FuzzMultiThreadedContext

const FuzzMultiThreadedContext = struct {
    testing_buf: []u8,
    backing_buf: []u8,

    io: std.Io,
    ops: *ThreadOps,

    const n_threads = 4;

    const ThreadOps = struct {
        /// Switches between two values for each time a run starts.
        run: Run,
        /// While this can be calculated as `n_threads - (i -| ops.items.len)`,
        /// this also serves as `.release` synchronization for each thread.
        running: u32,

        instance: SafeAllocator,
        i: usize,
        items: []Op,

        const Run = packed struct(u32) {
            n: bool,
            pad: u31 = 0,

            fn wait(ptr: *Run, val: Run, io: std.Io) error{Canceled}!void {
                assert(val.pad == 0);
                while (true) {
                    // This cannot load a previous value since this thread previously loaded the
                    // latest value.
                    const prev = @atomicLoad(Run, ptr, .acquire);
                    assert(prev.pad == 0);
                    if (prev == val) break;

                    try io.futexWait(Run, ptr, prev);
                }
            }

            fn next(r: Run) Run {
                assert(r.pad == 0);
                return .{ .n = !r.n };
            }
        };

        const Op = union(fuzz_probs.Op) {
            alloc: struct {
                len: usize,
                alignment: Alignment,

                splat: ?u8,
                /// Not embeded directly in the struct as a workaround for tsan since a
                /// switch directly on `Op` loads the entire value non-atomically.
                result: *MemoryDependency,
            },
            free: struct {
                memory: *MemoryDependency,
                alignment: Alignment,

                splat: ?u8,
            },
            resize: Realloc,
            remap: Realloc,

            const Realloc = struct {
                memory: *MemoryDependency,
                alignment: Alignment,
                new_len: usize,

                splat: ?u8,
                /// Not embeded directly in the struct as a workaround for tsan since a
                /// switch directly on `Op` loads the entire value non-atomically.
                result: *MemoryDependency,
            };

            const MemoryDependency = struct {
                ready: std.Io.Event,
                /// Null if the memory failed to be allocated
                memory: ?[]u8,

                const init: MemoryDependency = .{
                    .ready = .unset,
                    .memory = undefined,
                };

                fn get(dep: *MemoryDependency, io: std.Io) ?[]u8 {
                    dep.ready.waitUncancelable(io);
                    return dep.memory;
                }
            };
        };
    };
}