fileMemoryMapSetLength

fn fileMemoryMapSetLength(
    userdata: ?*anyopaque,
    mm: *File.MemoryMap,
    new_len: usize,
) File.MemoryMap.SetLengthError!void {
    const ev: *Evented = @ptrCast(@alignCast(userdata));

    const page_size = std.heap.pageSize();
    const alignment: Alignment = .fromByteUnits(page_size);
    const page_align = std.heap.page_size_min;
    const old_memory = mm.memory;

    if (alignment.forward(new_len) == alignment.forward(old_memory.len)) {
        mm.memory.len = new_len;
        return;
    }
    const flags: linux.MREMAP = .{ .MAYMOVE = true };
    const addr_hint: ?[*]const u8 = null;
    var sync: CancelRegion.Sync = try .init(ev);
    defer sync.deinit(ev);
    const new_memory = while (true) {
        try sync.cancel_region.await(.nothing);
        const rc = linux.mremap(old_memory.ptr, old_memory.len, new_len, flags, addr_hint);
        switch (linux.errno(rc)) {
            .SUCCESS => break @as([*]align(page_align) u8, @ptrFromInt(rc))[0..new_len],
            .INTR => {},
            .AGAIN => return error.LockedMemoryLimitExceeded,
            .NOMEM => return error.OutOfMemory,
            .INVAL => |err| return errnoBug(err),
            .FAULT => |err| return errnoBug(err),
            else => |err| return unexpectedErrno(err),
        }
    };
    mm.memory = new_memory;
}