show mark-queue on GC critical error

[d-netto]

Re-adds the capability of showing the mark-queue on a GC critical error.

Example:

a = Ref(1)
ptr = pointer_from_objref(a) - 8
unsafe_store!(Ptr{Int}(ptr), ptr)
GC.gc()

produces:

GC error (probable corruption)
Allocations: 15137 (Pool: 15119; Big: 18); GC: 0
Core.ReturnNode(val=SSAValue(1))

thread 0 ptr queue:
~~~~~~~~~~ ptr queue top ~~~~~~~~~~
...
Core.Binding(value=#<null>, globalref=Main.:(+), owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.println, owner=Core.Binding(value=Base.println, globalref=Base.println, owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.print, owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.stdout, owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.:(=>), owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.IOContext, owner=Core.Binding(value=Base.IOContext{IO_t} where IO_t<:IO, globalref=Base.IOContext, owner=<circular reference @-1>, ty=Any, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.:(/), owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.show, owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.DEPOT_PATH, owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.Sys, owner=Core.Binding(value=Base.Sys, globalref=Base.Sys, owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.Int64, owner=#<null>, ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.:(@warn), owner=Core.Binding(value=Base.CoreLogging.var"@warn", globalref=Base.CoreLogging.:(@warn), owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.Threads, owner=Core.Binding(value=Base.Threads, globalref=Base.Threads, owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.:(!=), owner=Core.Binding(value=Base.:(!=), globalref=Base.:(!=), owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.:(!==), owner=Core.Binding(value=Base.:(!==), globalref=Base.:(!==), owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.IOBuffer, owner=Core.Binding(value=Base.GenericIOBuffer{Array{UInt8, 1}}, globalref=Base.IOBuffer, owner=<circular reference @-1>, ty=Any, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=#<null>, globalref=Main.time_ns, owner=Core.Binding(value=Base.time_ns, globalref=Base.time_ns, owner=<circular reference @-1>, ty=#<null>, flags=0x03), ty=#<null>, flags=0x00)
==========
Core.Binding(value=Main.var"#1#2"{pre_output_time} where pre_output_time, globalref=Main.:(#1#2), owner=<circular reference @-1>, ty=Any, flags=0x01)
==========
Core.Binding(value=#<null>, globalref=Main.Ref, owner=Core.Binding(value=Ref{T} where T, globalref=Core.Ref, owner=<circular reference @-1>, ty=Any, flags=0x03), ty=#<null>, flags=0x00)
==========
~~~~~~~~~~ ptr queue bottom ~~~~~~~~~~

[vtjnash]

The implementation seems fine. Though I think this is never actually a stack now of the parent path (as it once was), so I don't know if this still comes in useful?

[d-netto]

It's no longer a path from parent to corrupted object.

Having some information about siblings of corrupted object would be better than nothing IMO.