add header to serialization format. fixes #22888

base/serialize.jl

@@ -24,7 +24,8 @@ SerializationState(io::IO) = SerializationState{typeof(io)}(io)
 # types AbstractSerializer and Serializer  # defined in dict.jl
 
 const n_int_literals = 33
-const n_reserved_slots = 12
+const n_reserved_slots = 25
+const n_reserved_tags = 12
 
 const TAGS = Any[
     Symbol, Int8, UInt8, Int16, UInt16, Int32, UInt32, Int64, UInt64, Int128, UInt128,
@@ -48,6 +49,8 @@ const TAGS = Any[
     Symbol, # OBJECT_TAG
     Symbol, # REF_OBJECT_TAG
     Symbol, # FULL_GLOBALREF_TAG
+    Symbol, # HEADER_TAG
+    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,
@@ -64,9 +67,9 @@ const TAGS = Any[
     (Int64(0):Int64(n_int_literals-1))...
 ]
 
-@assert length(TAGS) <= 255
+@assert length(TAGS) == 255
 
-const ser_version = 6 # do not make changes without bumping the version #!
+const ser_version = 7 # do not make changes without bumping the version #!
 
 const NTAGS = length(TAGS)
 
@@ -120,6 +123,7 @@ 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)
 
 writetag(s::IO, tag) = write(s, UInt8(tag))
 
@@ -638,15 +642,54 @@ function serialize_any(s::AbstractSerializer, @nospecialize(x))
 end
 
 """
-    serialize(stream, value)
+    Serializer.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, b"\x00\x00\x00")  # 3 reserved bytes
+    nothing
+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. In
-general, this process will not work if the reading and writing are done by different
+[`deserialize`](@ref). The read-back value will be as identical as possible to the original.
+In general, this process will not work if the reading and writing are done by different
 versions of Julia, or an instance of Julia with a different system image. `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 `SerializationState` and use it as the first argument to `serialize` instead.
+See also [`Serializer.writeheader`](@ref).
 """
-serialize(s::IO, x) = serialize(SerializationState(s), x)
+function serialize(s::IO, x)
+    ss = SerializationState(s)
+    writeheader(ss)
+    serialize(ss, x)
+end
 
 ## deserializing values ##
 
@@ -745,6 +788,11 @@ function handle_deserialize(s::AbstractSerializer, b::Int32)
         return s.table[Int(id)]
     elseif b == LONGSYMBOL_TAG
         return deserialize_symbol(s, Int(read(s.io, Int32)::Int32))
+    elseif b == HEADER_TAG
+        for _ = 1:7
+            read(s.io, UInt8)
+        end
+        return deserialize(s)
     end
     t = desertag(b)
     if t.mutable && length(t.types) > 0  # manual specialization of fieldcount

doc/src/stdlib/io-network.md

@@ -35,6 +35,7 @@ Base.isreadable
 Base.isopen
 Base.Serializer.serialize
 Base.Serializer.deserialize
+Base.Serializer.writeheader
 Base.Grisu.print_shortest
 Base.fd
 Base.redirect_stdout

test/serialize.jl

@@ -4,8 +4,8 @@ using Base.Test
 
 # Check that serializer hasn't gone out-of-frame
 @test Serializer.sertag(Symbol) == 1
-@test Serializer.sertag(()) == 55
-@test Serializer.sertag(false) == 63
+@test Serializer.sertag(()) == 68
+@test Serializer.sertag(false) == 76
 
 function create_serialization_stream(f::Function)
     s = IOBuffer()
@@ -480,3 +480,17 @@ let x = T20324[T20324(1) for i = 1:2]
     @test isa(y,Vector{T20324})
     @test y == x
 end
+
+# serializer header
+let io = IOBuffer()
+    serialize(io, ())
+    seekstart(io)
+    b = read(io)
+    @test b[1] == Serializer.HEADER_TAG
+    @test b[2:3] == b"JL"
+    @test b[4] == Serializer.ser_version
+    @test (b[5] & 0x3) == (ENDIAN_BOM == 0x01020304)
+    @test ((b[5] & 0xc)>>2) == (sizeof(Int) == 8)
+    @test (b[5] & 0xf0) == 0
+    @test all(b[6:8] .== 0)
+end