Introduce a recursive cycle check when writing (#345)

I noticed that writing can blow up when there are recusrive objects that reference each other.
(For example, in HTTP.jl, `Response` and `Request` reference each other)
This PR proposes a simple API for the `CompactContext` and `PrettyContext` where
`objectid` of objects will be tracked recursively when writing and it's configurable
what should be written out when a recursive cycle is detected.

Custom contexts can "hook in" to this behavior by subtyping `RecursiveCheckContext` and
including the required fields (see docs for new context). Otherwise, there shouldn't
be any functional change to APIs in any way.

src/Writer.jl

@@ -83,27 +83,43 @@ abstract type JSONContext <: StructuralContext end
 """
 Internal implementation detail.
 
+To handle recursive references in objects/arrays when writing, by default we want
+to track references to objects seen and break recursion cycles to avoid stack overflows.
+Subtypes of `RecursiveCheckContext` must include two fields in order to allow recursive
+cycle checking to work properly when writing:
+  * `objectids::Set{UInt64}`: set of object ids in the current stack of objects being written
+  * `recursive_cycle_token::Any`: Any string, `nothing`, or object to be written when a cycle is detected
+"""
+abstract type RecursiveCheckContext <: JSONContext end
+
+"""
+Internal implementation detail.
+
 Keeps track of the current location in the array or object, which winds and
 unwinds during serialization.
 """
-mutable struct PrettyContext{T<:IO} <: JSONContext
+mutable struct PrettyContext{T<:IO} <: RecursiveCheckContext
     io::T
     step::Int     # number of spaces to step
     state::Int    # number of steps at present
     first::Bool   # whether an object/array was just started
+    objectids::Set{UInt64}
+    recursive_cycle_token
 end
-PrettyContext(io::IO, step) = PrettyContext(io, step, 0, false)
+PrettyContext(io::IO, step, recursive_cycle_token=nothing) = PrettyContext(io, step, 0, false, Set{UInt64}(), recursive_cycle_token)
 
 """
 Internal implementation detail.
 
 For compact printing, which in JSON is fully recursive.
 """
-mutable struct CompactContext{T<:IO} <: JSONContext
+mutable struct CompactContext{T<:IO} <: RecursiveCheckContext
     io::T
     first::Bool
+    objectids::Set{UInt64}
+    recursive_cycle_token
 end
-CompactContext(io::IO) = CompactContext(io, false)
+CompactContext(io::IO, recursive_cycle_token=nothing) = CompactContext(io, false, Set{UInt64}(), recursive_cycle_token)
 
 """
 Internal implementation detail.
@@ -265,12 +281,26 @@ end
 show_json(io::SC, ::CS, ::Nothing) = show_null(io)
 show_json(io::SC, ::CS, ::Missing) = show_null(io)
 
+recursive_cycle_check(f, io, s, id) = f()
+
+function recursive_cycle_check(f, io::RecursiveCheckContext, s, id)
+    if id in io.objectids
+        show_json(io, s, io.recursive_cycle_token)
+    else
+        push!(io.objectids, id)
+        f()
+        delete!(io.objectids, id)
+    end
+end
+
 function show_json(io::SC, s::CS, x::Union{AbstractDict, NamedTuple})
-    begin_object(io)
-    for kv in pairs(x)
-        show_pair(io, s, kv)
+    recursive_cycle_check(io, s, objectid(x)) do
+        begin_object(io)
+        for kv in pairs(x)
+            show_pair(io, s, kv)
+        end
+        end_object(io)
     end
-    end_object(io)
 end
 
 function show_json(io::SC, s::CS, kv::Pair)
@@ -280,19 +310,23 @@ function show_json(io::SC, s::CS, kv::Pair)
 end
 
 function show_json(io::SC, s::CS, x::CompositeTypeWrapper)
-    begin_object(io)
-    for fn in x.fns
-        show_pair(io, s, fn, getproperty(x.wrapped, fn))
+    recursive_cycle_check(io, s, objectid(x.wrapped)) do
+        begin_object(io)
+        for fn in x.fns
+            show_pair(io, s, fn, getproperty(x.wrapped, fn))
+        end
+        end_object(io)
     end
-    end_object(io)
 end
 
 function show_json(io::SC, s::CS, x::Union{AbstractVector, Tuple})
-    begin_array(io)
-    for elt in x
-        show_element(io, s, elt)
+    recursive_cycle_check(io, s, objectid(x)) do
+        begin_array(io)
+        for elt in x
+            show_element(io, s, elt)
+        end
+        end_array(io)
     end
-    end_array(io)
 end
 
 """

test/runtests.jl

@@ -85,4 +85,36 @@ end
     end
 end
 
+mutable struct R1
+    id::Int
+    obj
+end
+
+struct MyCustomWriteContext <: JSON.Writer.RecursiveCheckContext
+    io
+    objectids::Set{UInt64}
+    recursive_cycle_token
+end
+MyCustomWriteContext(io) = MyCustomWriteContext(io, Set{UInt64}(), nothing)
+Base.print(io::MyCustomWriteContext, x::UInt8) = Base.print(io.io, x)
+for delegate in [:indent,
+    :delimit,
+    :separate,
+    :begin_array,
+    :end_array,
+    :begin_object,
+    :end_object]
+@eval JSON.Writer.$delegate(io::MyCustomWriteContext) = JSON.Writer.$delegate(io.io)
+end
+
+@testset "RecursiveCheckContext" begin
+    x = R1(1, nothing)
+    x.obj = x
+    str = JSON.json(x)
+    @test str == "{\"id\":1,\"obj\":null}"
+    io = IOBuffer()
+    str = JSON.show_json(MyCustomWriteContext(JSON.Writer.CompactContext(io)), JSON.Serializations.StandardSerialization(), x)
+    @test String(take!(io)) == "{\"id\":1,\"obj\":null}"
+end
+
 # Check that printing to the default stdout doesn't fail