Update message spec with details about attribute, prompts, etc.

From discussions over the past few days with Brian and Evan.

docs/source/development/messaging.txt

@@ -140,6 +140,11 b" inputs, and run all multiline input in 'exec' mode. This would preserve the"
 natural behavior of single-line inputs while allowing long cells to behave more
 likea a script.  This design will be refined as we complete the implementation.
 
+.. Note::
+
+   What today we call 'prompt requests' will be encoded in the
+   ``state_template`` field.
+
 Message type: ``execute_request``::
 
     content = {
@@ -148,17 +153,44 b' Message type: ``execute_request``::'
 
     # A boolean flag which, if True, signals the kernel to execute this
     # code as quietly as possible.  This means that the kernel will compile
-    # the code with 'exec' instead of 'single' (so sys.displayhook will not
-    # fire), and will *not*:
+    # the code witIPython/core/tests/h 'exec' instead of 'single' (so
+    # sys.displayhook will not fire), and will *not*:
     #   - broadcast exceptions on the PUB socket
     #   - do any logging
     #   - populate any history
+    #
     # The default is False.
     'silent' : bool,
+
+    # An optional string to request arbitrary state information from the
+    # kernel.  This string is evaluated via the itpl module, and it can
+    # therefore contain arbitrary code for execution.
+    
+    'state_template' : str,
     }
 
-Upon execution, the kernel *always* sends a reply, with a status code
-indicating what happened and additional data depending on the outcome.
+Execution semantics
+    Upon execution of the ``code`` field, the kernel *always* sends a reply,
+    with a status code indicating what happened and additional data depending
+    on the outcome.
+
+    Any code in the ``state_template`` string is evaluated, but full exceptions
+    that may occur are *not* propagated back.  If any error occurs during the
+    evaluation, the value of the string will simply be::
+
+       [ERROR in <contents of template>: ExceptionType - Exception message]
+
+    The user can simply send the same code contained in the template for normal
+    evaluation to see a regular traceback.
+
+Execution counter (old prompt number)
+    The kernel has a single, monotonically increasing counter of all execution
+    requests that are made with ``silent=False``.  This counter is used to
+    populate the ``In[n]``, ``Out[n]`` and ``_n`` variables, so clients will
+    likely want to display it in some form to the user, which will typically
+    (but not necessarily) be done in the prompts.  The value of this counter
+    will be returned as the ``execution_count`` field of all ``execute_reply```
+    messages.
 
 Message type: ``execute_reply``::
 
@@ -166,30 +198,25 b' Message type: ``execute_reply``::'
       # One of: 'ok' OR 'error' OR 'abort'
       'status' : str,
 
-      # This has the same structure as the output of a prompt request, but is
-      # for the client to set up the *next* prompt (with identical limitations
-      # to a prompt request)
-      'next_prompt' : {
-            'prompt_string' : str,
-            'prompt_number' : int,
-            'input_sep'     : str
-      },
+      # The global kernel counter that increases by one with each non-silent
+      # executed request.  This will typically be used by clients to display
+      # prompt numbers to the user.  If the request was a silent one, this will
+      # be the current value of the counter in the kernel.
+      'execution_count' : int,
 
-      # The prompt number of the actual execution for this code, which may be
-      # different from the one used when the code was typed, which was the
-      # 'next_prompt' field of the *previous* request.  They will differ in the
-      # case where there is more than one client talking simultaneously to a
-      # kernel, since the numbers can go out of sync.  GUI clients can use this
-      # to correct the previously written number in-place, terminal ones may
-      # re-print a corrected one if desired.
-      'prompt_number' : int,
+      # If the state_template was provided, this will contain the evaluated
+      # form of the template.
+      'state' : str,
     }
 
 When status is 'ok', the following extra fields are present::
 
     {
-      # The kernel will often transform the input provided to it.  This
-      # contains the transformed code, which is what was actually executed.
+      # The kernel will often transform the input provided to it.  If the
+      # '---->' transform had been applied, this is filled, otherwise it's the
+      # empty string.  So transformations like magics don't appear here, only
+      # autocall ones.
+    
       'transformed_code' : str,
 
       # The execution payload is a dict with string keys that may have been
@@ -235,31 +262,65 b" When status is 'error', the following extra fields are present::"
 When status is 'abort', there are for now no additional data fields.  This
 happens when the kernel was interrupted by a signal.
 
+Kernel attribute access
+-----------------------
+
+While this protocol does not specify full RPC access to arbitrary methods of
+the kernel object, the kernel does allow read (and in some cases write) access
+to certain attributes.
+
+The policy for which attributes can be read is: any attribute of the kernel, or
+its sub-objects, that belongs to a :class:`Configurable` object and has been
+declared at the class-level with Traits validation, is in principle accessible
+as long as its name does not begin with a leading underscore.  The attribute
+itself will have metadata indicating whether it allows remote read and/or write
+access.  The message spec follows for attribute read and write requests.
 
-Prompt
-------
+Message type: ``getattr_request``::
 
-A simple request for a current prompt string.
+    content = {
+        # The (possibly dotted) name of the attribute
+	'name' : str
+    }
 
-Message type: ``prompt_request``::
+When a ``getattr_request`` fails, there are two possible error types:
 
-    content = {}
+- AttributeError: this type of error was raised when trying to access the
+  given name by the kernel itself.  This means that the attribute likely
+  doesn't exist.
 
-In the reply, the prompt string comes back with the prompt number placeholder
-*unevaluated*.  The message format is:
+- AccessError: the attribute exists but its value is not readable remotely.
     
-Message type: ``prompt_reply``::
+
+Message type: ``getattr_reply``::
 
     content = {
-      'prompt_string' : str,
-      'prompt_number' : int,
-      'input_sep'     : str
+        # One of ['ok', 'AttributeError', 'AccessError'].
+        'status' : str
+	# If status is 'ok', a JSON object.
+	'value' : object
     }
 
-Clients can produce a prompt with ``prompt_string.format(prompt_number)``, but
-they should be aware that the actual prompt number for that input could change
-later, in the case where multiple clients are interacting with a single
-kernel. 
+Message type: ``setattr_request``::
+
+    content = {
+        # The (possibly dotted) name of the attribute
+	'name' : str
+	
+	# A JSON-encoded object, that will be validated by the Traits
+	# information in the kernel
+	'value' : object
+    }
+
+When a ``setattr_request`` fails, there are also two possible error types with
+similar meanings  as those of the ``getattr_request`` case, but for writing.
+    
+Message type: ``setattr_reply``::
+
+    content = {
+        # One of ['ok', 'AttributeError', 'AccessError'].
+        'status' : str
+    }
 
 
 Object information
@@ -315,9 +376,29 b' Message type: ``object_info_reply``::'
     'file' : str,
 
     # For pure Python callable objects, we can reconstruct the object
-    # definition line which provides its call signature
+    # definition line which provides its call signature.  For convenience this
+    # is returned as a single 'definition' field, but below the raw parts that
+    # compose it are also returned as the argspec field.
     'definition' : str,
 
+    # The individual parts that together form the definition string.  Clients
+    # with rich display capabilities may use this to provide a richer and more
+    # precise representation of the definition line (e.g. by highlighting
+    # arguments based on the user's cursor position).  For non-callable
+    # objects, this field is empty.
+    'argspec' : { # The names of all the arguments
+                  args : list,
+		  # The name of the varargs (*args), if any
+                  varargs : str,
+		  # The name of the varkw (**kw), if any
+		  varkw : str
+		  # The values (as strings) of all default arguments.  Note
+		  # that these must be matched *in reverse* with the 'args'
+		  # list above, since the first positional args have no default
+		  # value at all.
+		  func_defaults : list
+		  }
+
     # For instances, provide the constructor signature (the definition of
     # the __init__ method):
     'init_definition' : str,