# This file is a part of Julia. License is MIT: https://julialang.org/license

"""
Provide serialization of Julia objects via the functions
* [`serialize`](@ref)
* [`deserialize`](@ref)
"""
module Serialization

import Base: GMP, Bottom, unsafe_convert, uncompressed_ast
import Core: svec, SimpleVector
using Base: unaliascopy, unwrap_unionall, require_one_based_indexing, ntupleany
using Core.IR

export serialize, deserialize, AbstractSerializer, Serializer

abstract type AbstractSerializer end

mutable struct Serializer{I<:IO} <: AbstractSerializer
    io::I
    counter::Int
    table::IdDict{Any,Any}
    pending_refs::Vector{Int}
    known_object_data::Dict{UInt64,Any}
    version::Int
    Serializer{I}(io::I) where I<:IO = new(io, 0, IdDict(), Int[], Dict{UInt64,Any}(), ser_version)
end

Serializer(io::IO) = Serializer{typeof(io)}(io)

## serializing values ##

const n_int_literals = 33
const n_reserved_slots = 24
const n_reserved_tags = 8

const TAGS = Any[
    Symbol, Int8, UInt8, Int16, UInt16, Int32, UInt32, Int64, UInt64, Int128, UInt128,
    Float16, Float32, Float64, Char, DataType, Union, UnionAll, Core.TypeName, Tuple,
    Array, Expr, LineNumberNode, :__LabelNode__, GotoNode, QuoteNode, CodeInfo, TypeVar,
    Core.Box, Core.MethodInstance, Module, Task, String, SimpleVector, Method,
    GlobalRef, SlotNumber, TypedSlot, NewvarNode, SSAValue,

    # dummy entries for tags that don't correspond directly to types
    Symbol, # UNDEFREF_TAG
    Symbol, # BACKREF_TAG
    Symbol, # LONGBACKREF_TAG
    Symbol, # SHORTBACKREF_TAG
    Symbol, # LONGTUPLE_TAG
    Symbol, # LONGSYMBOL_TAG
    Symbol, # LONGEXPR_TAG
    Symbol, # LONGSTRING_TAG
    Symbol, # SHORTINT64_TAG
    Symbol, # FULL_DATATYPE_TAG
    Symbol, # WRAPPER_DATATYPE_TAG
    Symbol, # OBJECT_TAG
    Symbol, # REF_OBJECT_TAG
    Symbol, # FULL_GLOBALREF_TAG
    Symbol, # HEADER_TAG
    Symbol, # IDDICT_TAG
    Symbol, # SHARED_REF_TAG
    ReturnNode, GotoIfNot,
    fill(Symbol, n_reserved_tags)...,

    (), Bool, Any, Bottom, Core.TypeofBottom, Type, svec(), Tuple{}, false, true, nothing,
    :Any, :Array, :TypeVar, :Box, :Tuple, :Ptr, :return, :call, Symbol("::"), :Function,
    :(=), :(==), :(===), :gotoifnot, :A, :B, :C, :M, :N, :T, :S, :X, :Y, :a, :b, :c, :d, :e, :f,
    :g, :h, :i, :j, :k, :l, :m, :n, :o, :p, :q, :r, :s, :t, :u, :v, :w, :x, :y, :z, :add_int,
    :sub_int, :mul_int, :add_float, :sub_float, :new, :mul_float, :bitcast, :start, :done, :next,
    :indexed_iterate, :getfield, :meta, :eq_int, :slt_int, :sle_int, :ne_int, :push_loc, :pop_loc,
    :pop, :arrayset, :arrayref, :apply_type, :inbounds, :getindex, :setindex!, :Core, :!, :+,
    :Base, :static_parameter, :convert, :colon, Symbol("#self#"), Symbol("#temp#"), :tuple, Symbol(""),

    fill(:_reserved_, n_reserved_slots)...,

    (Int32(0):Int32(n_int_literals-1))...,
    (Int64(0):Int64(n_int_literals-1))...
]

@assert length(TAGS) == 255

const ser_version = 17 # do not make changes without bumping the version #!

format_version(::AbstractSerializer) = ser_version
format_version(s::Serializer) = s.version

const NTAGS = length(TAGS)

function sertag(@nospecialize(v))
    # NOTE: we use jl_value_ptr directly since we know at least one of the arguments
    # in the comparison below is a singleton.
    ptr = ccall(:jl_value_ptr, Ptr{Cvoid}, (Any,), v)
    ptags = convert(Ptr{Ptr{Cvoid}}, pointer(TAGS))
    # note: constant ints & reserved slots never returned here
    @inbounds for i in 1:(NTAGS-(n_reserved_slots+2*n_int_literals))
        ptr == unsafe_load(ptags,i) && return i%Int32
    end
    return Int32(-1)
end
desertag(i::Int32) = @inbounds(TAGS[i])

# tags >= this just represent themselves, their whole representation is 1 byte
const VALUE_TAGS = sertag(())
const ZERO32_TAG = Int32(NTAGS-(2*n_int_literals-1))
const ZERO64_TAG = Int64(NTAGS-(n_int_literals-1))
const TRUE_TAG = sertag(true)
const FALSE_TAG = sertag(false)
const EMPTYTUPLE_TAG = sertag(())
const TUPLE_TAG = sertag(Tuple)
const SIMPLEVECTOR_TAG = sertag(SimpleVector)
const SYMBOL_TAG = sertag(Symbol)
const INT8_TAG = sertag(Int8)
const ARRAY_TAG = findfirst(==(Array), TAGS)%Int32
const EXPR_TAG = sertag(Expr)
const MODULE_TAG = sertag(Module)
const METHODINSTANCE_TAG = sertag(Core.MethodInstance)
const METHOD_TAG = sertag(Method)
const TASK_TAG = sertag(Task)
const DATATYPE_TAG = sertag(DataType)
const TYPENAME_TAG = sertag(Core.TypeName)
const INT32_TAG = sertag(Int32)
const INT64_TAG = sertag(Int64)
const GLOBALREF_TAG = sertag(GlobalRef)
const BOTTOM_TAG = sertag(Bottom)
const UNIONALL_TAG = sertag(UnionAll)
const STRING_TAG = sertag(String)
const o0 = sertag(SSAValue)
const UNDEFREF_TAG         = Int32(o0+1)
const BACKREF_TAG          = Int32(o0+2)
const LONGBACKREF_TAG      = Int32(o0+3)
const SHORTBACKREF_TAG     = Int32(o0+4)
const LONGTUPLE_TAG        = Int32(o0+5)
const LONGSYMBOL_TAG       = Int32(o0+6)
const LONGEXPR_TAG         = Int32(o0+7)
const LONGSTRING_TAG       = Int32(o0+8)
const SHORTINT64_TAG       = Int32(o0+9)
const FULL_DATATYPE_TAG    = Int32(o0+10)
const WRAPPER_DATATYPE_TAG = Int32(o0+11)
const OBJECT_TAG           = Int32(o0+12)
const REF_OBJECT_TAG       = Int32(o0+13)
const FULL_GLOBALREF_TAG   = Int32(o0+14)
const HEADER_TAG           = Int32(o0+15)
const IDDICT_TAG           = Int32(o0+16)
const SHARED_REF_TAG       = Int32(o0+17)

writetag(s::IO, tag) = (write(s, UInt8(tag)); nothing)

function write_as_tag(s::IO, tag)
    tag < VALUE_TAGS && write(s, UInt8(0))
    write(s, UInt8(tag))
    nothing
end

# cycle handling
function serialize_cycle(s::AbstractSerializer, @nospecialize(x))
    offs = get(s.table, x, -1)::Int
    if offs != -1
        if offs <= typemax(UInt16)
            writetag(s.io, SHORTBACKREF_TAG)
            write(s.io, UInt16(offs))
        elseif offs <= typemax(Int32)
            writetag(s.io, BACKREF_TAG)
            write(s.io, Int32(offs))
        else
            writetag(s.io, LONGBACKREF_TAG)
            write(s.io, Int64(offs))
        end
        return true
    end
    s.table[x] = s.counter
    s.counter += 1
    return false
end

function serialize_cycle_header(s::AbstractSerializer, @nospecialize(x))
    serialize_cycle(s, x) && return true
    serialize_type(s, typeof(x), true)
    return false
end

function reset_state(s::AbstractSerializer)
    s.counter = 0
    empty!(s.table)
    empty!(s.pending_refs)
    s
end

serialize(s::AbstractSerializer, x::Bool) = x ? writetag(s.io, TRUE_TAG) :
                                                writetag(s.io, FALSE_TAG)

serialize(s::AbstractSerializer, p::Ptr) = serialize_any(s, oftype(p, C_NULL))

serialize(s::AbstractSerializer, ::Tuple{}) = writetag(s.io, EMPTYTUPLE_TAG)

function serialize(s::AbstractSerializer, t::Tuple)
    l = length(t)
    if l <= 255
        writetag(s.io, TUPLE_TAG)
        write(s.io, UInt8(l))
    else
        writetag(s.io, LONGTUPLE_TAG)
        write(s.io, Int32(l))
    end
    for x in t
        serialize(s, x)
    end
end

function serialize(s::AbstractSerializer, v::SimpleVector)
    writetag(s.io, SIMPLEVECTOR_TAG)
    write(s.io, Int32(length(v)))
    for x in v
        serialize(s, x)
    end
end

function serialize(s::AbstractSerializer, x::Symbol)
    tag = sertag(x)
    if tag > 0
        return write_as_tag(s.io, tag)
    end
    pname = unsafe_convert(Ptr{UInt8}, x)
    len = Int(ccall(:strlen, Csize_t, (Cstring,), pname))
    if len > 7
        serialize_cycle(s, x) && return
    end
    if len <= 255
        writetag(s.io, SYMBOL_TAG)
        write(s.io, UInt8(len))
    else
        writetag(s.io, LONGSYMBOL_TAG)
        write(s.io, Int32(len))
    end
    unsafe_write(s.io, pname, len)
    nothing
end

function serialize_array_data(s::IO, a)
    require_one_based_indexing(a)
    isempty(a) && return 0
    if eltype(a) === Bool
        last = a[1]::Bool
        count = 1
        for i = 2:length(a)
            if a[i]::Bool != last || count == 127
                write(s, UInt8((UInt8(last) << 7) | count))
                last = a[i]::Bool
                count = 1
            else
                count += 1
            end
        end
        write(s, UInt8((UInt8(last) << 7) | count))
    else
        write(s, a)
    end
end

function serialize(s::AbstractSerializer, a::Array)
    serialize_cycle(s, a) && return
    elty = eltype(a)
    writetag(s.io, ARRAY_TAG)
    if elty !== UInt8
        serialize(s, elty)
    end
    if ndims(a) != 1
        serialize(s, size(a))
    else
        serialize(s, length(a))
    end
    if isbitstype(elty)
        serialize_array_data(s.io, a)
    else
        sizehint!(s.table, div(length(a),4))  # prepare for lots of pointers
        @inbounds for i in eachindex(a)
            if isassigned(a, i)
                serialize(s, a[i])
            else
                writetag(s.io, UNDEFREF_TAG)
            end
        end
    end
end

function serialize(s::AbstractSerializer, a::SubArray{T,N,A}) where {T,N,A<:Array}
    # SubArray's copy only selects the relevant data (and reduces the size) but does not
    # preserve the type of the argument. This internal function does both:
    b = unaliascopy(a)
    serialize_any(s, b)
end

function serialize(s::AbstractSerializer, ss::String)
    len = sizeof(ss)
    if len > 7
        serialize_cycle(s, ss) && return
        writetag(s.io, SHARED_REF_TAG)
    end
    if len <= 255
        writetag(s.io, STRING_TAG)
        write(s.io, UInt8(len))
    else
        writetag(s.io, LONGSTRING_TAG)
        write(s.io, Int64(len))
    end
    write(s.io, ss)
    nothing
end

function serialize(s::AbstractSerializer, ss::SubString{String})
    # avoid saving a copy of the parent string, keeping the type of ss
    serialize_any(s, SubString(String(ss)))
end

# Don't serialize the pointers
function serialize(s::AbstractSerializer, r::Regex)
    serialize_type(s, typeof(r))
    serialize(s, r.pattern)
    serialize(s, r.compile_options)
    serialize(s, r.match_options)
end

function serialize(s::AbstractSerializer, n::BigInt)
    serialize_type(s, BigInt)
    serialize(s, string(n, base = 62))
end

function serialize(s::AbstractSerializer, ex::Expr)
    serialize_cycle(s, ex) && return
    l = length(ex.args)
    if l <= 255
        writetag(s.io, EXPR_TAG)
        write(s.io, UInt8(l))
    else
        writetag(s.io, LONGEXPR_TAG)
        write(s.io, Int32(l))
    end
    serialize(s, ex.head)
    for a in ex.args
        serialize(s, a)
    end
end

function serialize_dict_data(s::AbstractSerializer, d::AbstractDict)
    write(s.io, Int32(length(d)))
    for (k,v) in d
        serialize(s, k)
        serialize(s, v)
    end
end

function serialize(s::AbstractSerializer, d::Dict)
    serialize_cycle_header(s, d) && return
    serialize_dict_data(s, d)
end

function serialize(s::AbstractSerializer, d::IdDict)
    serialize_cycle(s, d) && return
    writetag(s.io, IDDICT_TAG)
    serialize_type_data(s, typeof(d))
    serialize_dict_data(s, d)
end

function serialize_mod_names(s::AbstractSerializer, m::Module)
    p = parentmodule(m)
    if p === m || m === Base
        key = Base.root_module_key(m)
        serialize(s, key.uuid === nothing ? nothing : key.uuid.value)
        serialize(s, Symbol(key.name))
    else
        serialize_mod_names(s, p)
        serialize(s, nameof(m))
    end
end

function serialize(s::AbstractSerializer, m::Module)
    writetag(s.io, MODULE_TAG)
    serialize_mod_names(s, m)
    writetag(s.io, EMPTYTUPLE_TAG)
end

# TODO: make this bidirectional, so objects can be sent back via the same key
const object_numbers = WeakKeyDict()
const obj_number_salt = Ref{UInt64}(0)
function object_number(s::AbstractSerializer, @nospecialize(l))
    global obj_number_salt, object_numbers
    if haskey(object_numbers, l)
        return object_numbers[l]
    end
    ln = obj_number_salt[]
    object_numbers[l] = ln
    obj_number_salt[] += 1
    return ln::UInt64
end

lookup_object_number(s::AbstractSerializer, n::UInt64) = nothing

remember_object(s::AbstractSerializer, @nospecialize(o), n::UInt64) = nothing

function lookup_object_number(s::Serializer, n::UInt64)
    return get(s.known_object_data, n, nothing)
end

function remember_object(s::Serializer, @nospecialize(o), n::UInt64)
    s.known_object_data[n] = o
    return nothing
end

function serialize(s::AbstractSerializer, meth::Method)
    serialize_cycle(s, meth) && return
    writetag(s.io, METHOD_TAG)
    write(s.io, object_number(s, meth))
    serialize(s, meth.module)
    serialize(s, meth.name)
    serialize(s, meth.file)
    serialize(s, meth.line)
    serialize(s, meth.sig)
    serialize(s, meth.slot_syms)
    serialize(s, meth.nargs)
    serialize(s, meth.isva)
    serialize(s, meth.is_for_opaque_closure)
    serialize(s, meth.constprop)
    serialize(s, meth.purity)
    if isdefined(meth, :source)
        serialize(s, Base._uncompressed_ast(meth, meth.source))
    else
        serialize(s, nothing)
    end
    if isdefined(meth, :generator)
        serialize(s, meth.generator)
    else
        serialize(s, nothing)
    end
    if isdefined(meth, :recursion_relation)
        serialize(s, method.recursion_relation)
    else
        serialize(s, nothing)
    end
    if isdefined(meth, :external_mt)
        error("cannot serialize Method objects with external method tables")
    end
    nothing
end

function serialize(s::AbstractSerializer, linfo::Core.MethodInstance)
    serialize_cycle(s, linfo) && return
    writetag(s.io, METHODINSTANCE_TAG)
    if isdefined(linfo, :uninferred)
        serialize(s, linfo.uninferred)
    else
        writetag(s.io, UNDEFREF_TAG)
    end
    serialize(s, nothing)  # for backwards compat
    serialize(s, linfo.sparam_vals)
    serialize(s, Any)  # for backwards compat
    serialize(s, linfo.specTypes)
    serialize(s, linfo.def)
    nothing
end

function serialize(s::AbstractSerializer, t::Task)
    serialize_cycle(s, t) && return
    if istaskstarted(t) && !istaskdone(t)
        error("cannot serialize a running Task")
    end
    writetag(s.io, TASK_TAG)
    serialize(s, t.code)
    serialize(s, t.storage)
    serialize(s, t.state)
    if t._isexception && (stk = Base.current_exceptions(t); !isempty(stk))
        # the exception stack field is hidden inside the task, so if there
        # is any information there make a CapturedException from it instead.
        # TODO: Handle full exception chain, not just the first one.
        serialize(s, CapturedException(stk[1].exception, stk[1].backtrace))
    else
        serialize(s, t.result)
    end
    serialize(s, t._isexception)
end

function serialize(s::AbstractSerializer, g::GlobalRef)
    if (g.mod === __deserialized_types__ ) ||
        (g.mod === Main && isdefined(g.mod, g.name) && isconst(g.mod, g.name))

        v = getfield(g.mod, g.name)
        unw = unwrap_unionall(v)
        if isa(unw,DataType) && v === unw.name.wrapper && should_send_whole_type(s, unw)
            # handle references to types in Main by sending the whole type.
            # needed to be able to send nested functions (#15451).
            writetag(s.io, FULL_GLOBALREF_TAG)
            serialize(s, v)
            return
        end
    end
    writetag(s.io, GLOBALREF_TAG)
    serialize(s, g.mod)
    serialize(s, g.name)
end

function serialize(s::AbstractSerializer, t::Core.TypeName)
    serialize_cycle(s, t) && return
    writetag(s.io, TYPENAME_TAG)
    write(s.io, object_number(s, t))
    serialize_typename(s, t)
end

function serialize_typename(s::AbstractSerializer, t::Core.TypeName)
    serialize(s, t.name)
    serialize(s, t.names)
    primary = unwrap_unionall(t.wrapper)
    serialize(s, primary.super)
    serialize(s, primary.parameters)
    serialize(s, primary.types)
    serialize(s, isdefined(primary, :instance))
    serialize(s, t.flags & 0x1 == 0x1) # .abstract
    serialize(s, t.flags & 0x2 == 0x2) # .mutable
    serialize(s, Int32(length(primary.types) - t.n_uninitialized))
    if isdefined(t, :mt) && t.mt !== Symbol.name.mt
        serialize(s, t.mt.name)
        serialize(s, collect(Base.MethodList(t.mt)))
        serialize(s, t.mt.max_args)
        if isdefined(t.mt, :kwsorter)
            serialize(s, t.mt.kwsorter)
        else
            writetag(s.io, UNDEFREF_TAG)
        end
    else
        writetag(s.io, UNDEFREF_TAG)
    end
    nothing
end

# decide whether to send all data for a type (instead of just its name)
function should_send_whole_type(s, t::DataType)
    tn = t.name
    if isdefined(tn, :mt)
        # TODO improve somehow
        # send whole type for anonymous functions in Main
        name = tn.mt.name
        mod = tn.module
        isanonfunction = mod === Main && # only Main
            t.super === Function && # only Functions
            unsafe_load(unsafe_convert(Ptr{UInt8}, tn.name)) == UInt8('#') && # hidden type
            (!isdefined(mod, name) || t != typeof(getfield(mod, name))) # XXX: 95% accurate test for this being an inner function
            # TODO: more accurate test? (tn.name !== "#" name)
        #TODO: iskw = startswith(tn.name, "#kw#") && ???
        #TODO: iskw && return send-as-kwftype
        return mod === __deserialized_types__ || isanonfunction
    end
    return false
end

function serialize_type_data(s, @nospecialize(t::DataType))
    whole = should_send_whole_type(s, t)
    iswrapper = (t === unwrap_unionall(t.name.wrapper))
    if whole && iswrapper
        writetag(s.io, WRAPPER_DATATYPE_TAG)
        serialize(s, t.name)
        return
    end
    serialize_cycle(s, t) && return
    if whole
        writetag(s.io, FULL_DATATYPE_TAG)
        serialize(s, t.name)
    else
        writetag(s.io, DATATYPE_TAG)
        tname = t.name.name
        serialize(s, tname)
        mod = t.name.module
        serialize(s, mod)
    end
    if !isempty(t.parameters)
        if iswrapper
            write(s.io, Int32(0))
        else
            write(s.io, Int32(length(t.parameters)))
            for p in t.parameters
                serialize(s, p)
            end
        end
    end
    nothing
end

function serialize(s::AbstractSerializer, t::DataType)
    tag = sertag(t)
    tag > 0 && return write_as_tag(s.io, tag)
    if t === Tuple
        # `sertag` is not able to find types === to `Tuple` because they
        # will not have been hash-consed. Plus `serialize_type_data` does not
        # handle this case correctly, since Tuple{} != Tuple. `Tuple` is the
        # only type with this property. issue #15849
        return write_as_tag(s.io, TUPLE_TAG)
    end
    serialize_type_data(s, t)
end

function serialize_type(s::AbstractSerializer, @nospecialize(t::DataType), ref::Bool = false)
    tag = sertag(t)
    tag > 0 && return writetag(s.io, tag)
    writetag(s.io, ref ? REF_OBJECT_TAG : OBJECT_TAG)
    serialize_type_data(s, t)
end

function serialize(s::AbstractSerializer, n::Int32)
    if 0 <= n <= (n_int_literals-1)
        write(s.io, UInt8(ZERO32_TAG+n))
    else
        writetag(s.io, INT32_TAG)
        write(s.io, n)
    end
    nothing
end

function serialize(s::AbstractSerializer, n::Int64)
    if 0 <= n <= (n_int_literals-1)
        write(s.io, UInt8(ZERO64_TAG+n))
    elseif typemin(Int32) <= n <= typemax(Int32)
        writetag(s.io, SHORTINT64_TAG)
        write(s.io, Int32(n))
    else
        writetag(s.io, INT64_TAG)
        write(s.io, n)
    end
    nothing
end

for i in 0:13
    tag = Int32(INT8_TAG + i)
    ty = TAGS[tag]
    (ty === Int32 || ty === Int64) && continue
    @eval serialize(s::AbstractSerializer, n::$ty) = (writetag(s.io, $tag); write(s.io, n); nothing)
end

serialize(s::AbstractSerializer, ::Type{Bottom}) = write_as_tag(s.io, BOTTOM_TAG)

function serialize(s::AbstractSerializer, u::UnionAll)
    writetag(s.io, UNIONALL_TAG)
    n = 0; t = u
    while isa(t, UnionAll)
        t = t.body
        n += 1
    end
    if isa(t, DataType) && t === unwrap_unionall(t.name.wrapper)
        write(s.io, UInt8(1))
        write(s.io, Int16(n))
        serialize(s, t)
    else
        write(s.io, UInt8(0))
        serialize(s, u.var)
        serialize(s, u.body)
    end
end

serialize(s::AbstractSerializer, @nospecialize(x)) = serialize_any(s, x)

function serialize_any(s::AbstractSerializer, @nospecialize(x))
    tag = sertag(x)
    if tag > 0
        return write_as_tag(s.io, tag)
    end
    t = typeof(x)::DataType
    nf = nfields(x)
    if nf == 0 && t.size > 0
        serialize_type(s, t)
        write(s.io, x)
    else
        if ismutable(x)
            serialize_cycle(s, x) && return
            serialize_type(s, t, true)
        else
            serialize_type(s, t, false)
        end
        for i in 1:nf
            if isdefined(x, i)
                serialize(s, getfield(x, i))
            else
                writetag(s.io, UNDEFREF_TAG)
            end
        end
    end
    nothing
end

"""
    Serialization.writeheader(s::AbstractSerializer)

Write an identifying header to the specified serializer. The header consists of
8 bytes as follows:

| Offset | Description                                     |
|:-------|:------------------------------------------------|
|   0    | tag byte (0x37)                                 |
|   1-2  | signature bytes "JL"                            |
|   3    | protocol version                                |
|   4    | bits 0-1: endianness: 0 = little, 1 = big       |
|   4    | bits 2-3: platform: 0 = 32-bit, 1 = 64-bit      |
|   5-7  | reserved                                        |
"""
function writeheader(s::AbstractSerializer)
    io = s.io
    writetag(io, HEADER_TAG)
    write(io, "JL")  # magic bytes
    write(io, UInt8(ser_version))
    endianness = (ENDIAN_BOM == 0x04030201 ? 0 :
                  ENDIAN_BOM == 0x01020304 ? 1 :
                  error("unsupported endianness in serializer"))
    machine = (sizeof(Int) == 4 ? 0 :
               sizeof(Int) == 8 ? 1 :
               error("unsupported word size in serializer"))
    write(io, UInt8(endianness) | (UInt8(machine) << 2))
    write(io, [0x00,0x00,0x00]) # 3 reserved bytes
    nothing
end

function readheader(s::AbstractSerializer)
    # Tag already read
    io = s.io
    m1 = read(io, UInt8)
    m2 = read(io, UInt8)
    if m1 != UInt8('J') || m2 != UInt8('L')
        error("Unsupported serialization format (got header magic bytes $m1 $m2)")
    end
    version    = read(io, UInt8)
    flags      = read(io, UInt8)
    reserved1  = read(io, UInt8)
    reserved2  = read(io, UInt8)
    reserved3  = read(io, UInt8)
    endianflag = flags & 0x3
    wordflag   = (flags >> 2) & 0x3
    wordsize = wordflag == 0 ? 4 :
               wordflag == 1 ? 8 :
               error("Unknown word size flag in header")
    endian_bom = endianflag == 0 ? 0x04030201 :
                 endianflag == 1 ? 0x01020304 :
                 error("Unknown endianness flag in header")
    # Check protocol compatibility.
    endian_bom == ENDIAN_BOM  || error("Serialized byte order mismatch ($(repr(endian_bom)))")
    # We don't check wordsize == sizeof(Int) here, as Int is encoded concretely
    # as Int32 or Int64, which should be enough to correctly deserialize a range
    # of data structures between Julia versions.
    if version > ser_version
        error("""Cannot read stream serialized with a newer version of Julia.
                 Got data version $version > current version $ser_version""")
    end
    s.version = version
    return
end

"""
    serialize(stream::IO, value)

Write an arbitrary value to a stream in an opaque format, such that it can be read back by
[`deserialize`](@ref). The read-back value will be as identical as possible to the original,
but note that `Ptr` values are serialized as all-zero bit patterns (`NULL`).

An 8-byte identifying header is written to the stream first. To avoid writing the header,
construct a `Serializer` and use it as the first argument to `serialize` instead.
See also [`Serialization.writeheader`](@ref).

The data format can change in minor (1.x) Julia releases, but files written by prior 1.x
versions will remain readable. The main exception to this is when the definition of a
type in an external package changes. If that occurs, it may be necessary to specify
an explicit compatible version of the affected package in your environment.
Renaming functions, even private functions, inside packages can also put existing files
out of sync. Anonymous functions require special care: because their names are automatically
generated, minor code changes can cause them to be renamed.
Serializing anonymous functions should be avoided in files intended for long-term storage.

In some cases, the word size (32- or 64-bit) of the reading and writing machines must match.
In rarer cases the OS or architecture must also match, for example when using packages
that contain platform-dependent code.
"""
function serialize(s::IO, x)
    ss = Serializer(s)
    writeheader(ss)
    serialize(ss, x)
end

"""
    serialize(filename::AbstractString, value)

Open a file and serialize the given value to it.

!!! compat "Julia 1.1"
    This method is available as of Julia 1.1.
"""
serialize(filename::AbstractString, x) = open(io->serialize(io, x), filename, "w")

## deserializing values ##

"""
    deserialize(stream)

Read a value written by [`serialize`](@ref). `deserialize` assumes the binary data read from
`stream` is correct and has been serialized by a compatible implementation of [`serialize`](@ref).
`deserialize` is designed for simplicity and performance, and so does not validate
the data read. Malformed data can result in process termination. The caller must ensure
the integrity and correctness of data read from `stream`.
"""
deserialize(s::IO) = deserialize(Serializer(s))

"""
    deserialize(filename::AbstractString)

Open a file and deserialize its contents.

!!! compat "Julia 1.1"
    This method is available as of Julia 1.1.
"""
deserialize(filename::AbstractString) = open(deserialize, filename)

function deserialize(s::AbstractSerializer)
    handle_deserialize(s, Int32(read(s.io, UInt8)::UInt8))
end

function deserialize_cycle(s::AbstractSerializer, @nospecialize(x))
    slot = pop!(s.pending_refs)
    s.table[slot] = x
    nothing
end

# optimized version of:
#     slot = s.counter; s.counter += 1
#     push!(s.pending_refs, slot)
#     slot = pop!(s.pending_refs)
#     s.table[slot] = x
function resolve_ref_immediately(s::AbstractSerializer, @nospecialize(x))
    s.table[s.counter] = x
    s.counter += 1
    nothing
end

function gettable(s::AbstractSerializer, id::Int)
    get(s.table, id) do
        errmsg = """Inconsistent Serializer state when deserializing.
            Attempt to access internal table with key $id failed.

            This might occur if the Serializer contexts when serializing and deserializing are inconsistent.
            In particular, if multiple serialize calls use the same Serializer object then
            the corresponding deserialize calls should also use the same Serializer object.
        """
        error(errmsg)
    end
end

# deserialize_ is an internal function to dispatch on the tag
# describing the serialized representation. the number of
# representations is fixed, so deserialize_ does not get extended.
function handle_deserialize(s::AbstractSerializer, b::Int32)
    if b == 0
        return desertag(Int32(read(s.io, UInt8)::UInt8))
    end
    if b >= VALUE_TAGS
        return desertag(b)
    elseif b == TUPLE_TAG
        return deserialize_tuple(s, Int(read(s.io, UInt8)::UInt8))
    elseif b == SHORTBACKREF_TAG
        id = read(s.io, UInt16)::UInt16
        return gettable(s, Int(id))
    elseif b == BACKREF_TAG
        id = read(s.io, Int32)::Int32
        return gettable(s, Int(id))
    elseif b == ARRAY_TAG
        return deserialize_array(s)
    elseif b == DATATYPE_TAG
        return deserialize_datatype(s, false)
    elseif b == FULL_DATATYPE_TAG
        return deserialize_datatype(s, true)
    elseif b == WRAPPER_DATATYPE_TAG
        tname = deserialize(s)::Core.TypeName
        return unwrap_unionall(tname.wrapper)
    elseif b == OBJECT_TAG
        t = deserialize(s)
        if t === Missing
            return missing
        end
        return deserialize(s, t)
    elseif b == REF_OBJECT_TAG
        slot = s.counter; s.counter += 1
        push!(s.pending_refs, slot)
        t = deserialize(s)
        return deserialize(s, t)
    elseif b == SHARED_REF_TAG
        slot = s.counter; s.counter += 1
        obj = deserialize(s)
        s.table[slot] = obj
        return obj
    elseif b == SYMBOL_TAG
        return deserialize_symbol(s, Int(read(s.io, UInt8)::UInt8))
    elseif b == SHORTINT64_TAG
        return Int64(read(s.io, Int32)::Int32)
    elseif b == EXPR_TAG
        return deserialize_expr(s, Int(read(s.io, UInt8)::UInt8))
    elseif b == MODULE_TAG
        return deserialize_module(s)
    elseif b == STRING_TAG
        return deserialize_string(s, Int(read(s.io, UInt8)::UInt8))
    elseif b == LONGSTRING_TAG
        return deserialize_string(s, Int(read(s.io, Int64)::Int64))
    elseif b == SIMPLEVECTOR_TAG
        return deserialize_svec(s)
    elseif b == GLOBALREF_TAG
        return GlobalRef(deserialize(s)::Module, deserialize(s)::Symbol)
    elseif b == FULL_GLOBALREF_TAG
        ty = deserialize(s)
        tn = unwrap_unionall(ty).name
        return GlobalRef(tn.module, tn.name)
    elseif b == LONGTUPLE_TAG
        return deserialize_tuple(s, Int(read(s.io, Int32)::Int32))
    elseif b == LONGEXPR_TAG
        return deserialize_expr(s, Int(read(s.io, Int32)::Int32))
    elseif b == LONGBACKREF_TAG
        id = read(s.io, Int64)::Int64
        return gettable(s, Int(id))
    elseif b == LONGSYMBOL_TAG
        return deserialize_symbol(s, Int(read(s.io, Int32)::Int32))
    elseif b == HEADER_TAG
        readheader(s)
        return deserialize(s)
    elseif b == INT8_TAG
        return read(s.io, Int8)
    elseif b == INT8_TAG+1
        return read(s.io, UInt8)
    elseif b == INT8_TAG+2
        return read(s.io, Int16)
    elseif b == INT8_TAG+3
        return read(s.io, UInt16)
    elseif b == INT32_TAG
        return read(s.io, Int32)
    elseif b == INT8_TAG+5
        return read(s.io, UInt32)
    elseif b == INT64_TAG
        return read(s.io, Int64)
    elseif b == INT8_TAG+7
        return read(s.io, UInt64)
    elseif b == INT8_TAG+8
        return read(s.io, Int128)
    elseif b == INT8_TAG+9
        return read(s.io, UInt128)
    elseif b == INT8_TAG+10
        return read(s.io, Float16)
    elseif b == INT8_TAG+11
        return read(s.io, Float32)
    elseif b == INT8_TAG+12
        return read(s.io, Float64)
    elseif b == INT8_TAG+13
        return read(s.io, Char)
    elseif b == IDDICT_TAG
        slot = s.counter; s.counter += 1
        push!(s.pending_refs, slot)
        t = deserialize(s)
        return deserialize_dict(s, t)
    end
    t = desertag(b)::DataType
    if ismutabletype(t) && length(t.types) > 0  # manual specialization of fieldcount
        slot = s.counter; s.counter += 1
        push!(s.pending_refs, slot)
    end
    return deserialize(s, t)
end

function deserialize_symbol(s::AbstractSerializer, len::Int)
    str = Base._string_n(len)
    unsafe_read(s.io, pointer(str), len)
    sym = Symbol(str)
    if len > 7
        resolve_ref_immediately(s, sym)
    end
    return sym
end

deserialize_tuple(s::AbstractSerializer, len) = ntupleany(i->deserialize(s), len)

function deserialize_svec(s::AbstractSerializer)
    n = read(s.io, Int32)
    svec(Any[ deserialize(s) for i=1:n ]...)
end

function deserialize_module(s::AbstractSerializer)
    mkey = deserialize(s)
    if isa(mkey, Tuple)
        # old version, TODO: remove
        if mkey === ()
            return Main
        end
        m = Base.root_module(mkey[1])
        for i = 2:length(mkey)
            m = getfield(m, mkey[i])::Module
        end
    else
        name = String(deserialize(s)::Symbol)
        pkg = (mkey === nothing) ? Base.PkgId(name) : Base.PkgId(Base.UUID(mkey), name)
        m = Base.root_module(pkg)
        mname = deserialize(s)
        while mname !== ()
            m = getfield(m, mname)::Module
            mname = deserialize(s)
        end
    end
    return m
end

function deserialize(s::AbstractSerializer, ::Type{Method})
    lnumber = read(s.io, UInt64)
    meth = lookup_object_number(s, lnumber)
    if meth !== nothing
        meth = meth::Method
        makenew = false
    else
        meth = ccall(:jl_new_method_uninit, Ref{Method}, (Any,), Main)
        makenew = true
    end
    deserialize_cycle(s, meth)
    mod = deserialize(s)::Module
    name = deserialize(s)::Symbol
    file = deserialize(s)::Symbol
    line = deserialize(s)::Int32
    sig = deserialize(s)::Type
    syms = deserialize(s)
    if syms isa SimpleVector
        # < v1.2
        _ambig = deserialize(s)
    else
        slot_syms = syms::String
    end
    nargs = deserialize(s)::Int32
    isva = deserialize(s)::Bool
    is_for_opaque_closure = false
    constprop = 0x00
    purity = 0x00
    template_or_is_opaque = deserialize(s)
    if isa(template_or_is_opaque, Bool)
        is_for_opaque_closure = template_or_is_opaque
        if format_version(s) >= 14
            constprop = deserialize(s)::UInt8
        end
        if format_version(s) >= 17
            purity = deserialize(s)::UInt8
        end
        template = deserialize(s)
    else
        template = template_or_is_opaque
    end
    generator = deserialize(s)
    recursion_relation = nothing
    if format_version(s) >= 15
        recursion_relation = deserialize(s)
    end
    if makenew
        meth.module = mod
        meth.name = name
        meth.file = file
        meth.line = line
        meth.sig = sig
        meth.nargs = nargs
        meth.isva = isva
        meth.is_for_opaque_closure = is_for_opaque_closure
        meth.constprop = constprop
        meth.purity = purity
        if template !== nothing
            # TODO: compress template
            meth.source = template::CodeInfo
            meth.pure = template.pure
            if !@isdefined(slot_syms)
                slot_syms = ccall(:jl_compress_argnames, Ref{String}, (Any,), meth.source.slotnames)
            end
        end
        meth.slot_syms = slot_syms
        if generator !== nothing
            meth.generator = generator
        end
        if recursion_relation !== nothing
            meth.recursion_relation = recursion_relation
        end
        if !is_for_opaque_closure
            mt = ccall(:jl_method_table_for, Any, (Any,), sig)
            if mt !== nothing && nothing === ccall(:jl_methtable_lookup, Any, (Any, Any, UInt), mt, sig, typemax(UInt))
                ccall(:jl_method_table_insert, Cvoid, (Any, Any, Ptr{Cvoid}), mt, meth, C_NULL)
            end
        end
        remember_object(s, meth, lnumber)
    end
    return meth
end

function deserialize(s::AbstractSerializer, ::Type{Core.MethodInstance})
    linfo = ccall(:jl_new_method_instance_uninit, Ref{Core.MethodInstance}, (Ptr{Cvoid},), C_NULL)
    deserialize_cycle(s, linfo)
    tag = Int32(read(s.io, UInt8)::UInt8)
    if tag != UNDEFREF_TAG
        linfo.uninferred = handle_deserialize(s, tag)::CodeInfo
    end
    tag = Int32(read(s.io, UInt8)::UInt8)
    if tag != UNDEFREF_TAG
        # for reading files prior to v1.2
        handle_deserialize(s, tag)
    end
    linfo.sparam_vals = deserialize(s)::SimpleVector
    _rettype = deserialize(s)  # for backwards compat
    linfo.specTypes = deserialize(s)
    linfo.def = deserialize(s)
    return linfo
end

function deserialize(s::AbstractSerializer, ::Type{Core.LineInfoNode})
    mod = deserialize(s)
    if mod isa Module
        method = deserialize(s)
    else
        # files post v1.2 and pre v1.6 are broken
        method = mod
        mod = Main
    end
    return Core.LineInfoNode(mod, method, deserialize(s)::Symbol, deserialize(s)::Int, deserialize(s)::Int)
end

function deserialize(s::AbstractSerializer, ::Type{PhiNode})
    edges = deserialize(s)
    if edges isa Vector{Any}
        edges = Vector{Int32}(edges)
    end
    values = deserialize(s)::Vector{Any}
    return PhiNode(edges, values)
end

function deserialize(s::AbstractSerializer, ::Type{CodeInfo})
    ci = ccall(:jl_new_code_info_uninit, Ref{CodeInfo}, ())
    deserialize_cycle(s, ci)
    code = deserialize(s)::Vector{Any}
    ci.code = code
    # allow older-style IR with return and gotoifnot Exprs
    for i in 1:length(code)
        stmt = code[i]
        if isa(stmt, Expr)
            ex = stmt::Expr
            if ex.head === :return
                code[i] = ReturnNode(isempty(ex.args) ? nothing : ex.args[1])
            elseif ex.head === :gotoifnot
                code[i] = GotoIfNot(ex.args[1], ex.args[2])
            end
        end
    end
    ci.codelocs = deserialize(s)::Vector{Int32}
    _x = deserialize(s)
    if _x isa Array || _x isa Int
        pre_12 = false
        ci.ssavaluetypes = _x
    else
        pre_12 = true
        # < v1.2
        ci.method_for_inference_limit_heuristics = _x
        ci.ssavaluetypes = deserialize(s)
        ci.linetable = deserialize(s)
    end
    ssaflags = deserialize(s)
    if length(ssaflags) ≠ length(code)
        # make sure the length of `ssaflags` matches that of `code`
        # so that the latest inference doesn't throw on IRs serialized from old versions
        ssaflags = UInt8[0x00 for _ in 1:length(code)]
    end
    ci.ssaflags = ssaflags
    if pre_12
        ci.slotflags = deserialize(s)
    else
        ci.method_for_inference_limit_heuristics = deserialize(s)
        ci.linetable = deserialize(s)
    end
    ci.slotnames = deserialize(s)
    if !pre_12
        ci.slotflags = deserialize(s)
        ci.slottypes = deserialize(s)
        ci.rettype = deserialize(s)
        ci.parent = deserialize(s)
        world_or_edges = deserialize(s)
        pre_13 = isa(world_or_edges, Integer)
        if pre_13
            ci.min_world = world_or_edges
        else
            ci.edges = world_or_edges
            ci.min_world = reinterpret(UInt, deserialize(s))
            ci.max_world = reinterpret(UInt, deserialize(s))
        end
    end
    ci.inferred = deserialize(s)
    ci.inlineable = deserialize(s)
    ci.propagate_inbounds = deserialize(s)
    ci.pure = deserialize(s)
    if format_version(s) >= 14
        ci.constprop = deserialize(s)::UInt8
    end
    if format_version(s) >= 17
        ci.purity = deserialize(s)::UInt8
    end
    return ci
end

if Int === Int64
const OtherInt = Int32
else
const OtherInt = Int64
end

function deserialize_array(s::AbstractSerializer)
    slot = s.counter; s.counter += 1
    d1 = deserialize(s)
    if isa(d1, Type)
        elty = d1
        d1 = deserialize(s)
    else
        elty = UInt8
    end
    if isa(d1, Int32) || isa(d1, Int64)
        if elty !== Bool && isbitstype(elty)
            a = Vector{elty}(undef, d1)
            s.table[slot] = a
            return read!(s.io, a)
        end
        dims = (Int(d1),)
    elseif d1 isa Dims
        dims = d1::Dims
    else
        dims = convert(Dims, d1::Tuple{Vararg{OtherInt}})::Dims
    end
    if isbitstype(elty)
        n = prod(dims)::Int
        if elty === Bool && n > 0
            A = Array{Bool, length(dims)}(undef, dims)
            i = 1
            while i <= n
                b = read(s.io, UInt8)::UInt8
                v = (b >> 7) != 0
                count = b & 0x7f
                nxt = i + count
                while i < nxt
                    A[i] = v
                    i += 1
                end
            end
        else
            A = read!(s.io, Array{elty}(undef, dims))
        end
        s.table[slot] = A
        return A
    end
    A = Array{elty, length(dims)}(undef, dims)
    s.table[slot] = A
    sizehint!(s.table, s.counter + div(length(A)::Int,4))
    deserialize_fillarray!(A, s)
    return A
end

function deserialize_fillarray!(A::Array{T}, s::AbstractSerializer) where {T}
    for i = eachindex(A)
        tag = Int32(read(s.io, UInt8)::UInt8)
        if tag != UNDEFREF_TAG
            @inbounds A[i] = handle_deserialize(s, tag)
        end
    end
    return A
end

function deserialize_expr(s::AbstractSerializer, len)
    e = Expr(:temp)
    resolve_ref_immediately(s, e)
    e.head = deserialize(s)::Symbol
    e.args = Any[ deserialize(s) for i = 1:len ]
    e
end

module __deserialized_types__ end

function deserialize(s::AbstractSerializer, ::Type{Core.TypeName})
    number = read(s.io, UInt64)
    return deserialize_typename(s, number)
end

function deserialize_typename(s::AbstractSerializer, number)
    name = deserialize(s)::Symbol
    tn = lookup_object_number(s, number)
    if tn !== nothing
        makenew = false
    else
        # reuse the same name for the type, if possible, for nicer debugging
        tn_name = isdefined(__deserialized_types__, name) ? gensym() : name
        tn = ccall(:jl_new_typename_in, Any, (Any, Any, Cint, Cint),
                   tn_name, __deserialized_types__, false, false)
        makenew = true
    end
    tn = tn::Core.TypeName
    remember_object(s, tn, number)
    deserialize_cycle(s, tn)

    names = deserialize(s)::SimpleVector
    super = deserialize(s)::Type
    parameters = deserialize(s)::SimpleVector
    types = deserialize(s)::SimpleVector
    attrs = Core.svec()
    has_instance = deserialize(s)::Bool
    abstr = deserialize(s)::Bool
    mutabl = deserialize(s)::Bool
    ninitialized = deserialize(s)::Int32

    if makenew
        # TODO: there's an unhanded cycle in the dependency graph at this point:
        # while deserializing super and/or types, we may have encountered
        # tn.wrapper and throw UndefRefException before we get to this point
        ndt = ccall(:jl_new_datatype, Any, (Any, Any, Any, Any, Any, Any, Any, Cint, Cint, Cint),
                    tn, tn.module, super, parameters, names, types, attrs,
                    abstr, mutabl, ninitialized)
        @assert tn == ndt.name
        ccall(:jl_set_const, Cvoid, (Any, Any, Any), tn.module, tn.name, tn.wrapper)
        ty = tn.wrapper
        if has_instance
            ty = ty::DataType
            if !isdefined(ty, :instance)
                singleton = ccall(:jl_new_struct, Any, (Any, Any...), ty)
                # use setfield! directly to avoid `fieldtype` lowering expecting to see a Singleton object already on ty
                ccall(:jl_set_nth_field, Cvoid, (Any, Csize_t, Any), ty, Base.fieldindex(DataType, :instance)-1, singleton)
            end
        end
    end

    tag = Int32(read(s.io, UInt8)::UInt8)
    if tag != UNDEFREF_TAG
        mtname = handle_deserialize(s, tag)
        defs = deserialize(s)
        maxa = deserialize(s)::Int
        if makenew
            mt = ccall(:jl_new_method_table, Any, (Any, Any), name, tn.module)
            if !isempty(parameters)
                mt.offs = 0
            end
            mt.name = mtname
            mt.max_args = maxa
            ccall(:jl_set_nth_field, Cvoid, (Any, Csize_t, Any), tn, Base.fieldindex(Core.TypeName, :mt)-1, mt)
            for def in defs
                if isdefined(def, :sig)
                    ccall(:jl_method_table_insert, Cvoid, (Any, Any, Ptr{Cvoid}), mt, def, C_NULL)
                end
            end
        end
        tag = Int32(read(s.io, UInt8)::UInt8)
        if tag != UNDEFREF_TAG
            kws = handle_deserialize(s, tag)
            if makenew
                tn.mt.kwsorter = kws
            end
        end
    elseif makenew
        mt = Symbol.name.mt
        ccall(:jl_set_nth_field, Cvoid, (Any, Csize_t, Any), tn, Base.fieldindex(Core.TypeName, :mt)-1, mt)
    end
    return tn
end

function deserialize_datatype(s::AbstractSerializer, full::Bool)
    slot = s.counter; s.counter += 1
    if full
        tname = deserialize(s)::Core.TypeName
        ty = tname.wrapper
    else
        name = deserialize(s)::Symbol
        mod = deserialize(s)::Module
        ty = getfield(mod,name)
    end
    if isa(ty,DataType) && isempty(ty.parameters)
        t = ty
    else
        np = Int(read(s.io, Int32)::Int32)
        if np == 0
            t = unwrap_unionall(ty)
        elseif ty === Tuple
            # note np==0 has its own tag
            if np == 1
                t = Tuple{deserialize(s)}
            elseif np == 2
                t = Tuple{deserialize(s), deserialize(s)}
            elseif np == 3
                t = Tuple{deserialize(s), deserialize(s), deserialize(s)}
            elseif np == 4
                t = Tuple{deserialize(s), deserialize(s), deserialize(s), deserialize(s)}
            else
                t = Tuple{Any[ deserialize(s) for i=1:np ]...}
            end
        else
            t = ty
            for i = 1:np
                t = t{deserialize(s)}
            end
        end
    end
    s.table[slot] = t
    return t
end

function deserialize(s::AbstractSerializer, ::Type{UnionAll})
    form = read(s.io, UInt8)
    if form == 0
        var = deserialize(s)
        body = deserialize(s)
        return UnionAll(var, body)
    else
        n = read(s.io, Int16)
        t = deserialize(s)::DataType
        w = t.name.wrapper
        k = 0
        while isa(w, UnionAll)
            w = w.body
            k += 1
        end
        w = t.name.wrapper
        k -= n
        while k > 0
            w = w.body
            k -= 1
        end
        return w
    end
end

function deserialize(s::AbstractSerializer, ::Type{Task})
    t = Task(()->nothing)
    deserialize_cycle(s, t)
    t.code = deserialize(s)
    t.storage = deserialize(s)
    state = deserialize(s)
    if state === :runnable
        t._state = Base.task_state_runnable
    elseif state === :done
        t._state = Base.task_state_done
    elseif state === :failed
        t._state = Base.task_state_failed
    else
        @assert false
    end
    t.result = deserialize(s)
    exc = deserialize(s)
    if exc === nothing
        t._isexception = false
    elseif exc isa Bool
        t._isexception = exc
    else
        t._isexception = true
        t.result = exc
    end
    t
end

function deserialize_string(s::AbstractSerializer, len::Int)
    out = ccall(:jl_alloc_string, Ref{String}, (Csize_t,), len)
    unsafe_read(s.io, pointer(out), len)
    return out
end

# default DataType deserializer
function deserialize(s::AbstractSerializer, t::DataType)
    nf = length(t.types)
    if nf == 0 && t.size > 0
        # bits type
        return read(s.io, t)
    elseif ismutabletype(t)
        x = ccall(:jl_new_struct_uninit, Any, (Any,), t)
        deserialize_cycle(s, x)
        for i in 1:nf
            tag = Int32(read(s.io, UInt8)::UInt8)
            if tag != UNDEFREF_TAG
                ccall(:jl_set_nth_field, Cvoid, (Any, Csize_t, Any), x, i-1, handle_deserialize(s, tag))
            end
        end
        return x
    elseif nf == 0
        return ccall(:jl_new_struct_uninit, Any, (Any,), t)
    else
        na = nf
        vflds = Vector{Any}(undef, nf)
        for i in 1:nf
            tag = Int32(read(s.io, UInt8)::UInt8)
            if tag != UNDEFREF_TAG
                f = handle_deserialize(s, tag)
                na >= i && (vflds[i] = f)
            else
                na >= i && (na = i - 1) # rest of tail must be undefined values
            end
        end
        return ccall(:jl_new_structv, Any, (Any, Ptr{Any}, UInt32), t, vflds, na)
    end
end

function deserialize_dict(s::AbstractSerializer, T::Type{<:AbstractDict})
    n = read(s.io, Int32)
    t = T(); sizehint!(t, n)
    deserialize_cycle(s, t)
    for i = 1:n
        k = deserialize(s)
        v = deserialize(s)
        t[k] = v
    end
    return t
end

function deserialize(s::AbstractSerializer, T::Type{Dict{K,V}}) where {K,V}
    return deserialize_dict(s, T)
end

deserialize(s::AbstractSerializer, ::Type{BigInt}) = parse(BigInt, deserialize(s), base = 62)

function deserialize(s::AbstractSerializer, t::Type{Regex})
    pattern = deserialize(s)
    compile_options = deserialize(s)
    match_options = deserialize(s)
    return Regex(pattern, compile_options, match_options)
end

## StackTraces

# provide a custom serializer that skips attempting to serialize the `outer_linfo`
# which is likely to contain complex references, types, and module references
# that may not exist on the receiver end
function serialize(s::AbstractSerializer, frame::Base.StackTraces.StackFrame)
    serialize_type(s, typeof(frame))
    serialize(s, frame.func)
    serialize(s, frame.file)
    write(s.io, frame.line)
    write(s.io, frame.from_c)
    write(s.io, frame.inlined)
    write(s.io, frame.pointer)
    nothing
end

function deserialize(s::AbstractSerializer, ::Type{Base.StackTraces.StackFrame})
    func = deserialize(s)
    file = deserialize(s)
    line = read(s.io, Int)
    from_c = read(s.io, Bool)
    inlined = read(s.io, Bool)
    pointer = read(s.io, UInt64)
    return Base.StackTraces.StackFrame(func, file, line, nothing, from_c, inlined, pointer)
end

function serialize(s::AbstractSerializer, lock::Base.AbstractLock)
    # assert_havelock(lock)
    serialize_cycle_header(s, lock)
    nothing
end

function deserialize(s::AbstractSerializer, ::Type{T}) where T<:Base.AbstractLock
    lock = T()
    deserialize_cycle(s, lock)
    return lock
end

function serialize(s::AbstractSerializer, cond::Base.GenericCondition)
    serialize_cycle_header(s, cond) && return
    serialize(s, cond.lock)
    nothing
end

function deserialize(s::AbstractSerializer, ::Type{T}) where T<:Base.GenericCondition
    lock = deserialize(s)
    cond = T(lock)
    deserialize_cycle(s, cond)
    return cond
end


end
