Segmentation fault in release mode with std.rand.Csprng.int(u8)

[pmwhite]

Here is a repro I've just tested with the most recent master from ziglang.org:

const std = @import("std");

pub fn main() anyerror!void {
    var seed = @intCast(u64, std.time.timestamp());
    var csprng = std.rand.DefaultCsprng.init(seed);
    var rand = csprng.random;
    _ = rand.int(u8);
}

In normal build mode, this runs fine. However, in release-safe build mode, I get the following error when I run the executable.

Segmentation fault at address 0x0
/.../zig/lib/zig/std/rand.zig:939:26: 0x214673 in std.rand.Isaac64.fill (repro)
                buf[i] = @truncate(u8, n);
                         ^
The process was killed by SIGABRT: Aborted

[LemonBoy]

var rand = csprng.random;

This creates a fresh copy of the random interface that's detached from csprng. When you call int you're basically mangling some random stack region, hence the segfault.

Use this instead:

var rand = &csprng.random;

[Tetralux]

This is the second time I've come across someone having this problem in a week.
Perhaps this interface could be improved.

My first thought: maybe random should be a member function, like writer() is on io.Writer?

[Rocknest]

Yeah looks like we need #3803 or some other protection

[jayschwa]

It looks like this is covered by #591 and related proposals.