py: Automatically provide weak links from "foo" to "ufoo" module name.

This commit implements automatic module weak links for all built-in
modules, by searching for "ufoo" in the built-in module list if "foo"
cannot be found.  This means that all modules named "ufoo" are always
available as "foo".  Also, a port can no longer add any other weak links,
which makes strict the definition of a weak link.

It saves some code size (about 100-200 bytes) on ports that previously had
lots of weak links.

Some changes from the previous behaviour:
- It doesn't intern the non-u module names (eg "foo" is not interned),
  which saves code size, but will mean that "import foo" creates a new qstr
  (namely "foo") in RAM (unless the importing module is frozen).
- help('modules') no longer lists non-u module names, only the u-variants;
  this reduces duplication in the help listing.

Weak links are effectively the same as having a set of symbolic links on
the filesystem that is searched last.  So an "import foo" will search
built-in modules first, then all paths in sys.path, then weak links last,
importing "ufoo" if it exists.  Thus a file called "foo.py" somewhere in
sys.path will still have precedence over the weak link of "foo" to "ufoo".

See issues: #1740, #4449, #5229, #5241.

5 files changed, 32 insertions, 34 deletions

diff --git a/py/builtinhelp.c b/py/builtinhelp.c
index a7fede00a..8f162d885 100644
--- a/py/builtinhelp.c
+++ b/py/builtinhelp.c
@@ -80,10 +80,6 @@ STATIC void mp_help_print_modules(void) {
 
     mp_help_add_from_map(list, &mp_builtin_module_map);
 
-    #if MICROPY_MODULE_WEAK_LINKS
-    mp_help_add_from_map(list, &mp_builtin_module_weak_links_map);
-    #endif
-
     #if MICROPY_MODULE_FROZEN_STR
     extern const char mp_frozen_str_names[];
     mp_help_add_from_names(list, mp_frozen_str_names);
diff --git a/py/builtinimport.c b/py/builtinimport.c
index 008a21dcf..b9f6c2ab2 100644
--- a/py/builtinimport.c
+++ b/py/builtinimport.c
@@ -3,7 +3,7 @@
  *
  * The MIT License (MIT)
  *
- * Copyright (c) 2013, 2014 Damien P. George
+ * Copyright (c) 2013-2019 Damien P. George
  * Copyright (c) 2014 Paul Sokolovsky
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -381,21 +381,18 @@ mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
             DEBUG_printf("Current path: %.*s\n", vstr_len(&path), vstr_str(&path));
 
             if (stat == MP_IMPORT_STAT_NO_EXIST) {
+                module_obj = MP_OBJ_NULL;
                 #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_map, MP_OBJ_NEW_QSTR(mod_name), MP_MAP_LOOKUP);
-                    if (el == NULL) {
-                        goto no_exist;
+                    module_obj = mp_module_search_umodule(mod_str);
+                    if (module_obj != MP_OBJ_NULL) {
+                        // found weak linked module
+                        mp_module_call_init(mod_name, module_obj);
                     }
-                    // found weak linked module
-                    module_obj = el->value;
-                    mp_module_call_init(mod_name, module_obj);
-                } else {
-                    no_exist:
-                #else
-                {
+                }
                 #endif
+                if (module_obj == MP_OBJ_NULL) {
                     // couldn't find the file, so fail
                     if (MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE) {
                         mp_raise_msg(&mp_type_ImportError, "module not found");
@@ -492,11 +489,11 @@ mp_obj_t mp_builtin___import__(size_t n_args, const mp_obj_t *args) {
 
     #if MICROPY_MODULE_WEAK_LINKS
     // Check if there is a weak link to this module
-    mp_map_elem_t *el = mp_map_lookup((mp_map_t*)&mp_builtin_module_weak_links_map, MP_OBJ_NEW_QSTR(module_name_qstr), MP_MAP_LOOKUP);
-    if (el != NULL) {
+    module_obj = mp_module_search_umodule(qstr_str(module_name_qstr));
+    if (module_obj != MP_OBJ_NULL) {
         // Found weak-linked module
-        mp_module_call_init(module_name_qstr, el->value);
-        return el->value;
+        mp_module_call_init(module_name_qstr, module_obj);
+        return module_obj;
     }
     #endif
 
diff --git a/py/mpconfig.h b/py/mpconfig.h
index 4172b5fcf..e46da3e83 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -1392,11 +1392,6 @@ typedef double mp_float_t;
 #define MICROPY_PORT_BUILTIN_MODULES
 #endif
 
-// Any module weak links - see objmodule.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
diff --git a/py/objmodule.c b/py/objmodule.c
index 4a07913c5..d725bb6a1 100644
--- a/py/objmodule.c
+++ b/py/objmodule.c
@@ -3,7 +3,7 @@
  *
  * The MIT License (MIT)
  *
- * Copyright (c) 2013, 2014 Damien P. George
+ * Copyright (c) 2013-2019 Damien P. George
  * Copyright (c) 2014-2015 Paul Sokolovsky
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
@@ -26,6 +26,7 @@
  */
 
 #include <stdlib.h>
+#include <string.h>
 #include <assert.h>
 
 #include "py/objmodule.h"
@@ -235,14 +236,6 @@ STATIC const mp_rom_map_elem_t mp_builtin_module_table[] = {
 
 MP_DEFINE_CONST_MAP(mp_builtin_module_map, mp_builtin_module_table);
 
-#if MICROPY_MODULE_WEAK_LINKS
-STATIC const mp_rom_map_elem_t mp_builtin_module_weak_links_table[] = {
-    MICROPY_PORT_BUILTIN_MODULE_WEAK_LINKS
-};
-
-MP_DEFINE_CONST_MAP(mp_builtin_module_weak_links_map, mp_builtin_module_weak_links_table);
-#endif
-
 // returns MP_OBJ_NULL if not found
 mp_obj_t mp_module_get(qstr module_name) {
     mp_map_t *mp_loaded_modules_map = &MP_STATE_VM(mp_loaded_modules_dict).map;
@@ -267,6 +260,21 @@ void mp_module_register(qstr qst, mp_obj_t module) {
     mp_map_lookup(mp_loaded_modules_map, MP_OBJ_NEW_QSTR(qst), MP_MAP_LOOKUP_ADD_IF_NOT_FOUND)->value = module;
 }
 
+#if MICROPY_MODULE_WEAK_LINKS
+// Search for u"foo" in built-in modules, return MP_OBJ_NULL if not found
+mp_obj_t mp_module_search_umodule(const char *module_str) {
+    for (size_t i = 0; i < MP_ARRAY_SIZE(mp_builtin_module_table); ++i) {
+        const mp_map_elem_t *entry = (const mp_map_elem_t*)&mp_builtin_module_table[i];
+        const char *key = qstr_str(MP_OBJ_QSTR_VALUE(entry->key));
+        if (key[0] == 'u' && strcmp(&key[1], module_str) == 0) {
+            return (mp_obj_t)entry->value;
+        }
+
+    }
+    return MP_OBJ_NULL;
+}
+#endif
+
 #if MICROPY_MODULE_BUILTIN_INIT
 void mp_module_call_init(qstr module_name, mp_obj_t module_obj) {
     // Look for __init__ and call it if it exists
diff --git a/py/objmodule.h b/py/objmodule.h
index b7702ec50..33a0ff07e 100644
--- a/py/objmodule.h
+++ b/py/objmodule.h
@@ -3,7 +3,7 @@
  *
  * The MIT License (MIT)
  *
- * Copyright (c) 2013, 2014 Damien P. George
+ * Copyright (c) 2013-2019 Damien P. George
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -34,6 +34,8 @@ extern const mp_map_t mp_builtin_module_weak_links_map;
 mp_obj_t mp_module_get(qstr module_name);
 void mp_module_register(qstr qstr, mp_obj_t module);
 
+mp_obj_t mp_module_search_umodule(const char *module_str);
+
 #if MICROPY_MODULE_BUILTIN_INIT
 void mp_module_call_init(qstr module_name, mp_obj_t module_obj);
 #else