Add user-facing help about destinations

src/allmydata/scripts/runner.py

@@ -12,6 +12,7 @@ from allmydata.scripts import debug, create_node, cli, \
 from allmydata.util.encodingutil import quote_output, quote_local_unicode_path, get_io_encoding
 from allmydata.util.eliotutil import (
     opt_eliot_destination,
+    opt_help_eliot_destinations,
 )
 
 def GROUP(s):
@@ -93,6 +94,7 @@ class Options(usage.Options):
         self.no_command_needed = True
 
     opt_eliot_destination = opt_eliot_destination
+    opt_help_eliot_destinations = opt_help_eliot_destinations
 
     def __str__(self):
         return ("\nUsage: tahoe [global-options] <command> [command-options]\n"

src/allmydata/test/test_runner.py

@@ -168,6 +168,25 @@ class BinTahoe(common_util.SignalMixin, unittest.TestCase, RunBinTahoeMixin):
         d.addCallback(_cb)
         return d
 
+    @inlineCallbacks
+    def test_help_eliot_destinations(self):
+        out, err, rc_or_sig = yield self.run_bintahoe(["--help-eliot-destinations"])
+        self.assertIn("\tfile:<path>", out)
+        self.assertEqual(rc_or_sig, 0)
+
+    @inlineCallbacks
+    def test_eliot_destination(self):
+        out, err, rc_or_sig = yield self.run_bintahoe([
+            # Proves little but maybe more than nothing.
+            "--eliot-destination=file:-",
+            # Throw in *some* command or the process exits with error, making
+            # it difficult for us to see if the previous arg was accepted or
+            # not.
+            "--help",
+        ])
+        self.assertEqual(rc_or_sig, 0)
+
+
 
 class CreateNode(unittest.TestCase):
     # exercise "tahoe create-node", create-introducer,

src/allmydata/util/eliotutil.py

@@ -15,6 +15,7 @@ __all__ = [
     "inline_callbacks",
     "eliot_logging_service",
     "opt_eliot_destination",
+    "opt_help_eliot_destinations",
 ]
 
 from sys import (
@@ -190,11 +191,8 @@ def eliot_logging_service(reactor, destinations):
     Parse the given Eliot destination descriptions and return an ``IService``
     which will add them when started and remove them when stopped.
 
-    The following destinations are supported:
-
-    * ``file:<path>[:rotated_length=<bytes>][:max_rotated_files=<count>]``
-      Sensible defaults are supplied for rotated_length and max_rotated_files
-      if they are not given.
+    See ``--help-eliot-destinations`` for details about supported
+    destinations.
     """
     return _EliotLogging(destinations=list(
         get_destination(reactor)
@@ -214,6 +212,22 @@ def opt_eliot_destination(self, description):
     )
 
 
+def opt_help_eliot_destinations(self):
+    """
+    Emit usage information for --eliot-destination.
+    """
+    print(
+        "Available destinations:\n"
+        # Might want to generate this from some metadata someday but we just
+        # have one hard-coded destination type now, it's easier to hard-code
+        # the help.
+        "\tfile:<path>[:rotate_length=<bytes>][:max_rotated_files=<count>]\n"
+        "\tSensible defaults are supplied for rotate_length and max_rotated_files\n"
+        "\tif they are not given.\n",
+        file=self.stdout,
+    )
+    raise SystemExit(0)
+
 
 class _EliotLogging(Service):
     """