Support OSError attributes

This adds support for the OSError attributes : errno, strerror, filename and filename2.
CPython only sets errno if 2 arguments has been passed in. This has not been implemented here.

CPython OSError.args is capped at 2 items for backward compatibility reasons. This has not been
implemented here.

MICROPY_CPYTHON_COMPAT has to be enabled to get these attributes.

mp_common_errno_to_str() has been extended to check mp_errno_to_str() as well. This is done to ease
reuse for the strerror argument.

Author: notro

py/moduerrno.c

@@ -136,19 +136,28 @@ const char* mp_errno_to_str(mp_obj_t errno_val) {
 #endif //MICROPY_PY_UERRNO
 
 
-// For commonly encountered errors, return human readable strings
-const compressed_string_t* mp_common_errno_to_str(mp_obj_t errno_val) {
-    if (MP_OBJ_IS_SMALL_INT(errno_val)) {
-        switch (MP_OBJ_SMALL_INT_VALUE(errno_val)) {
-            case EPERM:  return translate("Permission denied");
-            case ENOENT: return translate("No such file/directory");
-            case EIO:    return translate("Input/output error");
-            case EACCES: return translate("Permission denied");
-            case EEXIST: return translate("File exists");
-            case ENODEV: return translate("Unsupported operation");
-            case EINVAL: return translate("Invalid argument");
-            case EROFS:  return translate("Read-only filesystem");
-        }
+// For commonly encountered errors, return human readable strings, otherwise try errno name
+const char *mp_common_errno_to_str(mp_obj_t errno_val, char *buf, size_t len) {
+    if (!MP_OBJ_IS_SMALL_INT(errno_val)) {
+        return NULL;
+    }
+
+    const compressed_string_t* desc = NULL;
+    switch (MP_OBJ_SMALL_INT_VALUE(errno_val)) {
+        case EPERM:  desc = translate("Permission denied"); break;
+        case ENOENT: desc = translate("No such file/directory"); break;
+        case EIO:    desc = translate("Input/output error"); break;
+        case EACCES: desc = translate("Permission denied"); break;
+        case EEXIST: desc = translate("File exists"); break;
+        case ENODEV: desc = translate("Unsupported operation"); break;
+        case EINVAL: desc = translate("Invalid argument"); break;
+        case EROFS:  desc = translate("Read-only filesystem"); break;
     }
-    return NULL;
+    if (desc != NULL && desc->length <= len) {
+        decompress(desc, buf);
+        return buf;
+    }
+
+    const char *msg = mp_errno_to_str(errno_val);
+    return msg[0] != '\0' ? msg : NULL;
 }

py/mperrno.h

@@ -141,7 +141,6 @@
 #endif
 
 const char* mp_errno_to_str(mp_obj_t errno_val);
-// For commonly encountered errors, return compressed human readable strings
-const compressed_string_t* mp_common_errno_to_str(mp_obj_t errno_val);
+const char *mp_common_errno_to_str(mp_obj_t errno_val, char *buf, size_t len);
 
 #endif // MICROPY_INCLUDED_PY_MPERRNO_H

py/objexcept.c

@@ -114,17 +114,11 @@ void mp_obj_exception_print(const mp_print_t *print, mp_obj_t o_in, mp_print_kin
             return;
         } else if (o->args->len == 1) {
             // try to provide a nice OSError error message
-            if (o->base.type == &mp_type_OSError && MP_OBJ_IS_SMALL_INT(o->args->items[0])) {
-                const compressed_string_t* common = mp_common_errno_to_str(o->args->items[0]);
-                const char* msg;
+            if (MP_OBJ_IS_SMALL_INT(o->args->items[0]) &&
+                mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(o->base.type), MP_OBJ_FROM_PTR(&mp_type_OSError))) {
                 char decompressed[50];
-                if (common != NULL && common->length <= 50) {
-                    decompress(common, decompressed);
-                    msg = decompressed;
-                } else {
-                    msg = mp_errno_to_str(o->args->items[0]);
-                }
-                if (msg[0] != '\0') {
+                const char *msg = mp_common_errno_to_str(o->args->items[0], decompressed, sizeof(decompressed));
+                if (msg != NULL) {
                     mp_printf(print, "[Errno " INT_FMT "] %s", MP_OBJ_SMALL_INT_VALUE(o->args->items[0]), msg);
                     return;
                 }
@@ -215,6 +209,31 @@ void mp_obj_exception_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
         dest[0] = MP_OBJ_FROM_PTR(self->args);
     } else if (self->base.type == &mp_type_StopIteration && attr == MP_QSTR_value) {
         dest[0] = mp_obj_exception_get_value(self_in);
+    #if MICROPY_CPYTHON_COMPAT
+    } else if (mp_obj_is_subclass_fast(MP_OBJ_FROM_PTR(self->base.type), MP_OBJ_FROM_PTR(&mp_type_OSError))) {
+        if (attr == MP_QSTR_errno) {
+            dest[0] = mp_obj_exception_get_value(self_in);
+        } else if (attr == MP_QSTR_strerror) {
+            if (self->args->len > 1) {
+                dest[0] = self->args->items[1];
+            } else if (self->args->len > 0) {
+                char decompressed[50];
+                const char *msg = mp_common_errno_to_str(self->args->items[0], decompressed, sizeof(decompressed));
+                if (msg != NULL) {
+                    dest[0] = mp_obj_new_str(msg, strlen(msg));
+                } else {
+                    dest[0] = mp_const_none;
+                }
+            } else {
+                dest[0] = mp_const_none;
+            }
+        } else if (attr == MP_QSTR_filename) {
+            dest[0] = self->args->len > 2 ? self->args->items[2] : mp_const_none;
+        // skip winerror
+        } else if (attr == MP_QSTR_filename2) {
+            dest[0] = self->args->len > 4 ? self->args->items[4] : mp_const_none;
+        }
+    #endif
     }
 }