cacheprovider: do not write README/.gitignore to existing dir

Fixes https://github.com/pytest-dev/pytest/issues/4393.

changelog/4393.bugfix.rst

@@ -0,0 +1 @@
+Do not create ``.gitignore``/``README.md`` files in existing cache directories.

src/_pytest/cacheprovider.py

@@ -108,6 +108,10 @@ class Cache(object):
         """
         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)
         except (IOError, OSError):
             self.warn("could not create cache path {path}", path=path)
@@ -119,6 +123,7 @@ class Cache(object):
         else:
             with f:
                 json.dump(value, f, indent=2, sort_keys=True)
+            if not cache_dir_exists_already:
                 self._ensure_supporting_files()
 
     def _ensure_supporting_files(self):
@@ -128,8 +133,10 @@ class Cache(object):
             if not readme_path.is_file():
                 readme_path.write_text(README_CONTENT)
 
-            msg = u"# created by pytest automatically, do not change\n*"
-            self._cachedir.joinpath(".gitignore").write_text(msg, encoding="UTF-8")
+            gitignore_path = self._cachedir.joinpath(".gitignore")
+            if not gitignore_path.is_file():
+                msg = u"# Created by pytest automatically.\n*"
+                gitignore_path.write_text(msg, encoding="UTF-8")
 
 
 class LFPlugin(object):

testing/test_cacheprovider.py

@@ -899,5 +899,28 @@ def test_gitignore(testdir):
     config = testdir.parseconfig()
     cache = Cache.for_config(config)
     cache.set("foo", "bar")
-    msg = "# created by pytest automatically, do not change\n*"
-    assert cache._cachedir.joinpath(".gitignore").read_text(encoding="UTF-8") == msg
+    msg = "# Created by pytest automatically.\n*"
+    gitignore_path = cache._cachedir.joinpath(".gitignore")
+    assert gitignore_path.read_text(encoding="UTF-8") == msg
+
+    # Does not overwrite existing/custom one.
+    gitignore_path.write_text("")
+    cache.set("something", "else")
+    assert gitignore_path.read_text(encoding="UTF-8") == ""
+
+
+def test_does_not_create_boilerplate_in_existing_dirs(testdir):
+    from _pytest.cacheprovider import Cache
+
+    testdir.makeini(
+        """
+        [pytest]
+        cache_dir = .
+        """
+    )
+    config = testdir.parseconfig()
+    cache = Cache.for_config(config)
+    cache.set("foo", "bar")
+
+    assert not os.path.exists(".gitignore")
+    assert not os.path.exists("README.md")