Start writing some docs, in particular, an "intro to Mojo".

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.
+
+## Messages
+
+A *message* consists of two things:
+* a finite sequence of bytes, and
+* a finite sequence of Mojo handles.
+
+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()`.
+
+## Asynchronous operation and queueing
+
+Message pipes are *asynchronous* in the sense that sent messages do not have
+intrinsic response messages mediated/enforced by the system. (This is different
+from saying that message write/read are asynchronous operations: these
+operations are actually synchronous and complete "immediately". However, note
+that reading a message is "nonblocking" in the sense that it will fail if a
+message is not available to be read. Thus a message must be waited for, and the
+combined wait-then-read may form an asynchronous pattern.)
+
+To allow message writes to complete immediately, messages are queued. Indeed,
+one can understand a message pipe as a pair of queues, one in each direction.
+Each endpoint has opposite notions of incoming and outgoing queues (recall that
+message pipes have a pair of endpoints).
+
+Writing a message to an endpoint then simply entails enqueueing that message on
+that endpoint's outgoing queue (which is the *peer* endpoint's incoming queue).
+Reading a message from an endpoint is just dequeueing a message from that
+endpoint's incoming queue (which is the peer endpoint's outgoing queue).
+
+Queueing is unlimited. Why? The problem is that limiting queueing exposes Mojo
+programs to complex deadlock problems:
+* One way of limiting queue sizes is to block the sender if the queue is "full".
+  However, the receiver may not be able or willing to consume messages until the
+  sender does something else (and this is often the case in asynchronous
+  programming). For example, perhaps the putative "receiver" does not yet even
+  have a handle to the endpoint yet, and that handle is sent in a message (over
+  some other message pipe).
+* Another way would be to have the write fail if the queue is full. Then the
+  sender would want to additionally queue on its side. The thread would continue
+  running and, e.g., run its message loop. However, sender-side queueing
+  basically makes it impossible for the sender to transfer that endpoint
+  (handle), at least until the sender-side queue is cleared. However, the
+  receiver may not be able/willing to proceed until the sender has transferred
+  the aforementioned endpoint.
+
+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)**