extmod/vfs_posix: Fix getcwd() on non-root VFS.

The unwritten API contract expected of a VFS.getcwd() by mp_vfs_getcwd()
is that its return value should be either "" or "/" when the CWD is at
the root of the VFS and otherwise start with a slash and not end with a
slash. This was not correctly implemented in VfsPosix for instances with
a non-empty root - the required leading slash, if any, was cut off
because the root length includes a trailing slash. This would result in
missing slashes in the middle of the return value of os.getcwd() or in
uninitialized garbage from beyond a string's null terminator when the
CWD was at the VFS root.

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

extmod/vfs_posix.c

@@ -186,7 +186,14 @@ STATIC mp_obj_t vfs_posix_getcwd(mp_obj_t self_in) {
     if (ret == NULL) {
         mp_raise_OSError(errno);
     }
-    ret += self->root_len;
+    if (self->root_len > 0) {
+        ret += self->root_len - 1;
+        #ifdef _WIN32
+        if (*ret == '\\') {
+            *(char *)ret = '/';
+        }
+        #endif
+    }
     return mp_obj_new_str(ret, strlen(ret));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(vfs_posix_getcwd_obj, vfs_posix_getcwd);

tests/extmod/vfs_posix_paths.py

@@ -68,6 +68,25 @@ vfs.rmdir("/subdir")
 # done with vfs, restore CWD
 os.chdir(curdir)
 
+# some integration tests with a mounted VFS
+os.mount(os.VfsPosix(temp_dir_abs), "/mnt")
+os.mkdir("/mnt/dir")
+print('chdir("/mnt/dir"):', os.chdir("/mnt/dir"))
+print("getcwd():", os.getcwd())
+print('chdir("/mnt"):', os.chdir("/mnt"))
+print("getcwd():", os.getcwd())
+print('chdir("/"):', os.chdir("/"))
+print("getcwd():", os.getcwd())
+print('chdir("/mnt/dir"):', os.chdir("/mnt/dir"))
+print("getcwd():", os.getcwd())
+print('chdir(".."):', os.chdir(".."))
+print("getcwd():", os.getcwd())
+os.rmdir("/mnt/dir")
+os.umount("/mnt")
+
+# restore CWD
+os.chdir(curdir)
+
 # rmdir
 os.rmdir(temp_dir)
 print(temp_dir in os.listdir())

tests/extmod/vfs_posix_paths.py.exp

@@ -2,7 +2,7 @@ listdir("/"): ['subdir']
 listdir("."): ['subdir']
 getcwd() in {"", "/"}: True
 chdir("subdir"): None
-getcwd(): subdir
+getcwd(): /subdir
 mkdir("two"): None
 listdir("/"): ['subdir']
 listdir("/subdir"): ['file.py', 'one', 'two']
@@ -10,4 +10,14 @@ listdir("."): ['file.py', 'one', 'two']
 print('hello')
 hello
 <module 'file' from 'file.py'>
+chdir("/mnt/dir"): None
+getcwd(): /mnt/dir
+chdir("/mnt"): None
+getcwd(): /mnt
+chdir("/"): None
+getcwd(): /
+chdir("/mnt/dir"): None
+getcwd(): /mnt/dir
+chdir(".."): None
+getcwd(): /mnt
 False