PROTON-2879: [Python] Added accessors to unsettled deliveries

Added unsettled delivery accessors to high level python binding.
Including a generator which should make iterating over all unsettled
deliveries of a link straightforward.

Author: astitcher

python/proton/_delivery.py

@@ -48,7 +48,8 @@
                      pn_transactional_disposition_get_id,
                      pn_transactional_disposition_set_id,
                      pn_transactional_disposition_get_outcome_type,
-                     pn_transactional_disposition_set_outcome_type)
+                     pn_transactional_disposition_set_outcome_type,
+                     pn_unsettled_next)
 
 from ._condition import cond2obj, obj2cond, Condition
 from ._data import dat2obj, obj2dat
@@ -659,6 +660,14 @@ def settle(self) -> None:
         """
         pn_delivery_settle(self._impl)
 
+    @property
+    def unsettled_next(self) -> Optional['Delivery']:
+        """
+        The next unsettled delivery on the link or ``None`` if there are
+        no more unsettled deliveries.
+        """
+        return Delivery.wrap(pn_unsettled_next(self._impl))
+
     @property
     def aborted(self) -> bool:
         """

python/proton/_endpoints.py

@@ -54,7 +54,7 @@
     pn_terminus_is_dynamic, pn_terminus_outcomes, pn_terminus_properties, pn_terminus_set_address, \
     pn_terminus_set_distribution_mode, pn_terminus_set_durability, pn_terminus_set_dynamic, \
     pn_terminus_set_expiry_policy, pn_terminus_set_timeout, pn_terminus_set_type, \
-    pn_link_properties, pn_link_remote_properties
+    pn_link_properties, pn_link_remote_properties, pn_unsettled_head
 
 from ._condition import cond2obj, obj2cond
 from ._data import Data, dat2obj, obj2dat, PropertyDict, SymbolList
@@ -63,7 +63,7 @@
 from ._handler import Handler
 from ._transport import Transport
 from ._wrapper import Wrapper
-from typing import Any, Dict, List, Optional, Union, TYPE_CHECKING
+from typing import Any, Dict, Generator, List, Optional, Union, TYPE_CHECKING
 
 if TYPE_CHECKING:
     from ._condition import Condition
@@ -888,6 +888,28 @@ def unsettled(self) -> int:
         """
         return pn_link_unsettled(self._impl)
 
+    @property
+    def unsettled_head(self) -> Optional[Delivery]:
+        """
+        The first unsettled delivery for this link.
+
+        This operation will return the first unsettled delivery on the
+        link, or ``None`` if there are no unsettled deliveries.
+        """
+        return Delivery.wrap(pn_unsettled_head(self._impl))
+
+    @property
+    def unsettled_deliveries(self) -> Generator[Delivery]:
+        """
+        Returns a generator of unsettled deliveries for this link.
+
+        :return: Generator of unsettled deliveries.
+        """
+        delivery = self.unsettled_head
+        while delivery:
+            yield delivery
+            delivery = delivery.unsettled_next
+
     @property
     def credit(self) -> int:
         """