util: declare wire protocol support of compression engines

util: declare wire protocol support of compression engines

This patch implements a new compression engine API allowing
compression engines to declare support for the wire protocol.

Support is declared by returning a compression format string
identifier that will be added to payloads to signal the compression
type of data that follows and default integer priorities of the
engine.

Accessor methods have been added to the compression engine manager
class to facilitate use.

Note that the "none" and "bz2" engines declare wire protocol support
but aren't enabled by default due to their priorities being 0. It
is essentially free from a coding perspective to support these
compression formats, so we do it in case anyone may derive use from
it.

--- a/mercurial/help/internals/wireprotocol.txt	Sat Dec 24 13:56:36 2016 -0700
+++ b/mercurial/help/internals/wireprotocol.txt	Sat Dec 24 13:51:12 2016 -0700
@@ -269,6 +269,17 @@
 supported compression formats. The order of the compression formats is in
 server-preferred order, most preferred first.
 
+The identifiers used by the official Mercurial distribution are:
+
+bzip2
+   bzip2
+none
+   uncompressed / raw data
+zlib
+   zlib (no gzip header)
+zstd
+   zstd
+
 This capability was introduced in Mercurial 4.1 (released February 2017).
 
 getbundle

--- a/mercurial/util.py	Sat Dec 24 13:56:36 2016 -0700
+++ b/mercurial/util.py	Sat Dec 24 13:51:12 2016 -0700
@@ -2957,6 +2957,13 @@
 
 # compression code
 
+SERVERROLE = 'server'
+CLIENTROLE = 'client'
+
+compewireprotosupport = collections.namedtuple(u'compenginewireprotosupport',
+                                               (u'name', u'serverpriority',
+                                                u'clientpriority'))
+
 class compressormanager(object):
     """Holds registrations of various compression engines.
 
@@ -2973,6 +2980,8 @@
         self._bundlenames = {}
         # Internal bundle identifier to engine name.
         self._bundletypes = {}
+        # Wire proto identifier to engine name.
+        self._wiretypes = {}
 
     def __getitem__(self, key):
         return self._engines[key]
@@ -3014,6 +3023,16 @@
 
             self._bundletypes[bundletype] = name
 
+        wiresupport = engine.wireprotosupport()
+        if wiresupport:
+            wiretype = wiresupport.name
+            if wiretype in self._wiretypes:
+                raise error.Abort(_('wire protocol compression %s already '
+                                    'registered by %s') %
+                                  (wiretype, self._wiretypes[wiretype]))
+
+            self._wiretypes[wiretype] = name
+
         self._engines[name] = engine
 
     @property
@@ -3050,6 +3069,38 @@
                               engine.name())
         return engine
 
+    def supportedwireengines(self, role, onlyavailable=True):
+        """Obtain compression engines that support the wire protocol.
+
+        Returns a list of engines in prioritized order, most desired first.
+
+        If ``onlyavailable`` is set, filter out engines that can't be
+        loaded.
+        """
+        assert role in (SERVERROLE, CLIENTROLE)
+
+        attr = 'serverpriority' if role == SERVERROLE else 'clientpriority'
+
+        engines = [self._engines[e] for e in self._wiretypes.values()]
+        if onlyavailable:
+            engines = [e for e in engines if e.available()]
+
+        def getkey(e):
+            # Sort first by priority, highest first. In case of tie, sort
+            # alphabetically. This is arbitrary, but ensures output is
+            # stable.
+            w = e.wireprotosupport()
+            return -1 * getattr(w, attr), w.name
+
+        return list(sorted(engines, key=getkey))
+
+    def forwiretype(self, wiretype):
+        engine = self._engines[self._wiretypes[wiretype]]
+        if not engine.available():
+            raise error.Abort(_('compression engine %s could not be loaded') %
+                              engine.name())
+        return engine
+
 compengines = compressormanager()
 
 class compressionengine(object):
@@ -3090,6 +3141,31 @@
         """
         return None
 
+    def wireprotosupport(self):
+        """Declare support for this compression format on the wire protocol.
+
+        If this compression engine isn't supported for compressing wire
+        protocol payloads, returns None.
+
+        Otherwise, returns ``compenginewireprotosupport`` with the following
+        fields:
+
+        * String format identifier
+        * Integer priority for the server
+        * Integer priority for the client
+
+        The integer priorities are used to order the advertisement of format
+        support by server and client. The highest integer is advertised
+        first. Integers with non-positive values aren't advertised.
+
+        The priority values are somewhat arbitrary and only used for default
+        ordering. The relative order can be changed via config options.
+
+        If wire protocol compression is supported, the class must also implement
+        ``compressstream`` and ``decompressorreader``.
+        """
+        return None
+
     def compressstream(self, it, opts=None):
         """Compress an iterator of chunks.
 
@@ -3118,6 +3194,9 @@
     def bundletype(self):
         return 'gzip', 'GZ'
 
+    def wireprotosupport(self):
+        return compewireprotosupport('zlib', 20, 20)
+
     def compressstream(self, it, opts=None):
         opts = opts or {}
 
@@ -3151,6 +3230,11 @@
     def bundletype(self):
         return 'bzip2', 'BZ'
 
+    # We declare a protocol name but don't advertise by default because
+    # it is slow.
+    def wireprotosupport(self):
+        return compewireprotosupport('bzip2', 0, 0)
+
     def compressstream(self, it, opts=None):
         opts = opts or {}
         z = bz2.BZ2Compressor(opts.get('level', 9))
@@ -3199,6 +3283,12 @@
     def bundletype(self):
         return 'none', 'UN'
 
+    # Clients always support uncompressed payloads. Servers don't because
+    # unless you are on a fast network, uncompressed payloads can easily
+    # saturate your network pipe.
+    def wireprotosupport(self):
+        return compewireprotosupport('none', 0, 10)
+
     def compressstream(self, it, opts=None):
         return it
 
@@ -3229,6 +3319,9 @@
     def bundletype(self):
         return 'zstd', 'ZS'
 
+    def wireprotosupport(self):
+        return compewireprotosupport('zstd', 50, 50)
+
     def compressstream(self, it, opts=None):
         opts = opts or {}
         # zstd level 3 is almost always significantly faster than zlib