fileMemoryMapSetLength

fn fileMemoryMapSetLength(
    userdata: ?*anyopaque,
    mm: *File.MemoryMap,
    new_len: usize,
) File.MemoryMap.SetLengthError!void {
    const t: *Threaded = @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 (mm.section) |section| {
        _ = section;
        if (alignment.forward(new_len) == alignment.forward(old_memory.len)) {
            mm.memory.len = new_len;
            return;
        }
        switch (native_os) {
            .wasi => unreachable,
            .linux => {
                const flags: posix.MREMAP = .{ .MAYMOVE = true };
                const addr_hint: ?[*]const u8 = null;
                const new_memory = while (true) {
                    const syscall: Syscall = try .start();
                    const rc = posix.system.mremap(old_memory.ptr, old_memory.len, new_len, flags, addr_hint);
                    syscall.finish();
                    const err: posix.E = if (builtin.link_libc) e: {
                        if (rc != std.c.MAP_FAILED) break @as([*]align(page_align) u8, @ptrCast(@alignCast(rc)))[0..new_len];
                        break :e @fromBackingInt(@intCast(posix.system._errno().*));
                    } else e: {
                        const err = posix.errno(rc);
                        if (err == .SUCCESS) break @as([*]align(page_align) u8, @ptrFromInt(rc))[0..new_len];
                        break :e err;
                    };
                    switch (err) {
                        .SUCCESS => unreachable,
                        .INTR => continue,
                        .AGAIN => return error.LockedMemoryLimitExceeded,
                        .NOMEM => return error.OutOfMemory,
                        .INVAL => return errnoBug(err),
                        .FAULT => return errnoBug(err),
                        else => return posix.unexpectedErrno(err),
                    }
                };
                mm.memory = new_memory;
                return;
            },
            else => return error.OperationUnsupported,
        }
    } else {
        const gpa = t.allocator;
        if (gpa.rawRemap(old_memory, alignment, new_len, @returnAddress())) |new_ptr| {
            mm.memory = @alignCast(new_ptr[0..new_len]);
        } else {
            const new_ptr: [*]align(page_align) u8 = @alignCast(
                gpa.rawAlloc(new_len, alignment, @returnAddress()) orelse return error.OutOfMemory,
            );
            const copy_len = @min(new_len, old_memory.len);
            @memcpy(new_ptr[0..copy_len], old_memory[0..copy_len]);
            mm.memory = new_ptr[0..new_len];
            gpa.rawFree(old_memory, alignment, @returnAddress());
        }
    }
}