cli/discover: add implicit config to pair for collection creation

Adds support for auto-creating collections when they exist only on
one side and `implicit = 'create'` is set in the pair config.

CHANGELOG.rst

@@ -73,6 +73,10 @@ Version 0.19.0
 - Add a new ``showconfig`` status. This prints *some* configuration values as
   JSON. This is intended to be used by external tools and helpers that interact
   with ``vdirsyncer``, and considered experimental.
+- Add ``implicit`` option to the :ref:`pair section <pair_config>`. When set to
+  "create", it implicitly creates missing collections during sync without user
+  prompts. This simplifies workflows where collections should be automatically
+  created on both sides.
 - Update TLS-related tests that were failing due to weak MDs. :gh:`903`
 - ``pytest-httpserver`` and ``trustme`` are now required for tests.
 - ``pytest-localserver`` is no longer required for tests.

docs/config.rst

@@ -128,6 +128,16 @@ Pair Section
 
   The ``conflict_resolution`` parameter applies for these properties too.
 
+.. _implicit_def:
+
+- ``implicit``: Opt into implicitly creating collections. Example::
+
+      implicit = "create"
+
+  When set to "create", missing collections are automatically created on both
+  sides during sync without prompting the user. This simplifies workflows where
+  all collections should be synchronized bidirectionally.
+
 .. _storage_config:
 
 Storage Section

tests/system/cli/test_config.py

@@ -222,3 +222,62 @@ def test_validate_collections_param():
     x([["c", None, "b"]])
     x([["c", "a", None]])
     x([["c", None, None]])
+
+
+def test_invalid_implicit_value(read_config):
+    expected_message = "`implicit` parameter must be 'create' or absent"
+    with pytest.raises(exceptions.UserError) as excinfo:
+        read_config(
+            """
+            [general]
+            status_path = "/tmp/status/"
+
+            [pair my_pair]
+            a = "my_a"
+            b = "my_b"
+            collections = null
+            implicit = "invalid"
+
+            [storage my_a]
+            type = "filesystem"
+            path = "{base}/path_a/"
+            fileext = ".txt"
+
+            [storage my_b]
+            type = "filesystem"
+            path = "{base}/path_b/"
+            fileext = ".txt"
+            """
+        )
+
+    assert expected_message in str(excinfo.value)
+
+
+def test_implicit_create_only(read_config):
+    """Test that implicit create works."""
+    errors, c = read_config(
+        """
+        [general]
+        status_path = "/tmp/status/"
+
+        [pair my_pair]
+        a = "my_a"
+        b = "my_b"
+        collections = ["from a", "from b"]
+        implicit = "create"
+
+        [storage my_a]
+        type = "filesystem"
+        path = "{base}/path_a/"
+        fileext = ".txt"
+
+        [storage my_b]
+        type = "filesystem"
+        path = "{base}/path_b/"
+        fileext = ".txt"
+        """
+    )
+
+    assert not errors
+    pair = c.pairs["my_pair"]
+    assert pair.implicit == "create"

vdirsyncer/cli/config.py

@@ -96,6 +96,14 @@ def _validate_collections_param(collections):
             raise ValueError(f"`collections` parameter, position {i}: {e!s}")
 
 
+def _validate_implicit_param(implicit):
+    if implicit is None:
+        return
+
+    if implicit != "create":
+        raise ValueError("`implicit` parameter must be 'create' or absent.")
+
+
 class _ConfigReader:
     def __init__(self, f: IO[Any]):
         self._file: IO[Any] = f
@@ -230,6 +238,7 @@ class PairConfig:
         self.name: str = name
         self.name_a: str = options.pop("a")
         self.name_b: str = options.pop("b")
+        self.implicit = options.pop("implicit", None)
 
         self._partial_sync: str | None = options.pop("partial_sync", None)
         self.metadata: str | tuple[()] = options.pop("metadata", ())
@@ -248,6 +257,7 @@ class PairConfig:
             )
         else:
             _validate_collections_param(self.collections)
+            _validate_implicit_param(self.implicit)
 
         if options:
             raise ValueError("Unknown options: {}".format(", ".join(options)))

vdirsyncer/cli/discover.py

@@ -93,6 +93,13 @@ async def collections_for_pair(
             connector=connector,
         )
 
+    async def _handle_collection_not_found(
+        config, collection, e=None, implicit_create=False
+    ):
+        return await handle_collection_not_found(
+            config, collection, e=e, implicit_create=pair.implicit == "create"
+        )
+
     # We have to use a list here because the special None/null value would get
     # mangled to string (because JSON objects always have string keys).
     rv = await aiostream.stream.list(  #  type: ignore[assignment]
@@ -102,7 +109,7 @@ async def collections_for_pair(
             config_b=pair.config_b,
             get_a_discovered=a_discovered.get_self,
             get_b_discovered=b_discovered.get_self,
-            _handle_collection_not_found=handle_collection_not_found,
+            _handle_collection_not_found=_handle_collection_not_found,
         )
     )
 

vdirsyncer/cli/utils.py

@@ -286,7 +286,7 @@ async def storage_instance_from_config(
     except exceptions.CollectionNotFound as e:
         if create:
             config = await handle_collection_not_found(
-                config, config.get("collection", None), e=str(e)
+                config, config.get("collection", None), e=str(e), implicit_create=True
             )
             return await storage_instance_from_config(
                 config,
@@ -341,7 +341,9 @@ def assert_permissions(path: str, wanted: int) -> None:
         os.chmod(path, wanted)
 
 
-async def handle_collection_not_found(config, collection, e=None):
+async def handle_collection_not_found(
+    config, collection, e=None, implicit_create=False
+):
     storage_name = config.get("instance_name", None)
 
     cli_logger.warning(
@@ -350,7 +352,7 @@ async def handle_collection_not_found(config, collection, e=None):
         )
     )
 
-    if click.confirm("Should vdirsyncer attempt to create it?"):
+    if implicit_create or click.confirm("Should vdirsyncer attempt to create it?"):
         storage_type = config["type"]
         cls, config = storage_class_from_config(config)
         config["collection"] = collection