Patchwork [3,of,3] bundle2: handle long error params on the unbundling side

diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -1556,11 +1556,37 @@  def handlereplycaps(op, inpart):
 class AbortFromPart(error.Abort):
     """Sub-class of Abort that denotes an error from a bundle2 part."""
 
+def _errorparams(inpart):
+    """Read error parameters from the payload if so specified.
+
+    This should only be used for errors generated via bundleerrorpart."""
+    params = inpart.params
+    if params.get('longparams') != 'payload':
+        return params
+    # avoid modifying the params dict in inpart
+    params = params.copy()
+    # don't use inpart._unpack/_readexact because they read the underlying
+    # stream (with payload chunk sizes etc) -- instead read from the
+    # higher-level stream
+    lensize = struct.calcsize(_ferrorlongparamsize)
+    def readsize():
+        return struct.unpack(_ferrorlongparamsize,
+                             changegroup.readexactly(inpart, lensize))[0]
+
+    numparams = readsize()
+    for _param in xrange(numparams):
+        namesize = readsize()
+        name = changegroup.readexactly(inpart, namesize)
+        valuesize = readsize()
+        value = changegroup.readexactly(inpart, valuesize)
+        params[name] = value
+    return params
+
 @parthandler('error:abort', ('message', 'hint'))
 def handleerrorabort(op, inpart):
     """Used to transmit abort error over the wire"""
-    raise AbortFromPart(inpart.params['message'],
-                        hint=inpart.params.get('hint'))
+    params = _errorparams(inpart)
+    raise AbortFromPart(params['message'], hint=params.get('hint'))
 
 @parthandler('error:pushkey', ('namespace', 'key', 'new', 'old', 'ret',
                                'in-reply-to'))
@@ -1576,11 +1602,12 @@  def handleerrorpushkey(op, inpart):
 @parthandler('error:unsupportedcontent', ('parttype', 'params'))
 def handleerrorunsupportedcontent(op, inpart):
     """Used to transmit unknown content error over the wire"""
+    params = _errorparams(inpart)
     kwargs = {}
-    parttype = inpart.params.get('parttype')
+    parttype = params.get('parttype')
     if parttype is not None:
         kwargs['parttype'] = parttype
-    params = inpart.params.get('params')
+    params = params.get('params')
     if params is not None:
         kwargs['params'] = params.split('\0')
 
@@ -1589,7 +1616,8 @@  def handleerrorunsupportedcontent(op, in
 @parthandler('error:pushraced', ('message',))
 def handleerrorpushraced(op, inpart):
     """Used to transmit push race error over the wire"""
-    raise error.ResponseError(_('push failed:'), inpart.params['message'])
+    params = _errorparams(inpart)
+    raise error.ResponseError(_('push failed:'), params['message'])
 
 @parthandler('listkeys', ('namespace',))
 def handlelistkeys(op, inpart):
diff --git a/tests/test-bundle2-exchange.t b/tests/test-bundle2-exchange.t
--- a/tests/test-bundle2-exchange.t
+++ b/tests/test-bundle2-exchange.t
@@ -464,6 +464,10 @@  Setting up
   >         bundler.newpart('test:abort')
   >     if reason == 'abort-long':
   >         bundler.newpart('test:abort-long')
+  >     if reason == 'abort-legacy':
+  >         # Use bundler.newpart rather than bundler.newerrorpart to create this part.
+  >         # This simulates a Mercurial server < 4.2.
+  >         bundler.newpart('error:abort', [('message', 'Abandon ship too!')])
   >     if reason == 'unknown':
   >         bundler.newpart('test:unknown')
   >     if reason == 'race':
@@ -480,9 +484,16 @@  Setting up
   >     # across payload chunks
   >     raise error.Abort('a' * 8192, hint="don't panic")
   > 
+  > def overrideerrorparams(orig, inpart):
+  >     # simulate Mercurial < 4.2 if this config is set
+  >     if inpart.ui.configbool('failpush', 'legacyerrorparams'):
+  >         return inpart.params
+  >     return orig(inpart)
+  > 
   > def uisetup(ui):
   >     exchange.b2partsgenmapping['failpart'] = _pushbundle2failpart
   >     exchange.b2partsgenorder.insert(0, 'failpart')
+  >     extensions.wrapfunction(bundle2, '_errorparams', overrideerrorparams)
   > 
   > EOF
 
@@ -543,6 +554,22 @@  Doing the actual push: Abort error (mess
   $ cat << EOF >> $HGRCPATH
   > [failpush]
   > reason = abort-long
+  > legacyerrorparams = false
+  > EOF
+
+  $ hg -R main push ssh://user@dummy/other -r e7ec4e813ba6
+  pushing to ssh://user@dummy/other
+  searching for changes
+  remote: (a){8192} (re)
+  remote: (don't panic)
+  abort: push failed on remote
+  [255]
+
+Abort error (simulate client Mercurial < 4.2 -- truncated message)
+
+  $ cat << EOF >> $HGRCPATH
+  > [failpush]
+  > legacyerrorparams = true
   > EOF
 
   $ hg -R main push ssh://user@dummy/other -r e7ec4e813ba6
@@ -553,6 +580,25 @@  Doing the actual push: Abort error (mess
   abort: push failed on remote
   [255]
 
+  $ cat << EOF >> $HGRCPATH
+  > [failpush]
+  > legacyerrorparams = false
+  > EOF
+
+Abort error (simulate server Mercurial < 4.2)
+
+  $ cat << EOF >> $HGRCPATH
+  > [failpush]
+  > reason = abort-legacy
+  > EOF
+
+  $ hg -R main push ssh://user@dummy/other -r e7ec4e813ba6
+  pushing to ssh://user@dummy/other
+  searching for changes
+  remote: Abandon ship too!
+  abort: push failed on remote
+  [255]
+
 Doing the actual push: unknown mandatory parts
 
   $ cat << EOF >> $HGRCPATH