py: Add module weak link support.

With this patch a port can enable module weak link support and provide
a dict of qstr->module mapping.  This mapping is looked up only if an
import fails to find the requested module in the filesystem.

This allows to have the builtin module named, eg, usocket, and provide
a weak link of "socket" to the same module, but this weak link can be
overridden if a file by the name "socket.py" is found in the import
path.

py/builtinimport.c

@@ -44,6 +44,7 @@
 #include "runtime0.h"
 #include "runtime.h"
 #include "builtin.h"
+#include "builtintables.h"
 
 #if 0 // print debugging info
 #define DEBUG_PRINT (1)
@@ -251,12 +252,29 @@ mp_obj_t mp_builtin___import__(mp_uint_t n_args, mp_obj_t *args) {
             }
             DEBUG_printf("Current path: %s\n", vstr_str(&path));
 
-            // fail if we couldn't find the file
             if (stat == MP_IMPORT_STAT_NO_EXIST) {
-                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError, "No module named '%s'", qstr_str(mod_name)));
+                #if MICROPY_MODULE_WEAK_LINKS
+                // check if there is a weak link to this module
+                if (i == mod_len) {
+                    mp_map_elem_t *el = mp_map_lookup((mp_map_t*)&mp_builtin_module_weak_links_dict_obj.map, MP_OBJ_NEW_QSTR(mod_name), MP_MAP_LOOKUP);
+                    if (el == NULL) {
+                        goto no_exist;
+                    }
+                    // found weak linked module
+                    module_obj = el->value;
+                } else {
+                    no_exist:
+                #else
+                {
+                #endif
+                    // couldn't find the file, so fail
+                    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_ImportError, "No module named '%s'", qstr_str(mod_name)));
+                }
+            } else {
+                // found the file, so get the module
+                module_obj = mp_module_get(mod_name);
             }
 
-            module_obj = mp_module_get(mod_name);
             if (module_obj == MP_OBJ_NULL) {
                 // module not already loaded, so load it!
 

py/builtintables.c

@@ -227,3 +227,20 @@ const mp_obj_dict_t mp_builtin_module_dict_obj = {
         .table = (mp_map_elem_t*)mp_builtin_module_table,
     },
 };
+
+#if MICROPY_MODULE_WEAK_LINKS
+STATIC const mp_map_elem_t mp_builtin_module_weak_links_table[] = {
+    MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
+};
+
+const mp_obj_dict_t mp_builtin_module_weak_links_dict_obj = {
+    .base = {&mp_type_dict},
+    .map = {
+        .all_keys_are_qstrs = 1,
+        .table_is_fixed_array = 1,
+        .used = MP_ARRAY_SIZE(mp_builtin_module_weak_links_table),
+        .alloc = MP_ARRAY_SIZE(mp_builtin_module_weak_links_table),
+        .table = (mp_map_elem_t*)mp_builtin_module_weak_links_table,
+    },
+};
+#endif

py/builtintables.h

@@ -26,3 +26,7 @@
 
 extern const mp_obj_dict_t mp_builtin_object_dict_obj;
 extern const mp_obj_dict_t mp_builtin_module_dict_obj;
+
+#if MICROPY_MODULE_WEAK_LINKS
+extern const mp_obj_dict_t mp_builtin_module_weak_links_dict_obj;
+#endif

py/mpconfig.h

@@ -269,6 +269,11 @@ typedef double mp_float_t;
 #define MICROPY_STREAMS_NON_BLOCK (0)
 #endif
 
+// Whether module weak links are supported
+#ifndef MICROPY_MODULE_WEAK_LINKS
+#define MICROPY_MODULE_WEAK_LINKS (0)
+#endif
+
 /*****************************************************************************/
 /* Fine control over Python builtins, classes, modules, etc                  */
 
@@ -411,6 +416,11 @@ typedef double mp_float_t;
 #define MICROPY_PORT_BUILTIN_MODULES
 #endif
 
+// Any module weak links - see builtintables.c:mp_builtin_module_weak_links_table.
+#ifndef MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
+#define MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
+#endif
+
 // Additional constant definitions for the compiler - see compile.c:mp_constants_table.
 #ifndef MICROPY_PORT_CONSTANTS
 #define MICROPY_PORT_CONSTANTS