wireprotov2: define type to represent pre-encoded object

wireprotov2: define type to represent pre-encoded object

An upcoming commit will introduce a caching layer to command
serving. This will require the ability to cache pre-encoded data.

This commit introduces a type to represent pre-encoded data and
teaches the output layer to not CBOR encode an instance of that
type.

Differential Revision: https://phab.mercurial-scm.org/D4772

--- a/mercurial/wireprotoframing.py	Wed Sep 26 15:53:49 2018 -0700
+++ b/mercurial/wireprotoframing.py	Wed Sep 26 17:16:27 2018 -0700
@@ -22,6 +22,7 @@
     encoding,
     error,
     util,
+    wireprototypes,
 )
 from .utils import (
     cborutil,
@@ -840,10 +841,22 @@
                         yield createcommandresponseokframe(stream, requestid)
                         emitted = True
 
-                    for chunk in cborutil.streamencode(o):
-                        for frame in emitter.send(chunk):
+                    # Objects emitted by command functions can be serializable
+                    # data structures or special types.
+                    # TODO consider extracting the content normalization to a
+                    # standalone function, as it may be useful for e.g. cachers.
+
+                    # A pre-encoded object is sent directly to the emitter.
+                    if isinstance(o, wireprototypes.encodedresponse):
+                        for frame in emitter.send(o.data):
                             yield frame
 
+                    # A regular object is CBOR encoded.
+                    else:
+                        for chunk in cborutil.streamencode(o):
+                            for frame in emitter.send(chunk):
+                                yield frame
+
                 except Exception as e:
                     for frame in createerrorframe(stream, requestid,
                                                   '%s' % e,

--- a/mercurial/wireprototypes.py	Wed Sep 26 15:53:49 2018 -0700
+++ b/mercurial/wireprototypes.py	Wed Sep 26 17:16:27 2018 -0700
@@ -10,6 +10,9 @@
     hex,
 )
 from .i18n import _
+from .thirdparty import (
+    attr,
+)
 from . import (
     error,
     util,
@@ -352,3 +355,15 @@
                           ', '.sorted(validnames))
 
     return compengines
+
+@attr.s
+class encodedresponse(object):
+    """Represents response data that is already content encoded.
+
+    Wire protocol version 2 only.
+
+    Commands typically emit Python objects that are encoded and sent over the
+    wire. If commands emit an object of this type, the encoding step is bypassed
+    and the content from this object is used instead.
+    """
+    data = attr.ib()