rust-mockstream

Stream (Read+Write traits) implementations to be used to mock real streams in tests.

Install

Just use Cargo.

Usage scenario

Wrap the stream you use into ADT and provide Read and Write traits implementation.

enum NetStream {
	Mocked(SharedMockStream),
	Tcp(TcpStream)
}

impl io::Read for NetStream {
	fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
		match *self {
			NetStream::Mocked(ref mut s) => s.read(buf),
			NetStream::Tcp(ref mut s) => s.read(buf),
		}
	}
}

impl io::Write for NetStream {
	fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
		match *self {
			NetStream::Mocked(ref mut s) => s.write(buf),
			NetStream::Tcp(ref mut s) => s.write(buf),
		}
	}

	fn flush(&mut self) -> io::Result<()> {
		match *self {
			NetStream::Mocked(ref mut s) => s.flush(),
			NetStream::Tcp(ref mut s) => s.flush(),
		}
	}
}

Then use this ADT instead of stream everywhere.

fn reverse4(s: &mut NetStream) -> io::Result<usize> {
	use std::io::{Read, Write};
	
	// read 4 bytes into v
	let mut v = [0; 4];
	let count = try![s.read(v.as_mut_slice())];
	assert_eq!(count, 4);

	// reverse them
	v.reverse();

	// write them back to network
	s.write(v.as_slice())
}

In tests, provide data to mock and verify the results after performing operations.

	let mut s = SharedMockStream::new();
	let mut e = NetStream::Mocked(s.clone());

	// provide data to mock
	s.push_bytes_to_read([1, 2, 3, 4].as_slice());
	// check if io succeeded
	assert_eq!(reverse4(&mut e), Ok(4));
	// verify the data returned
	assert_eq!(s.pop_bytes_written().as_slice(), [4, 3, 2, 1]);

I/O failures

A mock simulating I/O errors is provided too. Combined with a Chain, this allows to simulate errors between reading data. See the following example where read_data performs 3 retries on I/O errors. This is verified in the test function below.

struct CountIo {}

impl CountIo {
	fn read_data(&self, r: &mut Read) -> usize {
		let mut count: usize = 0;
		let mut retries = 3;

		loop {
			let mut buffer = [0; 5];
			match r.read(&mut buffer) {
				Err(_) => {
					if retries == 0 { break; }
					retries -= 1;
				},
				Ok(0) => break,
				Ok(n) => count += n,
			}
		}
		count
	}
}

#[test]
fn test_io_retries() {
	let mut c = Cursor::new(&b"1234"[..])
			.chain(FailingMockStream::new(ErrorKind::Other, "Failing", 3))
			.chain(Cursor::new(&b"5678"[..]));

	let sut = CountIo {};
	// this will fail unless read_data performs at least 3 retries on I/O errors
	assert_eq!(8, sut.read_data(&mut c));
}