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>

tests/extmod/vfs_posix.py

@@ -99,6 +99,19 @@ if hasattr(vfs, "statvfs"):
 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