Issue #13703: add a way to randomize the hash values of basic types (str, bytes, datetime)

in order to make algorithmic complexity attacks on (e.g.) web apps much more complicated.

The environment variable PYTHONHASHSEED and the new command line flag -R control this
behavior.

Doc/using/cmdline.rst

@@ -21,7 +21,7 @@ Command line
 
 When invoking Python, you may specify any of these options::
 
-    python [-bBdEhiOsSuvVWx?] [-c command | -m module-name | script | - ] [args]
+    python [-bBdEhiORsSuvVWx?] [-c command | -m module-name | script | - ] [args]
 
 The most common use case is, of course, a simple invocation of a script::
 
@@ -215,6 +215,29 @@ Miscellaneous options
    Discard docstrings in addition to the :option:`-O` optimizations.
 
 
+.. cmdoption:: -R
+
+   Turn on hash randomization, so that the :meth:`__hash__` values of str, bytes
+   and datetime objects are "salted" with an unpredictable random value.
+   Although they remain constant within an individual Python process, they are
+   not predictable between repeated invocations of Python.
+
+   This is intended to provide protection against a denial-of-service caused by
+   carefully-chosen inputs that exploit the worst case performance of a dict
+   insertion, O(n^2) complexity.  See
+   http://www.ocert.org/advisories/ocert-2011-003.html for details.
+
+   Changing hash values affects the order in which keys are retrieved from a
+   dict.  Although Python has never made guarantees about this ordering (and it
+   typically varies between 32-bit and 64-bit builds), enough real-world code
+   implicitly relies on this non-guaranteed behavior that the randomization is
+   disabled by default.
+
+   See also :envvar:`PYTHONHASHSEED`.
+
+   .. versionadded:: 3.1.5
+
+
 .. cmdoption:: -s
 
    Don't add user site directory to sys.path
@@ -314,6 +337,7 @@ Miscellaneous options
 
    .. note:: The line numbers in error messages will be off by one.
 
+
 Options you shouldn't use
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -328,6 +352,7 @@ Options you shouldn't use
     Reserved for alternative implementations of Python to use for their own
     purposes.
 
+
 .. _using-on-envvars:
 
 Environment variables
@@ -435,6 +460,27 @@ These environment variables influence Python's behavior.
    import of source modules.
 
 
+.. envvar:: PYTHONHASHSEED
+
+   If this variable is set to ``random``, the effect is the same as specifying
+   the :option:`-R` option: a random value is used to seed the hashes of str,
+   bytes and datetime objects.
+
+   If :envvar:`PYTHONHASHSEED` is set to an integer value, it is used as a fixed
+   seed for generating the hash() of the types covered by the hash
+   randomization.
+
+   Its purpose is to allow repeatable hashing, such as for selftests for the
+   interpreter itself, or to allow a cluster of python processes to share hash
+   values.
+
+   The integer must be a decimal number in the range [0,4294967295].  Specifying
+   the value 0 will lead to the same hash values as when hash randomization is
+   disabled.
+
+   .. versionadded:: 3.1.5
+
+
 .. envvar:: PYTHONIOENCODING
 
    Overrides the encoding used for stdin/stdout/stderr, in the syntax