extmod/vfs_posix: Fix relative root path.

A VfsPosix created with a relative root path would get confused when
chdir() was called on it and become unable to properly resolve absolute
paths, because changing directories effectively shifted its root. The
simplest fix for that would be to say "don't do that", but since the
unit tests themselves do it, fix it by making a relative path absolute
before storing it.

Signed-off-by: Christian Walther <cwalther@gmx.ch>

extmod/vfs_posix.c

@@ -46,6 +46,9 @@
 #ifdef _MSC_VER
 #include <direct.h> // For mkdir etc.
 #endif
+#ifdef _WIN32
+#include <windows.h>
+#endif
 
 typedef struct _mp_obj_vfs_posix_t {
     mp_obj_base_t base;
@@ -107,7 +110,28 @@ STATIC mp_obj_t vfs_posix_make_new(const mp_obj_type_t *type, size_t n_args, siz
     mp_obj_vfs_posix_t *vfs = mp_obj_malloc(mp_obj_vfs_posix_t, type);
     vstr_init(&vfs->root, 0);
     if (n_args == 1) {
-        vstr_add_str(&vfs->root, mp_obj_str_get_str(args[0]));
+        const char *root = mp_obj_str_get_str(args[0]);
+        // if the root is relative, make it absolute, otherwise we'll get confused by chdir
+        #ifdef _WIN32
+        char buf[MICROPY_ALLOC_PATH_MAX + 1];
+        DWORD result = GetFullPathNameA(root, sizeof(buf), buf, NULL);
+        if (result > 0 && result < sizeof(buf)) {
+            vstr_add_str(&vfs->root, buf);
+        } else {
+            mp_raise_OSError(GetLastError());
+        }
+        #else
+        if (root[0] != '\0' && root[0] != '/') {
+            char buf[MICROPY_ALLOC_PATH_MAX + 1];
+            const char *cwd = getcwd(buf, sizeof(buf));
+            if (cwd == NULL) {
+                mp_raise_OSError(errno);
+            }
+            vstr_add_str(&vfs->root, cwd);
+            vstr_add_char(&vfs->root, '/');
+        }
+        vstr_add_str(&vfs->root, root);
+        #endif
         vstr_add_char(&vfs->root, '/');
     }
     vfs->root_len = vfs->root.len;

tests/extmod/vfs_posix.py

@@ -99,6 +99,19 @@ def write_files_without_closing():
 print(type(list(vfs.ilistdir("."))[0][0]))
 print(type(list(vfs.ilistdir(b"."))[0][0]))
 
+# chdir should not affect absolute paths (regression test)
+vfs.mkdir("/subdir")
+vfs.mkdir("/subdir/micropy_test_dir")
+with vfs.open("/subdir/micropy_test_dir/test2", "w") as f:
+    f.write("wrong")
+vfs.chdir("/subdir")
+with vfs.open("/test2", "r") as f:
+    print(f.read())
+os.chdir(curdir)
+vfs.remove("/subdir/micropy_test_dir/test2")
+vfs.rmdir("/subdir/micropy_test_dir")
+vfs.rmdir("/subdir")
+
 # remove
 os.remove(temp_dir + "/test2")
 print(os.listdir(temp_dir))

tests/extmod/vfs_posix.py.exp

@@ -10,6 +10,7 @@ next_file_no <= base_file_no True
 <class 'tuple'>
 <class 'str'>
 <class 'bytes'>
+hello
 []
 remove OSError
 False