Add a decoded context manager.

This simplifies a common chore when modifying traffic - decoding the object,
modifying it, then re-encoding it with the same encoding afterwards. You can
now simply say:

with flow.decoded(request):
    request.content = "bar"

libmproxy/flow.py

@@ -199,6 +199,34 @@ class ODictCaseless(ODict):
         return s.lower()
 
 
+class decoded(object):
+    """
+
+        A context manager that decodes a request, response or error, and then
+        re-encodes it with the same encoding after execution of the block.
+
+        Example:
+
+        with decoded(request):
+            request.content = request.content.replace("foo", "bar")
+    """
+    def __init__(self, o):
+        self.o = o
+        ce = o.headers["content-encoding"]
+        if ce and ce[0] in encoding.ENCODINGS:
+            self.ce = ce[0]
+        else:
+            self.ce = None
+
+    def __enter__(self):
+        if self.ce:
+            self.o.decode()
+
+    def __exit__(self, type, value, tb):
+        if self.ce:
+            self.o.encode(self.ce)
+
+
 class HTTPMsg(controller.Msg):
     def decode(self):
         """

test/test_flow.py

@@ -996,6 +996,29 @@ class uODictCaseless(libpry.AutoTree):
         assert len(self.od) == 1
 
 
+class udecoded(libpry.AutoTree):
+    def test_del(self):
+        r = tutils.treq()
+        assert r.content == "content"
+        assert not r.headers["content-encoding"]
+        r.encode("gzip")
+        assert r.headers["content-encoding"]
+        assert r.content != "content"
+        with flow.decoded(r):
+            assert not r.headers["content-encoding"]
+            assert r.content == "content"
+        assert r.headers["content-encoding"]
+        assert r.content != "content"
+
+        with flow.decoded(r):
+            r.content = "foo"
+
+        assert r.content != "foo"
+        r.decode()
+        assert r.content == "foo"
+
+
+
 
 tests = [
     uStickyCookieState(),
@@ -1012,4 +1035,5 @@ tests = [
     uClientConnect(),
     uODict(),
     uODictCaseless(),
+    udecoded()
 ]