Assertion failed: Unexpected type encountered .function()

[Zon8Research]

It was observed that compared to default mode, hybrid and multi mode crash Fuzzilli quite regularly. After running fuzzilli in debug mode to figure out why, the following crash occurred:

Fuzzilli/ProgramBuilder.swift:468: Assertion failed: Unexpected type encountered .function()
Current stack trace:
0    libswiftCore.so                    0x00007f3c8ef00210 swift_reportError + 50
1    libswiftCore.so                    0x00007f3c8ef74fc0 _swift_stdlib_reportFatalErrorInFile + 112
2    libswiftCore.so                    0x00007f3c8ec5dc40 _assertionFailure(_:_:file:line:flags:) + 1329
3    FuzzilliCli                        0x000055fca15d63c8 <unavailable> + 1000392
4    FuzzilliCli                        0x000055fca15d37d7 <unavailable> + 989143
5    FuzzilliCli                        0x000055fca15d4980 <unavailable> + 993664
6    FuzzilliCli                        0x000055fca1555f36 <unavailable> + 474934
7    FuzzilliCli                        0x000055fca153e00c <unavailable> + 376844
8    FuzzilliCli                        0x000055fca153e041 <unavailable> + 376897
9    FuzzilliCli                        0x000055fca153e3c2 <unavailable> + 377794
10   FuzzilliCli                        0x000055fca15df156 <unavailable> + 1036630
11   FuzzilliCli                        0x000055fca15df9ff <unavailable> + 1038847
12   FuzzilliCli                        0x000055fca154c23c <unavailable> + 434748
13   FuzzilliCli                        0x000055fca15ec321 <unavailable> + 1090337
14   FuzzilliCli                        0x000055fca18640f7 <unavailable> + 3678455
15   FuzzilliCli                        0x000055fca15df68e <unavailable> + 1037966
16   FuzzilliCli                        0x000055fca15dfb0b <unavailable> + 1039115

It seems possible to stop the crash using the following addition to generateVariable(), though it would be good to get feedback on it to see if it makes sense:

if type.Is(.function()) {
    return definePlainFunction(withSignature: FunctionSignature(withParameterCount: Int.random(in: 2...5), hasRestParam: probability(0.1))) { _ in
    	generateRecursive()
    	doReturn(value: randVar())
    }
}

[saelo]

Cool, thanks for looking into this! Yeah I think this approach should be fine. Eventually, we'll probably want CodeGenerators to be able to create variables of the requested type, but no idea if and when this will happen. So in the meantime, this is the right approach. The only thing I would suggest is checking whether type.signature is not nil, and using it instead of the generic signature if it is:

let signature = type.signature ?? FunctionSignature(withParameterCount: Int.random(in: 2...5)
...