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.

start.py

@@ -7,9 +7,10 @@ import signal
 import sys
 
 from dsc import SeafileClient, const
-from dsc.errors import DscError, GracefulShutdown
+from dsc.errors import ConfigError, DscError, GracefulShutdown
 from dsc.misc import setup_uid, create_dir
 from dsc.paths import plan_lib_dirs
+from dsc.secrets import resolve_secret
 
 _lg = logging.getLogger('dsc')
 
@@ -82,6 +83,9 @@ def main():
     parser.add_argument("-s", "--server")
     parser.add_argument("-u", "--username")
     parser.add_argument("-p", "--password")
+    parser.add_argument("--password-file")
+    parser.add_argument("-T", "--token")
+    parser.add_argument("--token-file")
     parser.add_argument("-l", "--libraries")
     parser.add_argument("--uid", type=int)
     parser.add_argument("--gid", type=int)
@@ -94,6 +98,9 @@ def main():
         server=os.getenv("SERVER_HOST"),
         username=os.getenv("USERNAME"),
         password=os.getenv("PASSWORD"),
+        password_file=os.getenv("PASSWORD_FILE"),
+        token=os.getenv("TOKEN"),
+        token_file=os.getenv("TOKEN_FILE"),
         libraries=os.getenv("LIBRARY_ID"),
         uid=os.getenv("SEAFILE_UID", default=1000),
         gid=os.getenv("SEAFILE_GID", default=1000),
@@ -107,17 +114,28 @@ def main():
         parser.error("Seafile server is not specified")
     if not args.username:
         parser.error("username is not specified")
-    if not args.password:
-        parser.error("password is not specified")
     if not args.libraries:
         parser.error("library is not specified")
 
+    try:
+        password = resolve_secret("PASSWORD", args.password, args.password_file)
+        token = resolve_secret("TOKEN", args.token, args.token_file)
+    except ConfigError as err:
+        _lg.error("%s", err)
+        return 2
+    if not password and not token:
+        parser.error("neither a password nor a token is specified")
+    if password and token:
+        # seaf-cli behaves the same way: a token makes the password unused.
+        _lg.info("Both a password and a token are configured; using the token")
+
     install_signal_handlers()
 
     setup_uid(args.uid, args.gid)
     create_dir(const.DEFAULT_APP_DIR)
 
-    client = SeafileClient(args.server, args.username, args.password, const.DEFAULT_APP_DIR)
+    client = SeafileClient(args.server, args.username, password,
+                           const.DEFAULT_APP_DIR, token=token)
 
     # check for deprecated /data directory
     if os.path.isdir(const.DEPRECATED_LIBS_DIR):