decorator: Extract zerver/lib/profile.py.

This lets us remove this debugging function from our core
authentication codebase.

zerver/lib/profile.py

@@ -0,0 +1,36 @@
+from __future__ import absolute_import
+
+import cProfile
+
+from functools import wraps
+from typing import Any
+
+from zerver.decorator import FuncT
+
+def profiled(func):
+    # type: (FuncT) -> FuncT
+    """
+    This decorator should obviously be used only in a dev environment.
+    It works best when surrounding a function that you expect to be
+    called once.  One strategy is to write a backend test and wrap the
+    test case with the profiled decorator.
+
+    You can run a single test case like this:
+
+        # edit zerver/tests/test_external.py and place @profiled above the test case below
+        ./tools/test-backend zerver.tests.test_external.RateLimitTests.test_ratelimit_decrease
+
+    Then view the results like this:
+
+        ./tools/show-profile-results.py test_ratelimit_decrease.profile
+
+    """
+    @wraps(func)
+    def wrapped_func(*args, **kwargs):
+        # type: (*Any, **Any) -> Any
+        fn = func.__name__ + ".profile"
+        prof = cProfile.Profile()
+        retval = prof.runcall(func, *args, **kwargs)  # type: Any
+        prof.dump_stats(fn)
+        return retval
+    return wrapped_func  # type: ignore # https://github.com/python/mypy/issues/1927