I'd like to try my hand at this, assuming no one else minds. I expect to make progress on it by the end of January at the latest.

Some notes of mine:

fork and vfork are explicitly marked as unavailable on most Darwin platforms. But setjmp and other functions are not. Here's my test program, which compiles just fine:

// swift -frontend -c -primary-file 2394.swift -sdk `xcrun -sdk macosx10.12 --show-sdk-path` -module-name main -o 2394.o

import Darwin

var x: Int32 = 0
setjmp(&x)

I believe Clang contains some code that attributes functions like fork and setjmp with __attribute__ ((returns_twice)). By placing a breakpoint within a ClangImporter method, I can see that setjmp has that attribute set. So, as far as I understand, all I have to do is find a good spot in lib/ClangImporter to check for that attribute, then mark the symbol as unavailable if it's set. At that point, I think I can remove the explicit exclusion of fork and vfork, since they'll be unavailable anyway.

I wonder, though: this is a source breaking change, isn't it? The Swift code above used to compile, but as of this change, it will no longer compile. Is that acceptable, @belkadan?

I think #6513 resolves this issue, but this seems like an invasive, source-breaking change. Particularly disappointing for me is the fact that the Swift test suite runs on Android in part thanks to fork. Marking it unavailable will break those tests. I'll have to re-implement that portion in order to get them running again.

Oops, my bad, fork isn't a 'returns_twice' function, I don't think. Sorry about the confusion!

#6513 was merged, so I think this is done!