Repository: juju/python-libjuju (python)

Issue:
Machine.ssh and Machine._scp do not use JUJU_DATA if set

Pull request description:
get the config dir resolve logic into one place
### Description

This PR introduces two utility functions `juju_config_dir` and `juju_ssh_pey_paths` that holds the logic of discovering the configuration directory and the ssh keys in one place. It works by 1) checking `JUJU_DATA` env variable, 2) checking `XDG_DATA_HOME` variable, 3) manually sets it to `~/.local/share/juju` (in this order). Anyone wants to use a custom directory have multiple different options.

### QA Steps 

```
tox -e py3 -- tests/unit/test_utils.py
tox -e integration -- tests/integration/test_machine.py
tox -e integration -- tests/integration/test_unit.py
```

### Notes & Discussion

Fixes #554

Patch:
diff --git a/juju/client/jujudata.py b/juju/client/jujudata.py
index 8b844c2..629ad52 100644
--- a/juju/client/jujudata.py
+++ b/juju/client/jujudata.py
@@ -8,6 +8,7 @@ import yaml
 from juju import tag
 from juju.client.gocookies import GoCookieJar
 from juju.errors import JujuError
+from juju.utils import juju_config_dir
 
 
 class NoModelException(Exception):
@@ -121,8 +122,7 @@ class FileJujuData(JujuData):
     '''Provide access to the Juju client configuration files.
     Any configuration file is read once and then cached.'''
     def __init__(self):
-        self.path = os.environ.get('JUJU_DATA') or '~/.local/share/juju'
-        self.path = os.path.abspath(os.path.expanduser(self.path))
+        self.path = juju_config_dir()
         # _loaded keeps track of the loaded YAML from
         # the Juju data files so we don't need to load the same
         # file many times.
diff --git a/juju/machine.py b/juju/machine.py
index 2067a21..1e46199 100644
--- a/juju/machine.py
+++ b/juju/machine.py
@@ -1,7 +1,6 @@
 import asyncio
 import ipaddress
 import logging
-import os
 
 import pyrfc3339
 
@@ -9,6 +8,7 @@ from . import model, tag
 from .annotationhelper import _get_annotations, _set_annotations
 from .client import client
 from .errors import JujuError
+from juju.utils import juju_ssh_key_paths
 
 log = logging.getLogger(__name__)
 
@@ -124,9 +124,10 @@ class Machine(model.ModelEntity):
         """ Execute an scp command. Requires a fully qualified source and
         destination.
         """
+        _, id_path = juju_ssh_key_paths()
         cmd = [
             'scp',
-            '-i', os.path.expanduser('~/.local/share/juju/ssh/juju_id_rsa'),
+            '-i', id_path,
             '-o', 'StrictHostKeyChecking=no',
             '-q',
             '-B'
@@ -153,9 +154,10 @@ class Machine(model.ModelEntity):
             raise NotImplementedError('proxy option is not implemented')
         address = self.dns_name
         destination = "{}@{}".format(user, address)
+        _, id_path = juju_ssh_key_paths()
         cmd = [
             'ssh',
-            '-i', os.path.expanduser('~/.local/share/juju/ssh/juju_id_rsa'),
+            '-i', id_path,
             '-o', 'StrictHostKeyChecking=no',
             '-q',
             destination
diff --git a/juju/utils.py b/juju/utils.py
index 2321ffd..9fca71d 100644
--- a/juju/utils.py
+++ b/juju/utils.py
@@ -32,15 +32,50 @@ async def execute_process(*cmd, log=None, loop=None):
     return p.returncode == 0
 
 
+def juju_config_dir():
+    """Resolves and returns the path string to the juju configuration
+    folder for the juju CLI tool. Of the following items, returns the
+    first option that works (top to bottom):
+
+    * $JUJU_DATA
+    * $XDG_DATA_HOME/juju
+    * ~/.local/share/juju
+
+    """
+    # Check $JUJU_DATA first
+    config_dir = os.environ.get('JUJU_DATA', None)
+
+    # Second option: $XDG_DATA_HOME for ~/.local/share
+    if not config_dir:
+        config_dir = os.environ.get('XDG_DATA_HOME', None)
+
+    # Third option: just set it to ~/.local/share/juju
+    if not config_dir:
+        config_dir = '~/.local/share/juju'
+
+    return os.path.abspath(os.path.expanduser(config_dir))
+
+
+def juju_ssh_key_paths():
+    """Resolves and returns the path strings for public and private ssh
+    keys for juju CLI.
+
+    """
+    config_dir = juju_config_dir()
+    public_key_path = os.path.join(config_dir, 'ssh', 'juju_id_rsa.pub')
+    private_key_path = os.path.join(config_dir, 'ssh', 'juju_id_rsa')
+
+    return public_key_path, private_key_path
+
+
 def _read_ssh_key():
     '''
     Inner function for read_ssh_key, suitable for passing to our
     Executor.
 
     '''
-    default_data_dir = Path(Path.home(), ".local", "share", "juju")
-    juju_data = os.environ.get("JUJU_DATA", default_data_dir)
-    ssh_key_path = Path(juju_data, 'ssh', 'juju_id_rsa.pub')
+    public_key_path_str, _ = juju_ssh_key_paths()
+    ssh_key_path = Path(public_key_path_str)
     with ssh_key_path.open('r') as ssh_key_file:
         ssh_key = ssh_key_file.readlines()[0].strip()
     return ssh_key


Test patch:
diff --git a/tests/unit/test_utils.py b/tests/unit/test_utils.py
new file mode 100644
index 0000000..6a17339
--- /dev/null
+++ b/tests/unit/test_utils.py
@@ -0,0 +1,14 @@
+import unittest
+
+from juju.utils import juju_config_dir, juju_ssh_key_paths
+
+
+class TestDirResolve(unittest.TestCase):
+    def test_config_dir(self):
+        config_dir = juju_config_dir()
+        assert 'local/share/juju' in config_dir
+
+    def test_juju_ssh_key_paths(self):
+        public, private = juju_ssh_key_paths()
+        assert public.endswith('ssh/juju_id_rsa.pub')
+        assert private.endswith('ssh/juju_id_rsa')