OutStream in struct gives runtime unreachable in debug and release-safe

[numsim1415]

The following code compiles but crashes on runtime.
The crash happens only if the variable doesMatter is there and only if debug or --release-safe is used.

const io = @import("std").io;

pub const Mandelbrot = struct {
    out: @typeOf((io.getStdOut() catch unreachable).outStream().stream),
    
    pub fn init(m: *Mandelbrot) void{
        m.out = (io.getStdOut() catch unreachable).outStream().stream;
    }
};
pub fn main() void {
    var doesntMatter : usize = 1234; 
    var mb : Mandelbrot = undefined;
    Mandelbrot.init(&mb);
    
    // comment this variable and the runtime crash won't happen
    var doesMatter : usize = 42;
    
    _ = mb.out.print("{}\n",@intCast(usize,123));  // crash
}

By the way, it does not compile on 0.3, only on Master, see https://godbolt.org/z/0yVCLg .
Is my code flawed or is this a zig bug ?

The crash output is

reached unreachable code
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/os/index.zig:368:28: 0x21d3c1 in ??? (lkerrs4)
            posix.EBADF => unreachable, // always a race condition
                           ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/os/file.zig:411:30: 0x206493 in ??? (lkerrs4)
            try os.posixWrite(self.handle, bytes);
                             ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/os/file.zig:457:35: 0x2059f5 in ??? (lkerrs4)
            return self.file.write(bytes);
                                  ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/fmt/index.zig:755:22: 0x221270 in ??? (lkerrs4)
        return output(context, padded_buf);
                     ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/fmt/index.zig:683:33: 0x220c0a in ??? (lkerrs4)
        return formatIntUnsigned(value, base, uppercase, width, context, Errors, output);
                                ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/fmt/index.zig:317:21: 0x222cd1 in ??? (lkerrs4)
    return formatInt(value, radix, uppercase, width, context, Errors, output);
                    ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/fmt/index.zig:270:52: 0x222c69 in ??? (lkerrs4)
        builtin.TypeId.Int => return formatIntValue(value, fmt, context, Errors, output),
                                                   ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/fmt/index.zig:120:31: 0x222bc9 in ??? (lkerrs4)
            return formatValue(value, fmt, context, Errors, output);
                              ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/fmt/index.zig:53:35: 0x222aa9 in ??? (lkerrs4)
                    try formatType(args[next_arg], fmt[0..0], context, Errors, output);
                                  ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/io.zig:203:34: 0x222978 in ??? (lkerrs4)
            return std.fmt.format(self, Error, self.writeFn, format, args);
                                 ^
/scratch/tmp/lkazaz/lll/benchmarksgame-sourcecode/mandelbrot/lkerrs4.zig:25:21: 0x2227d4 in ??? (lkerrs4)
    _ = mb.out.print("{}\n",@intCast(usize,123));  // crash
                    ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/special/bootstrap.zig:86:22: 0x2226c9 in ??? (lkerrs4)
            root.main();
                     ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/special/bootstrap.zig:70:20: 0x222675 in ??? (lkerrs4)
    return callMain();
                   ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/special/bootstrap.zig:64:39: 0x2224d8 in ??? (lkerrs4)
    std.os.posix.exit(callMainWithArgs(argc, argv, envp));
                                      ^
/scratch/tmp/lkazaz/lll/ziginst/lib/zig/std/special/bootstrap.zig:37:5: 0x222390 in ??? (lkerrs4)
    @noInlineCall(posixCallMainAndExit);
    ^
Aborted

Thank you in advance.

[Hejsil]

This is related to #591.

Basically, what you're doing is copying a field you should never copy. File.OutStream is defined like this:

pub const OutStream = struct {
    file: File,
    stream: Stream,

    pub const Error = WriteError;
    pub const Stream = io.OutStream(Error);

    fn writeFn(out_stream: *Stream, bytes: []const u8) Error!void {
        const self = @fieldParentPtr(OutStream, "stream", out_stream);
        return self.file.write(bytes);
    }
};

The writeFn is what's important here. It's the function that io.OutStream uses to implement all its helper functions. This function expects, that out_stream is embedded in File.OutStream and therefore uses @fieldParentPtr to get to its implementation. You just copied stream out of its implementation, so @fieldParentPtr returns invalid memory.

[Hejsil]

This should fix your code (haven't tested):

const io = @import("std").io;

pub const Mandelbrot = struct {
    out: @typeOf((io.getStdOut() catch unreachable).outStream()),
    
    pub fn init(m: *Mandelbrot) void{
        m.out = (io.getStdOut() catch unreachable).outStream();
    }
};
pub fn main() void {
    var doesntMatter : usize = 1234; 
    var mb : Mandelbrot = undefined;
    Mandelbrot.init(&mb);
    
    // comment this variable and the runtime crash won't happen
    var doesMatter : usize = 42;
    
    _ = mb.out.stream.print("{}\n",@intCast(usize,123));  // crash
}

[numsim1415]

Thank you, it seems this is a known zig issue. It's funny how optimizations suppress the crash.