add an acceptance test for cache write errors

_pytest/cacheprovider.py

@@ -73,7 +73,7 @@ class Cache(object):
             path.dirpath().ensure_dir()
         except (py.error.EEXIST, py.error.EACCES):
             self.config.warn(
-                code='I9', message='cache could not create cache path %s' % (path,)
+                code='I9', message='could not create cache path %s' % (path,)
             )
             return
         try:

testing/test_cache.py

@@ -32,13 +32,26 @@ class TestNewAPI:
         cache = config.cache
         cache.set('test/broken', [])
 
-    def test_cache_writefail_permissions(selfself, testdir):
+    def test_cache_writefail_permissions(self, testdir):
         testdir.makeini("[pytest]")
         testdir.tmpdir.ensure_dir('.cache').chmod(0)
         config = testdir.parseconfigure()
         cache = config.cache
         cache.set('test/broken', [])
 
+    def test_cache_failure_warns(self, testdir):
+        testdir.tmpdir.ensure_dir('.cache').chmod(0)
+        testdir.makepyfile("""
+            def test_pass():
+                pass
+
+        """)
+        result = testdir.runpytest('-rw')
+        assert result.ret == 0
+        result.stdout.fnmatch_lines([
+            "*could not create cache path*",
+            "*1 pytest-warnings*",
+        ])
 
     def test_config_cache(self, testdir):
         testdir.makeconftest("""