Python API: move the really_get_paths function into the API

example/example.py

@@ -1,4 +1,4 @@
-import pyscion as sci
+import pyscion as scion
 import sys
 import time
 
@@ -7,27 +7,18 @@
 
 
 def main():
-    sci.init()
-    print('Local Address is {}'.format(sci.local_address()))
+    scion.init()
+    print('Local Address is {}'.format(scion.local_address()))
     for dest_addr, nbytes in parse_tasks_from_stdin():
         destination = "{}:12345".format(dest_addr) # send to port 12345
         print(' === TASK to destination {} : {}MB ==='.format(destination, int(nbytes/1024/1024)))
-        paths = really_get_paths(destination)
-        print('Got %d paths' % len(paths))
-        with sci.connect(destination, paths[0]) as fd:
+        paths = scion.get_paths(destination)
+        print('Got {} paths'.format(len(paths)))
+        with scion.connect(destination, paths[0]) as fd:
             for i in range(int(nbytes / 1000)+1):
                 fd.write(MTU*b'a')
 
 
-def really_get_paths(destination):
-    # getting paths is async, so let's just retry until we get some
-    while True:
-        try:
-            return sci.paths(destination)
-        except sci.SCIONException:
-            time.sleep(0.1)
-
-
 def parse_tasks_from_stdin():
     def parse_line(line):
         dest, nbytes = line.split()

example/pyscion.py

@@ -4,7 +4,7 @@
 - set_log_level(level)
 - init()
 - addr = local_address()
-- paths = paths(destination)
+- paths = get_paths(destination)
 - fd = connect(destination, path)
 - fd.close()
 - fd.write(buffer)
@@ -60,3 +60,7 @@ def paths(destination):
 
 def listen(port):
     return connect(None, None)
+
+
+def get_paths(destination, loop_till_have_paths=True):
+    return paths(destination)

go/example.py

@@ -8,7 +8,7 @@ def main():
     print('Local Address is %s' % local_address())
 
     destination = '17-ffaa:1:a,[127.0.0.1]:12345'
-    p = paths(destination)
+    p = get_paths(destination, loop_till_have_paths=False)
     print('Got %d paths:' % len(p))
     for i in range(len(p)):
         print('[%d] %s' % (i, str(p)))

go/pyscion.py

@@ -1,5 +1,23 @@
+"""Rudimentary Python API for SCION: wraps a simplified Go API.
+
+To use this, you need to build scion_api.go, which will give you a .so that
+this file provides bindings for. Place both the .so and this file into the same
+directory, somewhere in your Python path (e.g. into .).
+
+The API consists of:
+- set_log_level(level)
+- init()
+- addr = local_address()
+- paths = get_paths(destination)
+- fd = connect(destination, path)
+- fd.close()
+- fd.write(buffer)
+- fd = listen(port)
+- addr, n = fd.read(buffer)
+"""
+
 __all__ = ['SCIONException', 'HostInfo', 'FwdPathMeta', 'Interface', 'Path', 'connect',
-    'set_log_level', 'init', 'local_address', 'paths', 'listen']
+    'set_log_level', 'init', 'local_address', 'get_paths', 'listen']
 
 from ctypes import *
 from collections import namedtuple
@@ -164,7 +182,7 @@ def local_address():
 _lib.Paths.restype = c_char_p
 _lib.FreePathsMemory.argtypes = [POINTER(_PathReplyEntry), c_size_t]
 _lib.FreePathsMemory.restype = c_char_p
-def paths(destination):
+def _call_paths(destination):
     paths_n = c_size_t()
     paths = (POINTER(_PathReplyEntry))()
     err = _lib.Paths(byref(paths_n), byref(paths), _str_to_cstr(destination))
@@ -229,3 +247,32 @@ def listen(port):
     conn = connect(None, None)
     conn.fd = int(fd.value)
     return conn
+
+
+def get_paths(destination, loop_till_have_paths=True):
+    """Returns a list of SCION Paths.
+
+    Warning: This function will change! However, using it with only the
+    destination (and leaving all other arguments default) will have the same
+    semantics, i.e. will block until it can return a non-empty list of paths to
+    the destination.
+
+    Getting paths in SCION can be async/non-blocking: if you pass
+    loop_till_have_paths=False, this function will return immediately with
+    whatever paths are in the cache right now. If this is the first time you're
+    looking up this destination, the cache may be empty (and this will return
+    []).
+
+    If loop_till_have_paths=True, this function will retry until it gets
+    at least one path. (Note that if the destination is unreachable, this will
+    loop forever!)
+
+    Ideally, this function would be able to distinguish between "no paths in
+    cache" and "unreachable". That is a TODO.
+    """
+    while True:
+        try:
+            return _call_paths(destination)
+        except SCIONException as e:
+            if not loop_till_have_paths:
+                raise e