add an internal function to replace Main, use it in sysimg.jl

make the Core.Main binding non-constant, so uses of "Main" in general
refer to the *current* Main. Doing "import Main" will give you a
constant reference to the Main at that time.

fixes #5802

related to #1195

base/boot.jl

@@ -112,7 +112,6 @@
 #     runnable::Bool
 # end
 
-import Main
 import Core.Intrinsics.ccall
 
 export

base/sysimg.jl

@@ -1,3 +1,6 @@
+import Core.Intrinsics.ccall
+ccall(:jl_new_main_module, Any, ())
+
 baremodule Base
 
 eval(x) = Core.eval(Base,x)
@@ -247,4 +250,7 @@ end
 
 end # baremodule Base
 
+using Base
+importall Base.Operators
+
 Base.isfile("userimg.jl") && Base.include("userimg.jl")

src/dump.c

@@ -1022,6 +1022,7 @@ void jl_restore_system_image(char *fname)
     jl_array_type->env = jl_deserialize_value(&f);
 
     jl_main_module = (jl_module_t*)jl_deserialize_value(&f);
+    jl_internal_main_module = jl_main_module;
     jl_core_module = (jl_module_t*)jl_get_global(jl_main_module,
                                                  jl_symbol("Core"));
     jl_base_module = (jl_module_t*)jl_get_global(jl_main_module,

src/gc.c

@@ -813,6 +813,7 @@ static void gc_mark(void)
 
     // modules
     gc_push_root(jl_main_module, 0);
+    gc_push_root(jl_internal_main_module, 0);
     gc_push_root(jl_current_module, 0);
     if (jl_old_base_module) gc_push_root(jl_old_base_module, 0);
 

src/init.c

@@ -690,14 +690,13 @@ void julia_init(char *imageFile)
     jl_init_serializer();
 
     if (!imageFile) {
-        jl_main_module = jl_new_module(jl_symbol("Main"));
-        jl_main_module->parent = jl_main_module;
         jl_core_module = jl_new_module(jl_symbol("Core"));
-        jl_core_module->parent = jl_main_module;
-        jl_set_const(jl_main_module, jl_symbol("Core"),
-                     (jl_value_t*)jl_core_module);
-        jl_module_using(jl_main_module, jl_core_module);
+        jl_new_main_module();
+        jl_internal_main_module = jl_main_module;
+
         jl_current_module = jl_core_module;
+        jl_root_task->current_module = jl_current_module;
+
         jl_init_intrinsic_functions();
         jl_init_primitives();
         jl_load("boot.jl");

src/julia.h

@@ -766,6 +766,7 @@ DLLEXPORT jl_value_t *jl_apply_array_type(jl_datatype_t *type, size_t dim);
 
 // modules and global variables
 extern DLLEXPORT jl_module_t *jl_main_module;
+extern DLLEXPORT jl_module_t *jl_internal_main_module;
 extern DLLEXPORT jl_module_t *jl_core_module;
 extern DLLEXPORT jl_module_t *jl_base_module;
 extern DLLEXPORT jl_module_t *jl_current_module;
@@ -789,6 +790,7 @@ DLLEXPORT void jl_module_use(jl_module_t *to, jl_module_t *from, jl_sym_t *s);
 DLLEXPORT void jl_module_import(jl_module_t *to, jl_module_t *from, jl_sym_t *s);
 DLLEXPORT void jl_module_importall(jl_module_t *to, jl_module_t *from);
 DLLEXPORT void jl_module_export(jl_module_t *from, jl_sym_t *s);
+DLLEXPORT jl_module_t *jl_new_main_module(void);
 void jl_add_standard_imports(jl_module_t *m);
 STATIC_INLINE jl_function_t *jl_get_function(jl_module_t *m, const char *name)
 {

src/toplevel.c

@@ -21,18 +21,43 @@
 int jl_lineno = 0;
 
 jl_module_t *jl_old_base_module = NULL;
+// the Main we started with, in case it is switched
+jl_module_t *jl_internal_main_module = NULL;
 
 jl_value_t *jl_toplevel_eval_flex(jl_value_t *e, int fast);
 
 void jl_add_standard_imports(jl_module_t *m)
 {
+    assert(jl_base_module != NULL);
     // using Base
     jl_module_using(m, jl_base_module);
     // importall Base.Operators
     jl_module_importall(m, (jl_module_t*)jl_get_global(jl_base_module,
                                                        jl_symbol("Operators")));
 }
 
+jl_module_t *jl_new_main_module(void)
+{
+    // switch to a new top-level module
+    if (jl_current_module != jl_main_module && jl_current_module != NULL)
+        jl_error("Main can only be replaced from the top level");
+
+    jl_module_t *old_main = jl_main_module;
+
+    jl_main_module = jl_new_module(jl_symbol("Main"));
+    jl_main_module->parent = jl_main_module;
+    jl_core_module->parent = jl_main_module;
+    jl_set_const(jl_main_module, jl_symbol("Core"),
+                 (jl_value_t*)jl_core_module);
+    jl_set_global(jl_core_module, jl_symbol("Main"),
+                  (jl_value_t*)jl_main_module);
+
+    jl_current_module = jl_main_module;
+    jl_current_task->current_module = jl_main_module;
+
+    return old_main;
+}
+
 extern void jl_get_system_hooks(void);
 extern void jl_get_uv_hooks(int);
 extern int base_module_conflict;
@@ -63,7 +88,7 @@ jl_value_t *jl_eval_module_expr(jl_expr_t *ex)
         jl_old_base_module = jl_base_module;
         // pick up Base module during bootstrap
         jl_base_module = newm;
-        newbase = 1;
+        newbase = base_module_conflict;
     }
     // export all modules from Main
     if (parent_module == jl_main_module)