chore(sources): delay embedded sources disposal (#24)

Refactor `sghi.etl.commons.sources.GatherSource` to delay the disposal
of embedded sources. All embedded sources will be disposed of when the
`GatherSource` is disposed of. This will support streaming sources that
may need to remain "live" even after their `draw()` method returns.

src/sghi/etl/commons/sources.py

@@ -118,7 +118,7 @@ class GatherSource(Source[Sequence[_RDT]], Generic[_RDT]):
     Disposing instances of this class also disposes of their embedded sources.
 
     .. admonition:: Regarding retry safety
-        :class: tip
+        :class: caution
 
         Instances of this ``Source`` are **NOT SAFE** to retry.
     """
@@ -256,8 +256,8 @@ def draw(self) -> Sequence[_RDT]:
         """
         self._logger.info("Aggregating data from all available sources.")
 
-        with self._executor as executor:
-            futures = executor.execute(None)
+        executor = self._executor.__enter__()
+        futures = executor.execute(None)
 
         return tuple(self._result_gatherer(futures))
 
@@ -289,9 +289,9 @@ def dispose(self) -> None:
     def _source_to_task(self, s: Source[_RDT]) -> Supplier[_RDT]:
         @supplier
         def do_draw() -> _RDT:
-            with s as _s:
-                draw = self._retry_policy_factory().retry(_s.draw)
-                return draw()
+            _s = s.__enter__()
+            draw = self._retry_policy_factory().retry(_s.draw)
+            return draw()
 
         # noinspection PyTypeChecker
         return do_draw

test/sghi/etl/commons_tests/sources_tests.py

@@ -4,19 +4,52 @@
 from __future__ import annotations
 
 import time
-from collections.abc import Sequence
-from typing import TYPE_CHECKING, Any
+from collections.abc import Iterable, Sequence
+from typing import Any
 from unittest import TestCase
 
 import pytest
 from typing_extensions import override
 
-from sghi.disposable import ResourceDisposedError
+from sghi.disposable import ResourceDisposedError, not_disposed
 from sghi.etl.commons import GatherSource, source
 from sghi.etl.core import Source
 
-if TYPE_CHECKING:
-    from collections.abc import Iterable
+# =============================================================================
+# HELPERS
+# =============================================================================
+
+
+class _StreamingSource(Source[Iterable[int]]):
+    def __init__(self) -> None:
+        self._yielded: int = 0
+        self._is_disposed: bool = False
+
+    @property
+    @override
+    def is_disposed(self) -> bool:
+        return self._is_disposed
+
+    @not_disposed
+    @override
+    def draw(self) -> Iterable[int]:
+        for _ in range(3):
+            yield from self._do_yield()
+        self._yielded = 0
+
+    @override
+    def dispose(self) -> None:
+        self._is_disposed = True
+
+    @not_disposed
+    def _do_yield(self) -> Iterable[int]:
+        yield from range(self._yielded, self._yielded + 4)
+        self._yielded += 4
+
+
+# =============================================================================
+# TESTS
+# =============================================================================
 
 
 def test_source_decorator_delegates_to_the_wrapped_callable() -> None:
@@ -126,6 +159,7 @@ def supply_ints_slowly(
             get_greeting,
             supply_ints,
             supply_ints_slowly,
+            _StreamingSource(),
         ]
         self._instance: Source[Sequence[Any]] = GatherSource(
             sources=self._embedded_sources,
@@ -158,6 +192,7 @@ def test_draw_returns_the_expected_value(self) -> None:
         assert result[0] == "Hello, World!"
         assert tuple(result[1]) == (0, 1, 2, 3, 4)
         assert tuple(result[2]) == (0, 1, 2, 3, 4)
+        assert tuple(result[3]) == (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
 
     def test_instantiation_fails_on_an_empty_sources_arg(self) -> None:
         """Instantiating a :class:`GatherSource` with an empty ``sources``