rfc42: add credit based flow control (write direction)

Problem: rfc42 does not specify anything about channel flow control,
so it is difficult to design clients and servers that operate without
dropping data.

Add credit based flow control to the protocol in the write direction.

The client receives credit for writes to the subprocess via the
exec add-credit response.

spec_42.rst

@@ -85,6 +85,8 @@ Goals
 
 - Optionally forward additional I/O channels.
 
+- Provide flow control on channels.
+
 - Provide signal delivery capability.
 
 - Protect against unauthorized use.
@@ -121,6 +123,10 @@ are defined as follows:
     channel (4)
       Forward auxiliary channel output to the client.
 
+    write-credit (8)
+      Send ``add-credit`` exec responses when buffer space is available
+      for standard input or writable auxiliary channels.
+
 Several response types are distinguished by the type key:
 
 .. object:: exec started response
@@ -179,6 +185,25 @@ Several response types are distinguished by the type key:
 
     See `I/O Object`_ below.
 
+.. object:: exec add-credit response
+
+  The subprocess server has more buffer space available to receive data
+  on the specified channel.  The response SHALL consist of a JSON object
+  with the following keys:
+
+  .. object:: type
+
+    (*string*, REQUIRED) The response type with a value of ``add-credit``.
+
+  .. object:: channels
+
+    (*object*, REQUIRED) An object with channel names as keys and
+    integer credit counts as values.
+
+  The server's initial response contains the full buffer size(s).
+
+  These messages are suppressed if the write-credit flag was not specified.
+
 .. object:: exec error response
 
 The :program:`exec` response stream SHALL be terminated by an error
@@ -201,6 +226,12 @@ The :program:`write` RPC sends data to an I/O channel of a remote process.
 Valid I/O channel names MAY include ``stdin`` and auxiliary channel names
 specified in the exec request command object.
 
+A client MAY avoid overrunning the remote buffer by monitoring
+:program:`exec add-credit` responses and keeping a running total of
+available remote buffer space, subtracting the size of the decoded data
+contained with in the I/O object each time a :program:`write` request is sent,
+and never allowing the running total to become negative.
+
 .. object:: write request
 
   The request SHALL consist of a JSON object with the following keys: