fixes #14370 (#14371)

changelog.md

@@ -90,7 +90,7 @@
   hangs if a process had both reads from stdin and writes (eg to stdout).
 
 - The callback that is passed to `system.onThreadDestruction` must now be `.raises: []`.
-
+- The callback that is assigned to `system.onUnhandledException` must now be `.gcsafe`.
 
 - `osproc.execCmdEx` now takes an optional `input` for stdin.
 - `osproc.execCmdEx` now takes an optional `input` for stdin, `workingDir` and `env`

lib/system/excpt.nim

@@ -345,7 +345,7 @@ else:
   proc stackTraceAvailable*(): bool = result = false
 
 var onUnhandledException*: (proc (errorMsg: string) {.
-  nimcall.}) ## Set this error \
+  nimcall, gcsafe.}) ## Set this error \
   ## handler to override the existing behaviour on an unhandled exception.
   ##
   ## The default is to write a stacktrace to ``stderr`` and then call ``quit(1)``.

lib/system/threads.nim

@@ -107,7 +107,7 @@ template afterThreadRuns() =
 when not defined(boehmgc) and not hasSharedHeap and not defined(gogc) and not defined(gcRegions):
   proc deallocOsPages() {.rtl, raises: [].}
 
-proc threadTrouble() {.raises: [].}
+proc threadTrouble() {.raises: [], gcsafe.}
   ## defined in system/excpt.nim
 
 when defined(boehmgc):