testing/iotest: add two new readers

[josharian]

Just in case there is interest, here are two other badly-behaved reader types that might make useful additions to testing/iotest:

// ZeroNilReader returns 0, nil on every second read. Other calls to read succeed.
func ZeroNilReader(r io.Reader) io.Reader

// CorruptingReader returns a Reader that modifies any unused portion of the byte slice passed to Read.
// It is useful in conjunction with OneByteReader.
func CorruptingReader(r io.Reader) io.Reader

[nightlyone]

I am not sure, the CorruptingReader is still within the contract of an io.Reader. It appears quite common to pass the lower and the upper half of a buffer represented by a byte slice to two different Read() calls in order to save the copying.

Could you provide a usage sample demonstrating what kind of error/good behavior that would expose?

[josharian]

From the io.Reader docs: "Even if Read returns n < len(p), it may use all of p as scratch space during the call." The CorruptingReader would be checking that the caller doesn't rely on Read leaving the buffer untouched aside from the part containing newly read data.

[nightlyone]

Ah ok, that makes sense now.

I understood, you plan to corrupt p[len(p):cap(p)] instead.

[josharian]

Actually, the plan is to corrupt p[n:].

[nightlyone]

Both sound like very useful additions to me, if you clear up the comment on what portion "unused portion" really is.