diff mercurial/wireprotoframing.py @ 37052:8c3c47362934

wireproto: implement basic frame reading and processing

We just implemented support for writing frames. Now let's implement
support for reading them.

The bulk of the new code is for a class that maintains the state of
a server. Essentially, you construct an instance, feed frames to it,
and it tells you what you should do next. The design is inspired by
the "sans I/O" movement and the reactor pattern. We don't want to
perform I/O or any major blocking event during frame ingestion because
this arbitrarily limits ways that server pieces can be implemented.
For example, it makes it much harder to swap in an alternate
implementation based on asyncio or do crazy things like have requests
dispatch to other processes.

We do still implement readframe() which does I/O. But it is decoupled
from the server reactor. And important parsing of frame headers is
a standalone function. So I/O is only needed to obtain frame data.

Because testing server-side ingest is useful and difficult on running
servers, we create a new "debugreflect" endpoint that will echo back
to the client what was received and how it was interpreted. This could
be useful for a server admin, someone implementing a client. But
immediately, it is useful for testing: we're able to demonstrate that
frames are parsed correctly and turned into requests to run commands
without having to implement command dispatch on the server!

In addition, we implement Python level unit tests for the reactor.
This is vastly more efficient than sending requests to the
"debugreflect" endpoint and vastly more powerful for advanced
testing.

Differential Revision: https://phab.mercurial-scm.org/D2852

--- a/mercurial/wireprotoframing.py	Mon Mar 19 16:49:53 2018 -0700
+++ b/mercurial/wireprotoframing.py	Wed Mar 14 15:25:06 2018 -0700
@@ -13,7 +13,9 @@
 
 import struct
 
+from .i18n import _
 from . import (
+    error,
     util,
 )
 
@@ -105,6 +107,51 @@
 
     return makeframe(frametype, finalflags, payload)
 
+def parseheader(data):
+    """Parse a unified framing protocol frame header from a buffer.
+
+    The header is expected to be in the buffer at offset 0 and the
+    buffer is expected to be large enough to hold a full header.
+    """
+    # 24 bits payload length (little endian)
+    # 4 bits frame type
+    # 4 bits frame flags
+    # ... payload
+    framelength = data[0] + 256 * data[1] + 16384 * data[2]
+    typeflags = data[3]
+
+    frametype = (typeflags & 0xf0) >> 4
+    frameflags = typeflags & 0x0f
+
+    return frametype, frameflags, framelength
+
+def readframe(fh):
+    """Read a unified framing protocol frame from a file object.
+
+    Returns a 3-tuple of (type, flags, payload) for the decoded frame or
+    None if no frame is available. May raise if a malformed frame is
+    seen.
+    """
+    header = bytearray(FRAME_HEADER_SIZE)
+
+    readcount = fh.readinto(header)
+
+    if readcount == 0:
+        return None
+
+    if readcount != FRAME_HEADER_SIZE:
+        raise error.Abort(_('received incomplete frame: got %d bytes: %s') %
+                          (readcount, header))
+
+    frametype, frameflags, framelength = parseheader(header)
+
+    payload = fh.read(framelength)
+    if len(payload) != framelength:
+        raise error.Abort(_('frame length error: expected %d; got %d') %
+                          (framelength, len(payload)))
+
+    return frametype, frameflags, payload
+
 def createcommandframes(cmd, args, datafh=None):
     """Create frames necessary to transmit a request to run a command.
 
@@ -154,3 +201,195 @@
 
             if done:
                 break
+
+class serverreactor(object):
+    """Holds state of a server handling frame-based protocol requests.
+
+    This class is the "brain" of the unified frame-based protocol server
+    component. While the protocol is stateless from the perspective of
+    requests/commands, something needs to track which frames have been
+    received, what frames to expect, etc. This class is that thing.
+
+    Instances are modeled as a state machine of sorts. Instances are also
+    reactionary to external events. The point of this class is to encapsulate
+    the state of the connection and the exchange of frames, not to perform
+    work. Instead, callers tell this class when something occurs, like a
+    frame arriving. If that activity is worthy of a follow-up action (say
+    *run a command*), the return value of that handler will say so.
+
+    I/O and CPU intensive operations are purposefully delegated outside of
+    this class.
+
+    Consumers are expected to tell instances when events occur. They do so by
+    calling the various ``on*`` methods. These methods return a 2-tuple
+    describing any follow-up action(s) to take. The first element is the
+    name of an action to perform. The second is a data structure (usually
+    a dict) specific to that action that contains more information. e.g.
+    if the server wants to send frames back to the client, the data structure
+    will contain a reference to those frames.
+
+    Valid actions that consumers can be instructed to take are:
+
+    error
+       Indicates that an error occurred. Consumer should probably abort.
+
+    runcommand
+       Indicates that the consumer should run a wire protocol command. Details
+       of the command to run are given in the data structure.
+
+    wantframe
+       Indicates that nothing of interest happened and the server is waiting on
+       more frames from the client before anything interesting can be done.
+    """
+
+    def __init__(self):
+        self._state = 'idle'
+        self._activecommand = None
+        self._activeargs = None
+        self._activedata = None
+        self._expectingargs = None
+        self._expectingdata = None
+        self._activeargname = None
+        self._activeargchunks = None
+
+    def onframerecv(self, frametype, frameflags, payload):
+        """Process a frame that has been received off the wire.
+
+        Returns a dict with an ``action`` key that details what action,
+        if any, the consumer should take next.
+        """
+        handlers = {
+            'idle': self._onframeidle,
+            'command-receiving-args': self._onframereceivingargs,
+            'command-receiving-data': self._onframereceivingdata,
+            'errored': self._onframeerrored,
+        }
+
+        meth = handlers.get(self._state)
+        if not meth:
+            raise error.ProgrammingError('unhandled state: %s' % self._state)
+
+        return meth(frametype, frameflags, payload)
+
+    def _makeerrorresult(self, msg):
+        return 'error', {
+            'message': msg,
+        }
+
+    def _makeruncommandresult(self):
+        return 'runcommand', {
+            'command': self._activecommand,
+            'args': self._activeargs,
+            'data': self._activedata.getvalue() if self._activedata else None,
+        }
+
+    def _makewantframeresult(self):
+        return 'wantframe', {
+            'state': self._state,
+        }
+
+    def _onframeidle(self, frametype, frameflags, payload):
+        # The only frame type that should be received in this state is a
+        # command request.
+        if frametype != FRAME_TYPE_COMMAND_NAME:
+            self._state = 'errored'
+            return self._makeerrorresult(
+                _('expected command frame; got %d') % frametype)
+
+        self._activecommand = payload
+        self._activeargs = {}
+        self._activedata = None
+
+        if frameflags & FLAG_COMMAND_NAME_EOS:
+            return self._makeruncommandresult()
+
+        self._expectingargs = bool(frameflags & FLAG_COMMAND_NAME_HAVE_ARGS)
+        self._expectingdata = bool(frameflags & FLAG_COMMAND_NAME_HAVE_DATA)
+
+        if self._expectingargs:
+            self._state = 'command-receiving-args'
+            return self._makewantframeresult()
+        elif self._expectingdata:
+            self._activedata = util.bytesio()
+            self._state = 'command-receiving-data'
+            return self._makewantframeresult()
+        else:
+            self._state = 'errored'
+            return self._makeerrorresult(_('missing frame flags on '
+                                           'command frame'))
+
+    def _onframereceivingargs(self, frametype, frameflags, payload):
+        if frametype != FRAME_TYPE_COMMAND_ARGUMENT:
+            self._state = 'errored'
+            return self._makeerrorresult(_('expected command argument '
+                                           'frame; got %d') % frametype)
+
+        offset = 0
+        namesize, valuesize = ARGUMENT_FRAME_HEADER.unpack_from(payload)
+        offset += ARGUMENT_FRAME_HEADER.size
+
+        # The argument name MUST fit inside the frame.
+        argname = bytes(payload[offset:offset + namesize])
+        offset += namesize
+
+        if len(argname) != namesize:
+            self._state = 'errored'
+            return self._makeerrorresult(_('malformed argument frame: '
+                                           'partial argument name'))
+
+        argvalue = bytes(payload[offset:])
+
+        # Argument value spans multiple frames. Record our active state
+        # and wait for the next frame.
+        if frameflags & FLAG_COMMAND_ARGUMENT_CONTINUATION:
+            raise error.ProgrammingError('not yet implemented')
+            self._activeargname = argname
+            self._activeargchunks = [argvalue]
+            self._state = 'command-arg-continuation'
+            return self._makewantframeresult()
+
+        # Common case: the argument value is completely contained in this
+        # frame.
+
+        if len(argvalue) != valuesize:
+            self._state = 'errored'
+            return self._makeerrorresult(_('malformed argument frame: '
+                                           'partial argument value'))
+
+        self._activeargs[argname] = argvalue
+
+        if frameflags & FLAG_COMMAND_ARGUMENT_EOA:
+            if self._expectingdata:
+                self._state = 'command-receiving-data'
+                self._activedata = util.bytesio()
+                # TODO signal request to run a command once we don't
+                # buffer data frames.
+                return self._makewantframeresult()
+            else:
+                self._state = 'waiting'
+                return self._makeruncommandresult()
+        else:
+            return self._makewantframeresult()
+
+    def _onframereceivingdata(self, frametype, frameflags, payload):
+        if frametype != FRAME_TYPE_COMMAND_DATA:
+            self._state = 'errored'
+            return self._makeerrorresult(_('expected command data frame; '
+                                           'got %d') % frametype)
+
+        # TODO support streaming data instead of buffering it.
+        self._activedata.write(payload)
+
+        if frameflags & FLAG_COMMAND_DATA_CONTINUATION:
+            return self._makewantframeresult()
+        elif frameflags & FLAG_COMMAND_DATA_EOS:
+            self._activedata.seek(0)
+            self._state = 'idle'
+            return self._makeruncommandresult()
+        else:
+            self._state = 'errored'
+            return self._makeerrorresult(_('command data frame without '
+                                           'flags'))
+
+    def _onframeerrored(self, frametype, frameflags, payload):
+        return self._makeerrorresult(_('server already errored'))