Boehm GC does not scan thread-local storage

[stefantalpalaru]

That's how you get surprise deallocations for targets of pointers that are only in threadvars.

https://www.hboehm.info/gc/gcinterface.html :

"Thread users should also be aware that on many platforms objects reachable only from thread-local variables may be prematurely reclaimed. Thus objects pointed to by thread-local variables should also be pointed to by a globally visible data structure."

gc-8.0.4/README.QUICK :

"The collector does not guarantee to scan thread-local storage (e.g. of the kind accessed with pthread_getspecific()). The collector does scan thread stacks though, so generally the best solution is to ensure that any pointers stored in thread-local storage are also stored on the thread's stack for the duration of their lifetime."

---

I found that out the hard way (Valgrind output):

==20803== Jump to the invalid address stated on the next line
==20803==    at 0x10: ???
==20803==    by 0x2C38DF: zonedTimeFromTime__zT9bMgAwfe1o0G71u2tO24Q (times.nim:1064)
==20803==    by 0x2C38DF: inZone__o59cT0ssqq4BC09cuUGSdJiA (times.nim:1093)
==20803==    by 0x2C38DF: local__JDUz4bfwLSZvJxYV9aiNG9bg (times.nim:1235)
==20803==    by 0x2C38DF: now__YmAcAi9bz5G8OQqVFWHN2uA (times.nim:1263)
==20803==    by 0x38F0FE: rfcTimestamp__i2AqQvvKFsZl0RRd5ysaHA (log_output.nim:426)
==20803==    by 0x38F0FE: initLogRecord__AdFvJuSUXMySj9bot5WYdCQ (log_output.nim:438)
==20803==    by 0x58C8DF: generateDeposits__u9cJTyqoU2khEGAmzCsRniA (chronicles.nim:305)
==20803==    by 0x5AB7D3: main__kYRn29bPOsjfiAecYiv9b2ng (beacon_node.nim:1007)
==20803==    by 0x5B56C1: NimMain (mainchain_monitor.nim:490)
==20803==    by 0x12AD8C: main (mainchain_monitor.nim:497)
==20803==  Address 0x10 is not stack'd, malloc'd or (recently) free'd
==20803== 

lib/pure/times.nim:

var localInstance {.threadvar.}: Timezone

And Timezone is a ref object:

  Timezone* = ref object ## \
      ## Timezone interface for supporting `DateTime <#DateTime>`_\s of arbitrary
      ## timezones. The ``times`` module only supplies implementations for the
      ## systems local time and UTC.
    zonedTimeFromTimeImpl: proc (x: Time): ZonedTime
        {.tags: [], raises: [], benign.}
    zonedTimeFromAdjTimeImpl: proc (x: Time): ZonedTime
        {.tags: [], raises: [], benign.}
    name: string