Since std.ssh private_key_filename can take an absolute path, it can be used to assess whether or not a file exists on the executor's filesystem:
mistral run-action std.ssh '{"cmd":"ls","host":"127.0.0.1","username":"root","private_key_filename":"/notthere"}'
{"result": "Failed to run action [action_ex_id=None, action_cls='<class 'mistral.actions.action_factory.SSHAction'>', attributes='{}', params='{u'username': u'root', u'host': u'127.0.0.1', u'cmd': u'ls', u'private_key_filename': u'/notthere'}']\n Failed to execute ssh cmd
'ls' on [u'127.0.0.1']\nException: [Errno 2] No such file or directory: u'/notthere'"}
mistral run-action std.ssh '{"cmd":"ls","host":"127.0.0.1","username":"root","private_key_filename":"/etc/passwd"}'
{"result": "Failed to run action [action_ex_id=None, action_cls='<class 'mistral.actions.action_factory.SSHAction'>', attributes='{}', params='{u'username': u'root', u'host': u'127.0.0.1', u'cmd': u'ls', u'private_key_filename': u'/etc/passwd'}']\n Failed to execute ssh cmd 'ls' on [u'127.0.0.1']\nException: not a valid RSA private key file"}
This can help detect the nature of the underlying system:
mistral run-action std.ssh '{"cmd":"ls","host":"127.0.0.1","username":"root","private_key_filename":"/etc/debian_version"}'
{"result": "Failed to run action [action_ex_id=None, action_cls='<class 'mistral.actions.action_factory.SSHAction'>', attributes='{}', params='{u'username': u'root', u'host': u'127.0.0.1', u'cmd': u'ls', u'private_key_filename': u'/etc/debian_version'}']\n Failed to execute ssh cmd 'ls' on [u'127.0.0.1']\nException: not a valid RSA private key file"}
-> Hello Debian based system!
This behavior is not present in pike (haven't tested versions in between pike and master) because it does not allow absolute path.
I only see two options out of this problem:
1) Remove the possibility to use absolute path but this might break stuff
2) Put a generic error message for both (file does not exist or file exists but is not a valid key) that does not disclose the existence of the file. Something like "Cannot use private key [...]"