Handle fork(2) failures

[vasild]

Generate() and SpawnProcess() call fork(2) but did not check if it
failed (return -1) and continued execution with possible unexpected
results.

Handle fork(2) failure by throwing an exception.

[ryanofsky]

Tested ACK 16ebae8. Thanks for catching this, and also fixing more missing includes

[ryanofsky]

Interesting, I didn't know system_error took a third string argument. It'd be good to start using this more places.

[vasild]

Yes, it also occurred to me that if std::system_category() modifies errno then std::system_error() may get the wrong value of errno:

#include <errno.h>
#include <stdio.h>

int f1() {
    errno = 3;
    return 20;
}   

void f2(int e, int x) {
    printf("%d %d\n", e, x);
}   

int main(int, char**) {
    errno = 2; // resulted from fork() or whatever
    f2(errno, f1());
    return 0;
}

produces:

# clang 9.0.1
$ clang++90 t.cc -o t && ./t
2 20

# gcc 9.2.0
$ g++9 t.cc -o t && ./t
3 20

[ryanofsky]

It's a possible concern, but in practice if that constructor was setting errno, it would probably break a lot of things. Also in the worst case error message text would just be less clear

[vasild]

Yes, safe to ignore in this case. Just the pattern func1(x, func2_may_possibly_modify_x()); is better to be avoided.

[vasild]

system_error took a third string argument. It'd be good to start using this more places.

#23 Tell std::system_error() which function failed

Cheerz!