Ensure proper cache initialization before writing

Writing cache data is interruptible; this prevents a pathological case
where interrupting a cache write can cause the cache directory to never
be properly initialized with its supporting files.

Unify `Cache.mkdir` with `Cache.set` while I'm here so the former also
properly initializes the cache directory.

Fixes pytest-dev#12167.

src/_pytest/cacheprovider.py

@@ -123,6 +123,10 @@ def warn(self, fmt: str, *, _ispytest: bool = False, **args: object) -> None:
             stacklevel=3,
         )
 
+    def _mkdir(self, path: Path) -> None:
+        self._ensure_cache_dir_and_supporting_files()
+        path.mkdir(exist_ok=True, parents=True)
+
     def mkdir(self, name: str) -> Path:
         """Return a directory path object with the given name.
 
@@ -141,7 +145,7 @@ def mkdir(self, name: str) -> Path:
         if len(path.parts) > 1:
             raise ValueError("name is not allowed to contain path separators")
         res = self._cachedir.joinpath(self._CACHE_PREFIX_DIRS, path)
-        res.mkdir(exist_ok=True, parents=True)
+        self._mkdir(res)
         return res
 
     def _getvaluepath(self, key: str) -> Path:
@@ -178,20 +182,13 @@ def set(self, key: str, value: object) -> None:
         """
         path = self._getvaluepath(key)
         try:
-            if path.parent.is_dir():
-                cache_dir_exists_already = True
-            else:
-                cache_dir_exists_already = self._cachedir.exists()
-                path.parent.mkdir(exist_ok=True, parents=True)
+            self._mkdir(path.parent)
         except OSError as exc:
             self.warn(
                 f"could not create cache path {path}: {exc}",
                 _ispytest=True,
             )
             return
-        if not cache_dir_exists_already:
-            self._ensure_supporting_files()
-        data = json.dumps(value, ensure_ascii=False, indent=2)
         try:
             f = path.open("w", encoding="UTF-8")
         except OSError as exc:
@@ -201,19 +198,23 @@ def set(self, key: str, value: object) -> None:
             )
         else:
             with f:
-                f.write(data)
+                json.dumps(value, ensure_ascii=False, indent=2, fp=f)
 
-    def _ensure_supporting_files(self) -> None:
-        """Create supporting files in the cache dir that are not really part of the cache."""
+    def _ensure_cache_dir_and_supporting_files(self) -> None:
+        """Create the cache dir and its supporting files."""
+        self._cachedir.mkdir(exist_ok=True, parents=True)
         readme_path = self._cachedir / "README.md"
-        readme_path.write_text(README_CONTENT, encoding="UTF-8")
+        if not readme_path.exists():
+            readme_path.write_text(README_CONTENT, encoding="UTF-8")
 
         gitignore_path = self._cachedir.joinpath(".gitignore")
-        msg = "# Created by pytest automatically.\n*\n"
-        gitignore_path.write_text(msg, encoding="UTF-8")
+        if not gitignore_path.exists():
+            msg = "# Created by pytest automatically.\n*\n"
+            gitignore_path.write_text(msg, encoding="UTF-8")
 
         cachedir_tag_path = self._cachedir.joinpath("CACHEDIR.TAG")
-        cachedir_tag_path.write_bytes(CACHEDIR_TAG_CONTENT)
+        if not cachedir_tag_path.exists():
+            cachedir_tag_path.write_bytes(CACHEDIR_TAG_CONTENT)
 
 
 class LFPluginCollWrapper: