Add moar documentation.

docs/intro/message_pipes.md

 # Message pipes
 
 Message pipes are the primary communication mechanism between Mojo programs. A
 *message pipe* is a point-to-point (with each side being called a message pipe
 *endpoint*), bidirectional message passing mechanism, where messages may contain
 both data and Mojo handles.
 
 It's important to understand that a message pipe is a "transport": it provides a
 way for data and handles to be sent between Mojo programs. The system is unaware
 of the meaning of the data or of the handles (other than their intrinsic
 properties).
 
 That said, Mojo provides a *standard* way of communicating over message pipes,
-namely via a standardized protocol together with [Mojom IDL](mojom.md) files.
+namely via a standardized protocol together with [Mojom](mojom.md) IDL files.
 
 ## Messages
 
 A *message* consists of two things:
 * a finite sequence of bytes, and
-* a finite sequence of Mojo handles.
+* a finite sequence of [Mojo handles](handles.md).
 
 Both of these are determined when the message is sent (or *written*). Messages
 are *framed* in the sense that they are "atomic" units: they are sent and
 received (or *read*) as entire units, not just by Mojo programs themselves but
 by the system, which is aware of and enforces the message boundaries.
 
 (Note on terminology: We'll use "send" and "write" interchangeably, and
 similarly for "receive" and "read". "Write" and "read" correspond more closely
 to the names usually given to the basic Mojo operations, e.g.,
 `MojoWriteMessage()` and `MojoReadMessage()`.
@@ skipping 37 matching lines @@
 Thus we allow unlimited queueing. (**TODO(vtl)**: Probably we'll eventually
 allow "hard" queue limits to be set for the purposes of preventing
 denial-of-service. However, the response to overruns will be hard failures, in
 the sense that the message pipe may be destroyed, rather than soft,
 "recoverable" failures -- since those expose deadlock issues.) It is then up to
 Mojo programs to implement flow control in some way. (**TODO(vtl)**: Write more
 about this in the context of Mojom.)
 
 ## "Synchronous" operation
 
-**TODO(vtl)**
+Though message pipes are asynchronous as discussed above, they may be used in a
+synchronous fashion: immediately after sending a *request* message, one can then
+just block and wait for the *response* message (and then read it and process
+it). Of course, this requires that the protocol support this:
+* Message pipes must be used in a "directional" way: there must be fixed request
+  and response directions or, equivalently, one endpoint belongs to the *client*
+  and the other to the *server* (or *impl*). (Historical note: This is the case
+  for the current Mojom protocol, but not in previous versions.) The issue here
+  is that without this, the sender of the request messages may have to process
+  incoming request messages from its peer.
+* Request messages must have unique response messages. (In the Mojom protocol,
+  request messages have optional unique responses. For messages without
+  responses, one can just proceed immediately without waiting. However, without
+  response messages there may be flow control issues; again, see above.) The
+  important point is that for each request message, there is a well-defined
+  number of response messages for each request and not arbitrary "callback"
+  messages.
+* The sending of a response message must not depend on a future action of the
+  client. (This is a higher-level semantic that is not enforced by the Mojom
+  protocol. E.g., one may define a Mojom interface in which the response to a
+  message *Foo* isn't sent until the client sends a request *Bar*.)
+
+That said, whether one wants to, or even can, use this synchronous mode of
+operation may depend on a number of things:
+* For message-loop-centric programming languages, this mode is at best
+  undesirable (and possibly infeasible, depending on what facilities are exposed
+  to user code). (E.g., this is the case for JavaScript and Dart.)
+* Similarly, on a message-loop programming model (in which threads -- if there
+  are more than one -- are mostly coordinated by "message passing"), it is
+  typically undesirable to block any thread (with a message loop). Indeed, if
+  other message pipes are serviced by a message loop, blocking the thread may
+  result in deadlock. (E.g., this is the usual programming model for the
+  standard Mojom C++ bindings.)
+* Even when blocking is permissible, it may not be desirable to do so:
+  advancement of the program then relies on trusting the server to be
+  responsive and send responses in a timely fashion.
+* Mixing asynchronous and synchronous operation is problematic: one cannot send
+  a request and synchronously wait for a response while responses to other
+  messages are still pending. (Theoretically, one could buffer such other
+  responses until the response to particular request is received, and process
+  those other responses later, but this would be dubious at best.)