Various updates for the Julia integration

1. test against Julia 1.4.2 instead of 1.2.0
2. fix compiler constness warnings in weakptr.c
3. use `jl_threadid()` and `jl_get_current_task()` helpers to avoid access to
   `JuliaTLS` members

Point 3 is important because the layout of the `jl_ptls_t` struct  keeps
changing between Julia versions, meaning that one has to recompile GAP when
updating Julia. With this patch, we avoid accessing the `tid` and `current_task`
members.

However, we still access `root_task` and `safe_restore`. More work will be needed
to deal with those.

etc/ci-prepare.sh

@@ -30,7 +30,7 @@ fi
 if [[ $JULIA = yes ]]
 then
   pushd extern
-  wget https://julialang-s3.julialang.org/bin/linux/x64/1.2/julia-1.2.0-linux-x86_64.tar.gz
+  wget https://julialang-s3.julialang.org/bin/linux/x64/1.4/julia-1.4.2-linux-x86_64.tar.gz
   tar xvf julia-*.tar.gz
   rm julia-*.tar.gz
   cd julia-*

src/julia_gc.c

@@ -37,6 +37,9 @@
 #include <julia.h>
 #include <julia_gcext.h>
 
+// import jl_get_current_task from julia_internal.h, which unfortunately
+// isn't installed as part of a typical Julia installation
+JL_DLLEXPORT jl_value_t *jl_get_current_task(void);
 
 /****************************************************************************
 **
@@ -554,9 +557,9 @@ static void GapRootScanner(int full)
     // Module is a valid Julia object at this point, so further checks
     // in JMark() can be skipped.
     jl_gc_mark_queue_obj(JuliaTLS, (jl_value_t *)Module);
-    jl_task_t * task = JuliaTLS->current_task;
+    jl_task_t * task = (jl_task_t *)jl_get_current_task();
     size_t      size;
-    int         tid;
+    int         tid;    // unused
     // We figure out the end of the stack from the current task. While
     // `stack_bottom` is passed to InitBags(), we cannot use that if
     // current_task != root_task.
@@ -567,13 +570,13 @@ static void GapRootScanner(int full)
     //
     // 1. GAP is not being used as a library, but is the main program
     //    and in charge of the main() function.
-    // 2. The stack of the current task is that of the main task of the
-    //    main thread.
+    // 2. The stack of the current task is that of the root task of the
+    //    main thread (which has thread id 0).
     //
     // The reason is that if Julia is being initialized from GAP, it
     // cannot always reliably find the top of the stack for that task,
     // so we have to fall back to GAP for that.
-    if (!IsUsingLibGap() && JuliaTLS->tid == 0 &&
+    if (!IsUsingLibGap() && jl_threadid() == 0 &&
         JuliaTLS->root_task == task) {
         stackend = (char *)GapStackBottom;
     }
@@ -607,7 +610,7 @@ static void GapTaskScanner(jl_task_t * task, int root_task)
     char * stack = (char *)jl_task_stack_buffer(task, &size, &tid);
     // If it is the current task, it has been scanned by GapRootScanner()
     // already.
-    if (task == JuliaTLS->current_task)
+    if (task == (jl_task_t *)jl_get_current_task())
         return;
     int rescan = 1;
     if (!FullGC) {