Include reason in cache path warnings to aid debugging

src/_pytest/cacheprovider.py

@@ -179,16 +179,26 @@ def set(self, key: str, value: object) -> None:
             else:
                 cache_dir_exists_already = self._cachedir.exists()
                 path.parent.mkdir(exist_ok=True, parents=True)
-        except OSError:
-            self.warn("could not create cache path {path}", path=path, _ispytest=True)
+        except OSError as exc:
+            self.warn(
+                "could not create cache path {path}: {exc}",
+                path=path,
+                exc=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:
-            self.warn("cache could not write path {path}", path=path, _ispytest=True)
+        except OSError as exc:
+            self.warn(
+                "cache could not write path {path}: {exc}",
+                path=path,
+                exc=exc,
+                _ispytest=True,
+            )
         else:
             with f:
                 f.write(data)

testing/test_cacheprovider.py

@@ -87,7 +87,7 @@ def test_cache_failure_warns(
                 "*= warnings summary =*",
                 "*/cacheprovider.py:*",
                 "  */cacheprovider.py:*: PytestCacheWarning: could not create cache path "
-                f"{unwritable_cache_dir}/v/cache/nodeids",
+                f"{unwritable_cache_dir}/v/cache/nodeids: *",
                 '    config.cache.set("cache/nodeids", sorted(self.cached_nodeids))',
                 "*1 failed, 3 warnings in*",
             ]