[GC] Illegal storage access when collecting cycle

[arnetheduck]

The following causes a crash:

$ nim --version
Nim Compiler Version 0.15.0 (2016-10-03) [Linux: amd64]
Copyright (c) 2006-2016 by Andreas Rumpf

git hash: 6e1313515df4ee5808c77af5aeacfa77fce42474
$ nim c test ; ./test
Traceback (most recent call last)
test.nim(19)           test
system.nim(347)          prepareDealloc
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

$ nim c -d:useSysAssert test ; ./test
[SYSASSERT] CollectZCT: isAllocatedPtr

test.nim:

type
  PNode = ref TNode
  TNode = object
    le: PNode

proc finalizeNode(n: PNode) =
  var s = @[0]

proc returnTree() =
  var cycle: PNode
  new(cycle, finalizeNode)
  cycle.le = cycle

for i in 1..100:
  returnTree()

GC_fullCollect()
GC_fullCollect()

[yglukhov]

+1. Sometimes observed similar issue when used echo in a finalizer. Looks like anything that allocates inside finalizer may potentially crash the GC.

[genotrance]

Still crashes with latest devel.

[ghost]

Same behaviour on latest devel:

~/P/stuff ❯❯❯ nimq c seq.nim && ./seq
Traceback (most recent call last)
/home/dian/Projects/stuff/seq.nim(18) seq
/home/dian/Things/Nim/lib/system/gc_common.nim(417) prepareDealloc
SIGSEGV: Illegal storage access. (Attempt to read from nil?)

~/P/stuff ❯❯❯ nimq c -d:useSysAssert seq.nim && ./seq
[SYSASSERT] CollectZCT: isAllocatedPtr

Although seems to work just fine with --gc:orc :)

~/P/stuff ❯❯❯ nimq c --gc:orc -d:useMalloc seq.nim
~/P/stuff ❯❯❯ valgrind --leak-check=full ./seq
==23485== Memcheck, a memory error detector
==23485== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==23485== Using Valgrind-3.16.0.GIT and LibVEX; rerun with -h for copyright info
==23485== Command: ./seq
==23485== 
==23485== 
==23485== HEAP SUMMARY:
==23485==     in use at exit: 0 bytes in 0 blocks
==23485==   total heap usage: 203 allocs, 203 frees, 1,689,952 bytes allocated
==23485== 
==23485== All heap blocks were freed -- no leaks are possible
==23485== 
==23485== For lists of detected and suppressed errors, rerun with: -s
==23485== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)