Failing test produces slightly garbled output

[kristoff-it]

How to reproduce (from zig/master, on macOS):

Edit line 213 of https://github.com/kristoff-it/zig-cuckoofilter/blob/b5447d98282c780b396897ea50ca1ddafab901bb/cuckoofilter.zig from

cf.insert(2, 1) catch unreachable;

to

cf.insert(0, 1) catch unreachable;

This will cause the test to fail and show the following output:

1/5 test "Hx == (Hy XOR hash(fp))"...OK
2/5 test "is not completely broken"...OK
3/5 test "generics are not completely broken"...OK
4/5 test "too full when adding too many copies"...�`6,�Rʶ��t5i�p error: TooFull
...

The garbling happens also in 0.4.0.

[LemonBoy]

Oh, this is quite nasty. The culprit is this line:

var rand = std.rand.DefaultPrng.init(42).random;

Here you have:

• std.rand.DefaultPrng.init(42) that is a Xoroshiro128
• std.rand.DefaultPrng.init(42).random that is a Random

Now, rand is just the Random part of the Xoroshiro128 structure and that means that whenever you call the fillFn and it tries to access the s field you're actually corrupting random memory.

And it turns out that your code ends up corrupting the error message stored in global_array 🎉

So it turns out it's not much of a bug, it's rather a programming error.

On a side note, why don't we allocate the buffer & slice that are passed on to panic() on the stack? Global state always feels icky to me.

[andrewrk]

This is #591 which is a recognized design flaw in the language and it's planned to be a compile error once that design flaw is resolved.

[kristoff-it]

Understood & fixed in my code. Thanks!