Implement "realpath" to retrieve the real absolute path

This function will call "os.path.realpth" and this in turn could call
"os.getcwd". If the current path has been deleted by other application,
this method will raise FileNotFoundError exception. This new function
catches and unsets this exception, returning an empty string.

Change-Id: If70411d8d189de4a020e528cd54412a5678cfab9
Closes-Bug: #1864090

oslo_rootwrap/filters.py

@@ -33,6 +33,19 @@ def _getuid(user):
     return pwd.getpwnam(user).pw_uid
 
 
+def realpath(path):
+    """Return the real absolute path.
+
+    If the execution directory does not exist, os.getcwd() raises a
+    FileNotFoundError exception. In this case, unset the exception and return
+    an empty string.
+    """
+    try:
+        return os.path.realpath(path)
+    except FileNotFoundError:
+        return ''
+
+
 class CommandFilter(object):
     """Command filter only checking that the 1st argument matches exec_path."""
 
@@ -128,7 +141,7 @@ class PathFilter(CommandFilter):
             if not os.path.isabs(arg)  # arguments not specifying abs paths
         )
         paths_are_within_base_dirs = all(
-            os.path.commonprefix([arg, os.path.realpath(value)]) == arg
+            os.path.commonprefix([arg, realpath(value)]) == arg
             for arg, value in zip(self.args, arguments)
             if os.path.isabs(arg)  # arguments specifying abs paths
         )
@@ -143,7 +156,7 @@ class PathFilter(CommandFilter):
         command, arguments = userargs[0], userargs[1:]
 
         # convert path values to canonical ones; copy other args as is
-        args = [os.path.realpath(value) if os.path.isabs(arg) else value
+        args = [realpath(value) if os.path.isabs(arg) else value
                 for arg, value in zip(self.args, arguments)]
 
         return super(PathFilter, self).get_command([command] + args,