added system.onThreadCreation feature for safe thread local storage initializations

lib/system/threads.nim

@@ -307,6 +307,35 @@ type
                                        ## a pointer as a thread ID.
 {.deprecated: [TThread: Thread, TThreadId: ThreadId].}
 
+var
+  threadCreationHandlers: array[60, proc () {.nimcall, gcsafe.}]
+  countThreadCreationHandlers: int
+
+proc onThreadCreation*(handler: proc () {.nimcall, gcsafe.}) =
+  ## Registers a global handler that is called at thread creation.
+  ## This can be used to initialize thread local variables properly.
+  ## Note that the handler has to be .gcafe and so the typical usage
+  ## looks like:
+  ##
+  ## .. code-block:: nim
+  ##
+  ##  var
+  ##    someGlobal: string = "some string here"
+  ##    perThread {.threadvar.}: string
+  ##
+  ##  proc setPerThread() =
+  ##    {.gcsafe.}:
+  ##      deepCopy(perThread, someGlobal)
+  ##
+  ## **Note**: The registration is currently not threadsafe! Better
+  ## call ``onThreadCreation`` before any thread started its work!
+  threadCreationHandlers[countThreadCreationHandlers] = handler
+  inc countThreadCreationHandlers
+
+template beforeThreadRuns() =
+  for i in 0..countThreadCreationHandlers-1:
+    threadCreationHandlers[i]()
+
 when not defined(boehmgc) and not hasSharedHeap and not defined(gogc) and not defined(gcstack):
   proc deallocOsPages()
 
@@ -321,6 +350,7 @@ when defined(boehmgc):
 
   proc threadProcWrapDispatch[TArg](sb: pointer, thrd: pointer) {.noconv.} =
     boehmGC_register_my_thread(sb)
+    beforeThreadRuns()
     let thrd = cast[ptr Thread[TArg]](thrd)
     when TArg is void:
       thrd.dataFn()
@@ -329,6 +359,7 @@ when defined(boehmgc):
     boehmGC_unregister_my_thread()
 else:
   proc threadProcWrapDispatch[TArg](thrd: ptr Thread[TArg]) =
+    beforeThreadRuns()
     when TArg is void:
       thrd.dataFn()
     else:

tests/testament/categories.nim

@@ -200,6 +200,7 @@ proc threadTests(r: var TResults, cat: Category, options: string) =
   test "tthreadanalysis2"
   #test "tthreadanalysis3"
   test "tthreadheapviolation1"
+  test "tonthreadcreation"
 
 # ------------------------- IO tests ------------------------------------------
 

tests/threads/tonthreadcreation.nim

@@ -0,0 +1,22 @@
+discard """
+  output: '''some string here'''
+"""
+
+var
+  someGlobal: string = "some string here"
+  perThread {.threadvar.}: string
+
+proc setPerThread() =
+  {.gcsafe.}:
+    deepCopy(perThread, someGlobal)
+
+proc foo() {.thread.} =
+  echo perThread
+
+proc main =
+  onThreadCreation setPerThread
+  var t: Thread[void]
+  createThread[void](t, foo)
+  t.joinThread()
+
+main()