Greg Ewing <[EMAIL PROTECTED]> wrote:
>
> Ronald Oussoren wrote:
>
> > I'm slighly worried about this thread. Async I/O and "read exactly N
> > bytes" don't really match up. I don't know about the other mechanisms,
> > but at least with select and poll when the system says you can read
> > from a file descriptor you're only guaranteed that one call to
> > read(2)/recv(2)/... won't block. The implementation of a python read
> > method that returns exactly the number of bytes that you requested will
> > have to call the read system call in a loop and hence might block.
>
> This is one case where the callback model of async i/o may
> help. If there were a way to say "don't call me until you've
> got n bytes ready", the descriptor could become ready
> multiple times and multiple reads performed behind the
> scenes, then when enough bytes are there, your callback
> is called.
class ReadExactly:
def __init__(self, callback, current_count):
self.remaining = current_count
self.callback = callback
self.buffer = []
def __call__(self, data):
while data:
if len(data) >= self.remaining:
b, self.buffer = self.buffer, []
b.append(data[:self.remaining])
data = data[:self.remaining]
self.remaining = 0
self.callback(''.join(b), reader=self)
else:
self.buffer.append(data)
self.remaining -= len(data)
break
Generally though, it's a bit easier to handle the piecewise reading, etc.,
as part of the async socket class. The asynchat module uses,
handle_read(), collect_incoming_data(data), and found_terminator(); a
semantic I've borrowed for my own asynchronous socket classes and have
been fairly happy with. asynchat implements the handle_read() portion,
which knows about line-terminated protocols (pop, smtp, http, ...) as
well as protocols using the 'read X bytes semantic', where X can be
fixed or variable. (read 4 bytes, decode, read X bytes, decode, read 4...)
If the new asynchronous class had some equivalent functionality, and
some reasonable set of default behavior (overridable via subclass or
flags), you could get arbitrarily desired behavior; from "call this
thing whenever you get data", to "call this thing when you have gotten X
bytes", to "call this thing when you have found this ''line'' terminator
X", etc.
- Josiah
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