feat(sources): add a GatherSource

src/sghi/etl/commons/sources.py

@@ -3,7 +3,9 @@
 from __future__ import annotations
 
 import logging
-from collections.abc import Callable
+from collections.abc import Callable, Iterable, Sequence
+from concurrent.futures import Executor, Future, ThreadPoolExecutor
+from contextlib import ExitStack
 from functools import update_wrapper
 from logging import Logger
 from typing import Final, Generic, Self, TypeVar, final
@@ -12,7 +14,11 @@
 
 from sghi.disposable import not_disposed
 from sghi.etl.core import Source
-from sghi.utils import ensure_callable, type_fqn
+from sghi.retry import Retry, noop_retry
+from sghi.task import ConcurrentExecutor, Supplier, supplier
+from sghi.utils import ensure_callable, ensure_not_none_nor_empty, type_fqn
+
+from .utils import fail_fast
 
 # =============================================================================
 # TYPES
@@ -22,6 +28,10 @@
 _RDT = TypeVar("_RDT")
 """Raw Data Type."""
 
+_T = TypeVar("_T")
+
+_ResultGatherer = Callable[[Iterable[Future[_T]]], Iterable[_T]]
+
 _SourceCallable = Callable[[], _RDT]
 
 
@@ -80,6 +90,130 @@ def source(f: Callable[[], _RDT]) -> Source[_RDT]:
 # =============================================================================
 
 
+@final
+class GatherSource(Source[Sequence[_RDT]], Generic[_RDT]):
+    """A :class:`Source` that aggregates raw data from multiple sources."""
+
+    __slots__ = (
+        "_sources",
+        "_retry_policy_factory",
+        "_executor_factory",
+        "_result_gatherer",
+        "_is_disposed",
+        "_logger",
+        "_exit_stack",
+        "_executor",
+        "_prepped_sources",
+    )
+
+    def __init__(
+        self,
+        sources: Sequence[Source[_RDT]],
+        retry_policy_factory: Callable[[], Retry] = noop_retry,
+        executor_factory: Callable[[], Executor] = ThreadPoolExecutor,
+        result_gatherer: _ResultGatherer[_RDT] = fail_fast,
+    ) -> None:
+        super().__init__()
+        ensure_not_none_nor_empty(
+            value=sources,
+            message="'sources' MUST NOT be None or empty.",
+        )
+        self._sources: Sequence[Source[_RDT]] = tuple(sources)
+        self._retry_policy_factory: Callable[[], Retry] = ensure_callable(
+            value=retry_policy_factory,
+            message="'retry_policy_factory' MUST be a callable.",
+        )
+        self._executor_factory: Callable[[], Executor] = ensure_callable(
+            value=executor_factory,
+            message="'executor_factory' MUST be a callable.",
+        )
+        self._result_gatherer: _ResultGatherer[_RDT] = ensure_callable(
+            value=result_gatherer,
+            message="'result_gatherer' MUST be a callable.",
+        )
+        self._is_disposed: bool = False
+        self._logger: Logger = logging.getLogger(type_fqn(self.__class__))
+        self._exit_stack: ExitStack = ExitStack()
+        self._executor: ConcurrentExecutor[None, _RDT] = ConcurrentExecutor(
+            executor=self._executor_factory()
+        )
+
+        # Prepare embedded sources for execution by ensuring that they are all
+        # disposed of properly once this object is disposed.
+        self._prepped_sources: Sequence[Supplier[_RDT]] = tuple(
+            self._source_to_task(self._exit_stack.push(_source))
+            for _source in self._sources
+        )
+
+    @not_disposed
+    @override
+    def __enter__(self) -> Self:
+        """Return ``self`` upon entering the runtime context.
+
+        .. admonition:: Don't use after dispose
+            :class: error
+
+            Invoking this method on an instance that is disposed(i.e. the
+            :attr:`is_disposed` property on the instance is ``True``) will
+            result in a :exc:`ResourceDisposedError` being raised.
+
+        :return: This instance.
+
+        :raise ResourceDisposedError: If this source has already been disposed.
+        """
+        return super(Source, self).__enter__()
+
+    @property
+    @override
+    def is_disposed(self) -> bool:
+        return self._is_disposed
+
+    @not_disposed
+    @override
+    def draw(self) -> Sequence[_RDT]:
+        self._logger.info("Aggregating data from all available sources.")
+
+        with self._executor as executor:
+            futures = executor.execute(None)
+
+        return tuple(self._result_gatherer(futures))
+
+    @override
+    def dispose(self) -> None:
+        """Release any underlying resources contained by this source.
+
+        All embedded sources are also disposed. After this method returns
+        successfully, the :attr:`is_disposed` property should return ``True``.
+
+        .. note::
+            Unless otherwise specified, trying to use methods of a
+            ``Disposable`` instance decorated with the
+            :func:`~sghi.disposable.not_disposed` decorator after this method
+            returns should generally be considered a programming error and
+            should result in a :exc:`~sghi.disposable.ResourceDisposedError`
+            being raised.
+
+            This method should be idempotent allowing it to be called more
+            than once; only the first call, however, should have an effect.
+
+        :return: None.
+        """
+        self._is_disposed = True
+        self._exit_stack.close()
+        self._executor.dispose()
+        self._logger.info("Disposal complete.")
+
+    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()
+
+        # noinspection PyTypeChecker
+        return do_draw
+
+
 @final
 class _SourceOfCallable(Source[_RDT], Generic[_RDT]):
     __slots__ = ("_delegate_to", "_is_disposed", "_logger")