mingw: Add implementation of realpath()

The mingw port used _fullpath() until now, but the behaviour is not exactly
the same as realpath()'s on unix; major difference being that it doesn't
return an error for non-existing files, which would bypass main's error
checking and bail out without any error message.

Also realpath() will return forward slashes only since main() relies on that.

unix/main.c

@@ -394,11 +394,7 @@ int main(int argc, char **argv) {
                 return usage(argv);
             }
         } else {
-#ifdef __MINGW32__
-            char *basedir = _fullpath(NULL, argv[a], _MAX_PATH);
-#else
             char *basedir = realpath(argv[a], NULL);
-#endif
             if (basedir == NULL) {
                 fprintf(stderr, "%s: can't open file '%s': [Errno %d] ", argv[0], argv[1], errno);
                 perror("");

windows/Makefile

@@ -30,6 +30,7 @@ endif
 SRC_C = \
 	unix/main.c \
 	unix/file.c \
+	realpath.c \
 
 OBJ = $(PY_O) $(addprefix $(BUILD)/, $(SRC_C:.c=.o))
 

windows/mpconfigport.h

@@ -36,3 +36,5 @@ typedef const void *machine_const_ptr_t; // must be of pointer size
 extern const struct _mp_obj_fun_native_t mp_builtin_open_obj;
 #define MICROPY_EXTRA_BUILTINS \
     { MP_OBJ_NEW_QSTR(MP_QSTR_open), (mp_obj_t)&mp_builtin_open_obj },
+
+#include "realpath.h"

windows/realpath.c

+#include <stdlib.h>
+#include <errno.h>
+#include <io.h>
+
+#ifndef R_OK
+    #define R_OK 4
+#endif
+
+// Make sure a path only has forward slashes.
+char *to_unix_path(char *p) {
+    if (p != NULL) {
+        char *pp = p;
+        while (*pp != 0) {
+            if (*pp == '\\')
+                *pp = '/';
+            ++pp;
+        }
+    }
+    return p;
+}
+
+// Implement realpath() using _fullpath and make it use the same error codes as realpath() on unix.
+// Also have it return a path with forward slashes only as some code relies on this,
+// but _fullpath() returns backward slashes no matter what.
+char *realpath(const char *path, char *resolved_path) {
+    char *ret = NULL;
+    if (path == NULL) {
+        errno = EINVAL;
+    } else if (access(path, R_OK) == 0) {
+        ret = resolved_path;
+        if (ret == NULL)
+            ret = malloc(_MAX_PATH);
+        if (ret == NULL) {
+            errno = ENOMEM;
+        } else {
+            ret = _fullpath(ret, path, _MAX_PATH);
+            if (ret == NULL)
+                errno = EIO;
+        }
+    }
+    return to_unix_path(ret);
+}

windows/realpath.h

+
+extern char *realpath(const char *path, char *resolved_path);