1 files changed, 81 insertions, 0 deletions
diff --git a/indra/llmessage/tests/testrunner.py b/indra/llmessage/tests/testrunner.py
index b70ce91ee7..8ff13e0426 100644
--- a/indra/llmessage/tests/testrunner.py
+++ b/indra/llmessage/tests/testrunner.py
@@ -29,6 +29,8 @@ $/LicenseInfo$
 
 import os
 import sys
+import errno
+import socket
 
 def debug(*args):
     sys.stdout.writelines(args)
@@ -36,6 +38,85 @@ def debug(*args):
 # comment out the line below to enable debug output
 debug = lambda *args: None
 
+def freeport(portlist, expr):
+    """
+    Find a free server port to use. Specifically, evaluate 'expr' (a
+    callable(port)) until it stops raising EADDRINUSE exception.
+
+    Pass:
+
+    portlist: an iterable (e.g. xrange()) of ports to try. If you exhaust the
+    range, freeport() lets the socket.error exception propagate. If you want
+    unbounded, you could pass itertools.count(baseport), though of course in
+    practice the ceiling is 2^16-1 anyway. But it seems prudent to constrain
+    the range much more sharply: if we're iterating an absurd number of times,
+    probably something else is wrong.
+
+    expr: a callable accepting a port number, specifically one of the items
+    from portlist. If calling that callable raises socket.error with
+    EADDRINUSE, freeport() retrieves the next item from portlist and retries.
+
+    Returns: (expr(port), port)
+
+    port: the value from portlist for which expr(port) succeeded
+
+    Raises:
+
+    Any exception raised by expr(port) other than EADDRINUSE.
+
+    socket.error if, for every item from portlist, expr(port) raises
+    socket.error. The exception you see is the one from the last item in
+    portlist.
+
+    StopIteration if portlist is completely empty.
+
+    Example:
+
+    server, port = freeport(xrange(8000, 8010),
+                            lambda port: HTTPServer(("localhost", port),
+                                                    MyRequestHandler))
+    # pass 'port' to client code
+    # call server.serve_forever()
+    """
+    # If portlist is completely empty, let StopIteration propagate: that's an
+    # error because we can't return meaningful values. We have no 'port',
+    # therefore no 'expr(port)'.
+    portiter = iter(portlist)
+    port = portiter.next()
+
+    while True:
+        try:
+            # If this value of port works, return as promised.
+            return expr(port), port
+
+        except socket.error, err:
+            # Anything other than 'Address already in use', propagate
+            if err.args[0] != errno.EADDRINUSE:
+                raise
+
+            # Here we want the next port from portiter. But on StopIteration,
+            # we want to raise the original exception rather than
+            # StopIteration. So save the original exc_info().
+            type, value, tb = sys.exc_info()
+            try:
+                try:
+                    port = portiter.next()
+                except StopIteration:
+                    raise type, value, tb
+            finally:
+                # Clean up local traceback, see docs for sys.exc_info()
+                del tb
+
+        # Recap of the control flow above:
+        # If expr(port) doesn't raise, return as promised.
+        # If expr(port) raises anything but EADDRINUSE, propagate that
+        # exception.
+        # If portiter.next() raises StopIteration -- that is, if the port
+        # value we just passed to expr(port) was the last available -- reraise
+        # the EADDRINUSE exception.
+        # If we've actually arrived at this point, portiter.next() delivered a
+        # new port value. Loop back to pass that to expr(port).
+
 def run(*args, **kwds):
     """All positional arguments collectively form a command line, executed as
     a synchronous child process.