Concept ACK but this seems a bit… arbitrary. If you want to be 100% certain to not step on an existing one, could you open a (real) file descriptor, connected to something like a pipe() or /dev/null etc, and use that? Or do you use the fact that it needs to be a non-allocated file descriptor somewhere?

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Conflicts

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• fuzz: split FuzzedSock interface and implementation #21630 (fuzz: split FuzzedSock interface and implementation by vasild)

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... could you open a (real) file descriptor, connected to ...

Right! It does not need to be connected to anything. Just reuse this:

plus a close() in the FuzzedSocket destructor.

Concept ACK but this seems a bit… arbitrary. If you want to be 100% certain to not step on an existing one, could you open a (real) file descriptor, connected to something like a pipe() or /dev/null etc, and use that? Or do you use the fact that it needs to be a non-allocated file descriptor somewhere?

The nice thing about going with INVALID_SOCKET - 1 is that it doesn't involve slowdowns due to syscalls, and that reading or writing from fd INVALID_SOCKET - 1 is guaranteed to fail (unless the fuzzing harness has four billion open file descriptors :)).

Note that INVALID_SOCKET - 1 is 4 294 967 294, and that POSIX requires file descriptor id re-use on close.

Note that this PR is only a short-term bug fix to avoid file descriptors stdin (0), stdout (1), stderr (2) and other likely to be used file descriptors while awaiting vasild's suggested cleanup/refactoring.

The chosen constant INVALID_SOCKET - 1 is bigger than FD_SETSIZE, so those dummy sockets are not selectable:

I think that's ok. I am working on a change to convert IsSelectableSocket(bare SOCKET or sock.Get()) to sock.IsSelectable() so that the mocked sockets could override that.

#21700 reduces the usage of Sock::Get() to tests and CreateSockTCP() which is not to be mocked. It also changes IsSelectableSocket() to be a method of Sock() so that FuzzedSock can override it nicely.