Can't fetch tag.gz package on linux

[travisstaloch]

Zig Version

0.16.0-dev.1276+c58687225

Steps to Reproduce and Observed Behavior

I noticed my flatbuferz lib segfaulting since around 0.16.0-dev.1225 when Io.Threaded was introduced when running zig build. You should be able to reproduce a segfault (or similar stack trace with a debug compiler build) by running

git clone git@github.com:travisstaloch/flatbufferz.git
cd flatbufferz
zig build

The issue seems to happen while downloading the dep file https://github.com/google/flatbuffers/archive/refs/tags/v24.3.25.tar.gz

Maybe related to #25776. The following reproduction is a copy of @squeek502's repro here #25776 (comment) but with my dep file path.

// /tmp/tmp.zig
const std = @import("std");

pub fn main() !void {
    var debug_allocator: std.heap.DebugAllocator(.{}) = .{};
    defer std.debug.assert(debug_allocator.deinit() == .ok);
    const gpa = debug_allocator.allocator();

    var threaded: std.Io.Threaded = .init(gpa);
    defer threaded.deinit();
    const io = threaded.io();

    var client: std.http.Client = .{ .io = io, .allocator = gpa };
    defer client.deinit();

    var body: std.Io.Writer.Allocating = .init(gpa);
    defer body.deinit();

    const res = try client.fetch(.{
        .location = .{ .url = "https://github.com/google/flatbuffers/archive/refs/tags/v24.3.25.tar.gz" },
        .method = .GET,
        .response_writer = &body.writer,
    });

    std.debug.print("{}\n", .{res.status});
    std.debug.print("{s}\n", .{body.written()});
}

$ zig version
0.16.0-dev.1262+be4eaed7c
$ zig run /tmp/tmp.zig
thread 3796124 panic: reached unreachable code
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/debug.zig:416:14: 0x10575c9 in assert (std.zig)
    if (!ok) unreachable; // assertion failure
             ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Reader.zig:521:11: 0x10d21f8 in toss (std.zig)
    assert(r.seek <= r.end);
          ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Threaded.zig:5538:20: 0x119c946 in lookupHostsReader (std.zig)
        reader.toss(1);
                   ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Threaded.zig:5502:29: 0x118d4ba in lookupHosts (std.zig)
    return lookupHostsReader(t, host_name, resolved, options, &file_reader.interface) catch |err| switch (err) {
                            ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Threaded.zig:4841:20: 0x118de8a in netLookupFallible (std.zig)
        lookupHosts(t, host_name, resolved, options) catch |err| switch (err) {
                   ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Threaded.zig:4703:66: 0x118ebec in netLookup (std.zig)
    resolved.putOneUncancelable(t_io, .{ .end = netLookupFallible(t, host_name, resolved, options) });
                                                                 ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/net/HostName.zig:98:31: 0x151f1c2 in lookup (std.zig)
    return io.vtable.netLookup(io.userdata, host_name, resolved, options);
                              ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io.zig:1034:17: 0x14bfceb in start (std.zig)
                @call(.auto, function, args_casted.*);
                ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Threaded.zig:605:16: 0x109687a in start (std.zig)
        gc.func(group, gc.contextPointer());
               ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Io/Threaded.zig:188:26: 0x1100646 in worker (std.zig)
            closure.start(closure);
                         ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Thread.zig:558:13: 0x10d9ed0 in callFn__anon_16095 (std.zig)
            @call(.auto, f, args);
            ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/Thread.zig:1534:30: 0x10b66c0 in entryFn (std.zig)
                return callFn(f, self.fn_args);
                             ^
~/.local/share/zigup/0.16.0-dev.1262+be4eaed7c/files/lib/std/os/linux/x86_64.zig:105:5: 0x10d9f85 in clone (std.zig)
    asm volatile (
    ^
Aborted (core dumped)

Expected Behavior

I think the flatbufferz lib should build and the reproduction should also work. The flatbufferz lib built fine in 0.15 and in 0.16-dev versions prior to the introduction of Io.Threaded.