Authenticate from mounted secrets and stop passing the password to seaf-cli

The password could only be given through the PASSWORD variable, so Docker
Compose and Swarm secrets were unusable and the password sat in the container
environment.

PASSWORD_FILE and TOKEN_FILE read a secret from a mounted file. Only trailing
newlines are stripped, since secret files are usually written with one and a
password that silently carries it just fails to authenticate; everything else,
including surrounding spaces, belongs to the secret. Setting both a variable
and a file for one secret is rejected rather than resolved by precedence,
because the two are meant to be the same value and quietly picking one hides
the mistake until authentication fails.

The installed seaf-cli supports token authentication and ignores the password
when a token is given, so sync now passes -T instead of -p. The account
password no longer appears in any command line. The token does, because
seaf-cli accepts credentials only as arguments, but a token can be revoked on
the server. This is documented in the README.

Both secrets are masked before a command is logged.

Tests cover newline handling, empty and missing files, conflicting sources,
and that neither secret reaches the logs. Requests are now blocked in tests:
the client retries with backoff, so a test that reached the network appeared
to hang instead of failing.

dsc/client.py

@@ -12,8 +12,8 @@ from urllib3.util.retry import Retry
 import seafile
 
 from dsc import const
-from dsc.errors import DaemonError, DaemonTimeout
-from dsc.misc import create_dir, hide_password, user_cmd
+from dsc.errors import ConfigError, DaemonError, DaemonTimeout
+from dsc.misc import create_dir, hide_secrets, user_cmd
 
 _lg = logging.getLogger(__name__)
 
@@ -22,13 +22,16 @@ class SeafileClient:
     def __init__(self,
                  host: str,
                  user: str,
-                 passwd: str,
-                 app_dir: str = const.DEFAULT_APP_DIR):
+                 passwd: str = None,
+                 app_dir: str = const.DEFAULT_APP_DIR,
+                 token: str = None):
         self.user = user
         self.password = passwd
         self.app_dir = os.path.abspath(app_dir)
         self.rpc = seafile.RpcClient(os.path.join(self.app_dir, 'seafile-data', 'seafile.sock'))
-        self.__token = None
+        # A token supplied by the operator is used as is; otherwise one is
+        # fetched from the server with the password.
+        self.__token = token
 
         # determine server URL (assume HTTPS unless explicitly specified)
         if host.startswith('http://') or host.startswith('https://'):
@@ -86,6 +89,8 @@ class SeafileClient:
     @property
     def token(self):
         if self.__token is None:
+            if not self.password:
+                raise ConfigError("No password or token available to authenticate")
             url = f"{self.url}/api2/auth-token/"
             _lg.info("Fetching token: %s", url)
             r = self.session.post(url, data={"username": self.user, "password": self.password})
@@ -170,11 +175,14 @@ class SeafileClient:
             "-s", self.url,
             "-d", lib_dir,
             "-u", self.user,
-            "-p", self.password,
+            # The token, not the password: seaf-cli ignores the password when a
+            # token is given, and command arguments are visible to any process
+            # in the container. A token can also be revoked on the server.
+            "-T", self.token,
         ]
         _lg.info(
             "Syncing library %s into %s: %s", lib_id, lib_dir,
-            " ".join(hide_password(cmd, self.password)),
+            " ".join(hide_secrets(cmd, self.token, self.password)),
         )
         proc = self.__run(cmd, check=False)
         if proc.returncode != 0: