Fix the logic for scanning tasks in the Julia GC.

This fixes the test that tries to avoid scanning the stack of the
current thread twice.

src/julia_gc.c

@@ -199,6 +199,7 @@ static jl_datatype_t * datatype_largebag;
 static UInt            StackAlignBags;
 static Bag *           GapStackBottom;
 static jl_ptls_t       JuliaTLS, SaveTLS;
+static jl_task_t *     MainTask;
 static size_t          max_pool_obj_size;
 static UInt            YoungRef;
 static int             FullGC;
@@ -583,6 +584,9 @@ static void GapRootScanner(int full)
     // current_task != root_task.
     char * stackend = (char *)jl_task_stack_buffer(task, &size, &tid);
     stackend += size;
+    // We record the main task so that GapTaskScanner() will not scan
+    // it again.
+    MainTask = task;
     // The following test overrides the stackend if the following two
     // conditions hold:
     //
@@ -646,7 +650,10 @@ static void GapTaskScanner(jl_task_t * task, int root_task)
         if (tag->bits.gc & 2)
             rescan = 0;
     }
-    if (stack && tid < 0) {
+    // We only scan the task stack if there is a valid task stack and
+    // if the task is not the one that we already scanned in
+    // GapRootScanner().
+    if (stack && task != MainTask) {
         if (task->copy_stack) {
             // We know which part of the task stack is actually used,
             // so we shorten the range we have to scan.