make blocking.Wormhole into a context manager

docs/api.md

@@ -56,6 +56,11 @@ suffer longer invitation codes as a result. To encourage `close()`, the
 library will log an error if a Wormhole object is destroyed before being
 closed.
 
+To make it easier to call `close()`, the blocking Wormhole objects can be
+used as a context manager. Just put your code in the body of a `with
+Wormhole(ARGS) as w:` statement, and `close()` will automatically be called
+when the block exits (either successfully or due to an exception).
+
 ## Examples
 
 The synchronous+blocking flow looks like this:
@@ -64,12 +69,11 @@ The synchronous+blocking flow looks like this:
 from wormhole.blocking.transcribe import Wormhole
 from wormhole.public_relay import RENDEZVOUS_RELAY
 mydata = b"initiator's data"
-i = Wormhole(u"appid", RENDEZVOUS_RELAY)
+with Wormhole(u"appid", RENDEZVOUS_RELAY) as i:
     code = i.get_code()
     print("Invitation Code: %s" % code)
     i.send_data(mydata)
     theirdata = i.get_data()
-i.close()
     print("Their data: %s" % theirdata.decode("ascii"))
 ```
 
@@ -79,11 +83,10 @@ from wormhole.blocking.transcribe import Wormhole
 from wormhole.public_relay import RENDEZVOUS_RELAY
 mydata = b"receiver's data"
 code = sys.argv[1]
-r = Wormhole(u"appid", RENDEZVOUS_RELAY)
+with Wormhole(u"appid", RENDEZVOUS_RELAY) as r:
     r.set_code(code)
     r.send_data(mydata)
     theirdata = r.get_data()
-r.close()
     print("Their data: %s" % theirdata.decode("ascii"))
 ```
 

src/wormhole/blocking/transcribe.py

@@ -203,6 +203,12 @@ class Wormhole:
         self._got_data = set()
         self._closed = False
 
+    def __enter__(self):
+        return self
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.close()
+        return False
+
     def handle_welcome(self, welcome):
         if ("motd" in welcome and
             not self.motd_displayed):

src/wormhole/scripts/cmd_receive.py

@@ -12,8 +12,7 @@ def receive(args):
     from .progress import start_progress, update_progress, finish_progress
     assert isinstance(args.relay_url, type(u""))
 
-    if True:
+    with Wormhole(APPID, args.relay_url) as w:
-        w = Wormhole(APPID, args.relay_url)
         if args.zeromode:
             assert not args.code
             args.code = u"0-"
@@ -30,12 +29,10 @@ def receive(args):
             them_bytes = w.get_data()
         except WrongPasswordError as e:
             print("ERROR: " + e.explain(), file=sys.stderr)
-            w.close()
             return 1
         them_d = json.loads(them_bytes.decode("utf-8"))
         if "error" in them_d:
             print("ERROR: " + them_d["error"], file=sys.stderr)
-            w.close()
             return 1
 
         if "message" in them_d:
@@ -43,18 +40,15 @@ def receive(args):
             print(them_d["message"])
             data = json.dumps({"message_ack": "ok"}).encode("utf-8")
             w.send_data(data)
-            w.close()
             return 0
 
         if not "file" in them_d:
             print("I don't know what they're offering\n")
             print(them_d)
-            w.close()
             return 1
 
         if "error" in them_d:
             print("ERROR: " + data["error"], file=sys.stderr)
-            w.close()
             return 1
 
         file_data = them_d["file"]
@@ -68,7 +62,6 @@ def receive(args):
             print("Error: refusing to overwrite existing file %s" % (filename,))
             data = json.dumps({"error": "file already exists"}).encode("utf-8")
             w.send_data(data)
-            w.close()
             return 1
 
         print("Receiving file (%d bytes) into: %s" % (filesize, filename))
@@ -79,7 +72,6 @@ def receive(args):
             print("transfer rejected", file=sys.stderr)
             data = json.dumps({"error": "transfer rejected"}).encode("utf-8")
             w.send_data(data)
-            w.close()
             return 1
 
         transit_receiver = TransitReceiver(args.transit_helper)
@@ -91,7 +83,6 @@ def receive(args):
                 },
             }).encode("utf-8")
         w.send_data(data)
-        w.close()
 
     # now receive the rest of the owl
     tdata = them_d["transit"]

src/wormhole/scripts/cmd_send.py

@@ -46,8 +46,7 @@ def send(args):
                 },
             }
 
-    if True:
+    with Wormhole(APPID, args.relay_url) as w:
-        w = Wormhole(APPID, args.relay_url)
         if args.zeromode:
             assert not args.code
             args.code = u"0-"
@@ -78,7 +77,6 @@ def send(args):
                     reject_data = json.dumps({"error": "verification rejected",
                                               }).encode("utf-8")
                     w.send_data(reject_data)
-                    w.close()
                     return 1
 
         my_phase1_bytes = json.dumps(phase1).encode("utf-8")
@@ -87,30 +85,24 @@ def send(args):
             them_phase1_bytes = w.get_data()
         except WrongPasswordError as e:
             print("ERROR: " + e.explain(), file=sys.stderr)
-            w.close()
             return 1
         them_phase1 = json.loads(them_phase1_bytes.decode("utf-8"))
 
         if sending_message:
             if them_phase1["message_ack"] == "ok":
                 print("text message sent")
-                w.close()
                 return 0
             print("error sending text: %r" % (them_phase1,))
-            w.close()
             return 1
 
         if "error" in them_phase1:
             print("remote error: %s" % them_phase1["error"])
             print("transfer abandoned")
-            w.close()
             return 1
         if them_phase1.get("file_ack") != "ok":
             print("ambiguous response from remote: %s" % (them_phase1,))
             print("transfer abandoned")
-            w.close()
             return 1
-        w.close()
 
     tdata = them_phase1["transit"]
     transit_key = w.derive_key(APPID+"/transit-key")