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by jjnoakes
3603 days ago
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I'm not sure I follow. Why not test with something that implements the reader or writer interface, but shuffles bytes around in memory? That should alleviate the testing explosion. And whatever interface you do have must be putting bytes in our getting bytes out of the protocol layers. Why not call that interface reader or writer? I'm not seeing the distinction. |
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The reason that just having an in-memory Reader or Writer doesn't solve the problem is that the failure modes don't match up. An in-memory reader/writer has basically no failure modes beyond ENOMEM. That's why in the no-I/O implementation, this is exactly what we use: write to an in-memory buffer.
Real I/O on the other hand has many failure modes. For an example, consider timeouts. If your parser does I/O, you need to test timeouts at every location that your parser does I/O. You need to confirm it handles those timeouts appropriately. And you need to decide what "appropriately" means here: do you retry? Do you abort? Do you attempt to unwind that state transition?
All of these are expansions of your state space. This means your protocol parser has to handle this combinatorial explosion of possible outcomes: at every point you have a Read/Write you need to be ready and prepared to handle all possible error conditions that can come out of that.
If your parser does no I/O, though, and only writes to buffers, this problem does not exist. That allows you to have two totally isolated sections of code: one part manipulates bytes in memory (the parser), and another bit is responsible for getting those bytes to and from the network. Each can be tested separately. If we need `n` tests for the no-I/O parser, and `m` tests for the I/O without parser, then to achieve equivalent test coverage your combined code requires `n * m` tests to achieve equivalent logical coverage of the possibility space.
Small, isolated components are good.