openable_temporary_file
 
 
 @contextmanager
@@ -173,7 +173,7 @@ def run_snippet(name, code, root, **kwargs):
     # We work around that by returning the value out-of-band, in a temporary
     # file outside of the report's directory. That way it is completely
     # transparent to the caller
-    with tempfile.NamedTemporaryFile() as output:
+    with openable_temporary_file() as output:
         wrapped = f"""
 def body():
 {textwrap.indent(code, "    ")}