marshal regression with refs that alias the same object

[dylanagreen]

Example

import marshal
import streams

type
  Obj = ref object
    i: int
    b: bool

let
  strm = newStringStream()

var
  o = Obj(i: 1, b: false)
  t1 = @[o, o]
  t2: seq[Obj]

doAssert t1[0] == t1[1]

strm.store(t1)
strm.setPosition(0)
strm.load(t2)
strm.close()

doAssert t2[0] == t2[1]

Current Output

src/example.nim(21) example
.choosenim/toolchains/nim-#devel/lib/pure/marshal.nim(279) load
.choosenim/toolchains/nim-#devel/lib/pure/marshal.nim(266) loadAny
.choosenim/toolchains/nim-#devel/lib/pure/marshal.nim(186) loadAny
.choosenim/toolchains/nim-#devel/lib/pure/marshal.nim(204) loadAny
.choosenim/toolchains/nim-#devel/lib/core/typeinfo.nim(274) setPointer
.choosenim/toolchains/nim-#devel/lib/system/assign.nim(111) genericAssign
.choosenim/toolchains/nim-#devel/lib/system/assign.nim(106) genericAssignAux
.choosenim/toolchains/nim-#devel/lib/system/gc.nim(251) unsureAsgnRef
.choosenim/toolchains/nim-#devel/lib/system/gc.nim(184) incRef
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Expected Output

Nothing.

Additional Information

If o = Obj(i: 1, b: false) is replaced with o = Obj(), the above segfault will not occur. Instead, the marshaling will appear to succeed, but the second assertion will fail. The example code will run without issues if Obj is an object rather than a ref object. This may be related to issue #15934.

This code runs without issue in 1.2.6, but fails in 1.2.8 and devel.

git bisect on the version-1-2 branch indicates that the first bad commit is 2624de0

Output of nim -v for above stacktrace:

Nim Compiler Version 1.5.1 [MacOSX: amd64]
Compiled at 2020-12-22
Copyright (c) 2006-2020 by Andreas Rumpf

git hash: 297c8e403d110dd872e070563328f4e0c734cd01