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channel: truncate twrite messages based on msize While there are a few problems around handling of msize, the easiest to address and, arguably, the most problematic is that of Twrite. We now truncate Twrite.Data to the correct length if it will overflow the msize limit negotiated on the session. ErrShortWrite is returned by the `Session.Write` method if written data is truncated. In addition, we now reject incoming messages from `ReadFcall` that overflow the msize. Such messages are probably terminal in practice, but can be detected with the `Overflow` function. Tread is also handled accordingly, such that the Count field will be rewritten such that the response doesn't overflow the msize. Signed-off-by: Stephen J Day <stephen.day@docker.com>
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 1 0f5f58bb 2016-11-16 stephen.d package p9p

 2 0f5f58bb 2016-11-16 stephen.d 

 3 0f5f58bb 2016-11-16 stephen.d import "fmt"

 4 0f5f58bb 2016-11-16 stephen.d 

 5 0f5f58bb 2016-11-16 stephen.d // Overflow will return a positive number, indicating there was an overflow for

 6 0f5f58bb 2016-11-16 stephen.d // the error.

 7 0f5f58bb 2016-11-16 stephen.d func Overflow(err error) int {

 8 0f5f58bb 2016-11-16 stephen.d 	if of, ok := err.(overflow); ok {

 9 0f5f58bb 2016-11-16 stephen.d 		return of.Size()

10 0f5f58bb 2016-11-16 stephen.d 	}

11 0f5f58bb 2016-11-16 stephen.d 

12 0f5f58bb 2016-11-16 stephen.d 	// traverse cause, if above fails.

13 0f5f58bb 2016-11-16 stephen.d 	if causal, ok := err.(interface {

14 0f5f58bb 2016-11-16 stephen.d 		Cause() error

15 0f5f58bb 2016-11-16 stephen.d 	}); ok {

16 0f5f58bb 2016-11-16 stephen.d 		return Overflow(causal.Cause())

17 0f5f58bb 2016-11-16 stephen.d 	}

18 0f5f58bb 2016-11-16 stephen.d 

19 0f5f58bb 2016-11-16 stephen.d 	return 0

20 0f5f58bb 2016-11-16 stephen.d }

21 0f5f58bb 2016-11-16 stephen.d 

22 0f5f58bb 2016-11-16 stephen.d // overflow is a resolvable error type that can help callers negotiate

23 0f5f58bb 2016-11-16 stephen.d // session msize. If this error is encountered, no message was sent.

24 0f5f58bb 2016-11-16 stephen.d //

25 0f5f58bb 2016-11-16 stephen.d // The return value of `Size()` represents the number of bytes that would have

26 0f5f58bb 2016-11-16 stephen.d // been truncated if the message were sent. This IS NOT the optimal buffer size

27 0f5f58bb 2016-11-16 stephen.d // for operations like read and write.

28 0f5f58bb 2016-11-16 stephen.d //

29 0f5f58bb 2016-11-16 stephen.d // In the case of `Twrite`, the caller can Size() from the local size to get an

30 0f5f58bb 2016-11-16 stephen.d // optimally size buffer or the write can simply be truncated to `len(buf) -

31 0f5f58bb 2016-11-16 stephen.d // err.Size()`.

32 0f5f58bb 2016-11-16 stephen.d //

33 0f5f58bb 2016-11-16 stephen.d // For the most part, no users of this package should see this error in

34 0f5f58bb 2016-11-16 stephen.d // practice. If this escapes the Session interface, it is a bug.

35 0f5f58bb 2016-11-16 stephen.d type overflow interface {

36 0f5f58bb 2016-11-16 stephen.d 	Size() int // number of bytes overflowed.

37 0f5f58bb 2016-11-16 stephen.d }

38 0f5f58bb 2016-11-16 stephen.d 

39 0f5f58bb 2016-11-16 stephen.d type overflowErr struct {

40 0f5f58bb 2016-11-16 stephen.d 	size int // number of bytes overflowed

41 0f5f58bb 2016-11-16 stephen.d }

42 0f5f58bb 2016-11-16 stephen.d 

43 0f5f58bb 2016-11-16 stephen.d func (o overflowErr) Error() string {

44 0f5f58bb 2016-11-16 stephen.d 	return fmt.Sprintf("message overflowed %d bytes", o.size)

45 0f5f58bb 2016-11-16 stephen.d }

46 0f5f58bb 2016-11-16 stephen.d 

47 0f5f58bb 2016-11-16 stephen.d func (o overflowErr) Size() int {

48 0f5f58bb 2016-11-16 stephen.d 	return o.size

49 0f5f58bb 2016-11-16 stephen.d }