add classes and methods for alternative closure calculation

src/_pytest/fixtures.py

@@ -1,7 +1,9 @@
 import abc
+import copy
 import dataclasses
 import functools
 import inspect
+import itertools
 import os
 import warnings
 from collections import defaultdict
@@ -12,6 +14,7 @@ from typing import AbstractSet
 from typing import Any
 from typing import Callable
 from typing import cast
+from typing import Collection
 from typing import Dict
 from typing import Final
 from typing import final
@@ -20,7 +23,9 @@ from typing import Generic
 from typing import Iterable
 from typing import Iterator
 from typing import List
+from typing import Mapping
 from typing import MutableMapping
+from typing import MutableSet
 from typing import NoReturn
 from typing import Optional
 from typing import overload
@@ -340,6 +345,243 @@ class FuncFixtureInfo:
         self.names_closure[:] = sorted(closure, key=self.names_closure.index)
 
 
+# this exceptions is mainly intended for internal signaling
+class CyclicDependency(Exception):
+    pass
+
+
+# this exceptions is mainly intended for internal signaling
+class DuplicateDependency(Exception):
+    pass
+
+
+# keep the interface (mostly) backwards compatible to FuncFixtureInfo.
+# the topological sort/graph cycle detection algorithm is based on the paper
+# "A dynamic topological sort algorithm for directed acyclic graphs" by
+# David J. Pearce and Paul H. J. Kelly
+# doi.org/10.1145/1187436.1210590
+# whileydave.com/publications/pk07_jea
+class FuncFixtureInfo2:
+    # TODO:
+    # - check which sort/sorted calls need to be reversed by default.
+    # - maybe replace Tuple[str, int] by a NamedTuple?
+    # - make this a frozen dataclass again? this would make clone more difficult to implement!
+    __slots__ = (
+        "argnames",
+        "initialnames",
+        "name2fixturedefs",
+        "_closure_store",
+        "_incoming",
+        "_rank",
+        "_reuse",
+        "_outgoing",
+    )
+
+    # Fixture names that the item requests directly by function parameters.
+    argnames: Tuple[str, ...]
+    # Fixture names that the item immediately requires. These include
+    # argnames + fixture names specified via usefixtures and via autouse=True in
+    # fixture definitions.
+    initialnames: Tuple[str, ...]
+    # A map from a fixture name in the transitive closure to the FixtureDefs
+    # matching the name which are applicable to this function.
+    # There may be multiple overriding fixtures with the same name. The
+    # sequence is ordered from furthest to closes to the function.
+    name2fixturedefs: Dict[str, Sequence["FixtureDef[Any]"]]
+
+    # topological rank / does not honor scope
+    _rank: MutableMapping[Tuple[str, int], int]
+    # reusable ranks. get populated by removing a graph node
+    _reuse: MutableSet[int]
+    # outgoing edges of the graph
+    _outgoing: MutableMapping[Tuple[str, int], MutableMapping[Tuple[str, int], None]]
+    # incoming edges of the grpah
+    _incoming: MutableMapping[Tuple[str, int], MutableMapping[Tuple[str, int], None]]
+    # backing store for name_closure. this is only needed because several
+    # objects just take references to it.
+    _closure_store: List[str]
+
+    def __init__(self, argnames: Tuple[str, ...], initialnames: Tuple[str, ...]):
+        self.argnames = argnames
+        self.initialnames = initialnames
+        # populate while building
+        self.name2fixturedefs = {}
+
+        self._rank = defaultdict(
+            lambda: self._reuse.pop() if self._reuse else len(self._rank)
+        )
+        self._outgoing = defaultdict(defaultdict)
+        self._incoming = defaultdict(defaultdict)
+        self._reuse = set()
+
+    def clone(self) -> "FuncFixtureInfo2":
+        clone = FuncFixtureInfo2(self.argnames, copy.copy(self.initialnames))
+        clone.name2fixturedefs = self.name2fixturedefs
+        # maybe reassign ranks to tighten assigned ranks again and to empty reuse
+        # shallow copy is enough for now
+        clone._rank = copy.copy(self._rank)
+        clone._reuse = copy.copy(self._reuse)
+        # deepcopy these
+        clone._outgoing = copy.deepcopy(self._outgoing)
+        clone._outgoing = copy.deepcopy(self._outgoing)
+        return clone
+
+    # The transitive closure of the fixture names that the item requires.
+    # Note: can't include dynamic dependencies (`request.getfixturevalue` calls).
+    @property
+    def names_closure(self) -> List[str]:
+        self._closure_store[:] = [
+            name for name, _ in self.name_idx_closure(reverse=True)
+        ]
+        return self._closure_store
+
+    def remove_fixture(self, node: Tuple[str, int]) -> None:
+        # remove all edges from/to the node, remove the node and mark the rank
+        # of the node for reuse
+        for next in self._outgoing[node]:
+            del self._incoming[next][node]
+        for previous in self._incoming[node]:
+            del self._outgoing[previous][node]
+        del self._incoming[node]
+        del self._outgoing[node]
+        self._reuse.add(self._rank.pop(node))
+
+    def name_idx_closure(self, reverse: bool = False) -> Sequence[Tuple[str, int]]:
+        # closure in topological sort order / scope is not honored
+        return sorted(self._rank, key=self._rank.__getitem__)
+
+    def fixturedef_closure(self, reverse: bool = False) -> Sequence["FixtureDef[Any]"]:
+        # closure in topogical sort order and proper scope order
+        # unless the closure already was ordered by scope this is a different
+        # topocoligal sort order!
+        return sorted(
+            (self.name2fixturedefs[name][idx] for name, idx in self.name_idx_closure()),
+            key=lambda d: d._scope,
+        )
+
+    def add_node(self, node: Tuple[str, int]):
+        # triggers insertion if the node does not already exist
+        self._rank[node]
+
+    def add_dependency(self, source: Tuple[str, int], dest: Tuple[str, int]) -> None:
+        if source == dest:
+            raise CyclicDependency
+
+        if dest in self._outgoing[source] or source in self._incoming[dest]:
+            # if the invariant holds only one check is necessary since _incoming
+            # is the adjunct to _outgoing and so they mirror each other.
+            raise DuplicateDependency
+
+        # apprently nested default dicts behave a little funny.
+        self._outgoing[source][dest] = None
+        self._incoming[dest][source] = None
+
+        if self._rank[source] > self._rank[dest]:
+            # access to the nodes triggers insertion of the nodes if they do not exist already.
+            # these are kept whether the edge is inserted or not. Similarly, empty entries in
+            # _outgoing and _incoming are kept. To remove these orphaned/unconnected entries call `prune`
+            try:
+                delta_f = [
+                    *self._depth_first_traversal(
+                        dest, self._outgoing, self._fwd_cond(source)
+                    )
+                ]
+            except CyclicDependency:
+                self.remove_dependency(source, dest)
+                raise
+            else:
+                delta_b = [
+                    *self._depth_first_traversal(
+                        source, self._incoming, self._bwd_cond(dest)
+                    )
+                ]
+                delta_f.sort(key=self._rank.__getitem__)
+                delta_b.sort(key=self._rank.__getitem__)
+
+                ranks = sorted(
+                    self._rank[node] for node in itertools.chain(delta_f, delta_b)
+                )
+
+                for node, rank in zip(itertools.chain(delta_b, delta_f), ranks):
+                    self._rank[node] = rank
+
+    def remove_dependency(self, source: Tuple[str, int], dest: Tuple[str, int]) -> None:
+        del self._outgoing[source][dest]
+        del self._incoming[dest][source]
+
+    def _fwd_cond(self, node: Tuple[str, int]) -> Callable[[Tuple[str, int]], bool]:
+        bound = self._rank[node]
+
+        def predicate(next: Tuple[str, int]) -> bool:
+            rank = self._rank[next]
+            if rank == bound:
+                raise CyclicDependency(node, next)
+            return rank < bound
+
+        return predicate
+
+    def _bwd_cond(self, node: Tuple[str, int]) -> Callable[[Tuple[str, int]], bool]:
+        bound = self._rank[node]
+
+        def predicate(next: Tuple[str, int]) -> bool:
+            return self._rank[next] > bound
+
+        return predicate
+
+    def _prune(self, roots: Collection[Tuple[str, int]]):
+        # remove all nodes not connected to any root
+        while unconnected := {
+            node
+            for node, dests in self._incoming.items()
+            if node not in roots and not dests
+        }:
+            for node in unconnected:
+                self.remove_fixture(node)
+        for node in {
+            node
+            for node in self._rank
+            if node not in self._outgoing and node not in self._incoming
+        }:
+            self.remove_fixture(node)
+
+    def _compress(self):
+        for i, k in enumerate(self.name_idx_closure()):
+            self._rank[k] = i
+        self._reuse.clear()
+
+    def prune(self):
+        # usage index -1 mirrors current behaviour which has some bugs concerning autouse
+        self._prune({(f, -1) for f in self.initialnames})
+
+    # keep the old interface aroud
+    def prune_dependency_tree(self):
+        self.prune()
+        # update the names_closure store
+        _ = self.names_closure
+
+    @staticmethod
+    def _depth_first_traversal(
+        start: Tuple[str, int],
+        edges: Mapping[Tuple[str, int], Mapping[Tuple[str, int], None]],
+        valid: Callable[[Tuple[str, int]], bool] = lambda _: True,
+    ) -> Iterator[Tuple[str, int]]:
+        stack: list[Tuple[str, int]] = [start]
+        visited: set[Tuple[str, int]] = set()
+
+        while stack:
+            node = stack.pop()
+
+            if node in visited:
+                continue
+            visited.add(node)
+            yield node
+
+            # to keep the proper order we need to iterate in reverse
+            for next in reversed(edges[node].keys()):
+                if valid(next) and next not in visited:
+                    stack.append(next)
+
+
 class FixtureRequest(abc.ABC):
     """The type of the ``request`` fixture.
 
@@ -1493,6 +1735,36 @@ class FixtureManager:
 
         return FuncFixtureInfo(argnames, initialnames, names_closure, arg2fixturedefs)
 
+    def getfixtureinfo2(
+        self,
+        node: nodes.Item,
+        func: Callable[..., object],
+        cls: Optional[type],
+        funcargs: bool = True,
+    ) -> FuncFixtureInfo2:
+        """See above"""
+        if funcargs and not getattr(node, "nofuncargs", False):
+            argnames = getfuncargnames(func, name=node.name, cls=cls)
+        else:
+            argnames = ()
+
+        usefixtures = tuple(
+            arg for mark in node.iter_markers(name="usefixtures") for arg in mark.args
+        )
+        autousefixtures = tuple(self._getautousenames(node.nodeid))
+        # keep the previous order.
+        # #in most cases this results in a suboptimal order later on
+        initialnames = autousefixtures + usefixtures + argnames
+        # unique initialnames >=py3.6 / optimize later
+        initialnames = tuple(dict.fromkeys(initialnames))
+        # fixturenames to skip because they would be replaced later on anyway
+        # convert to set for faster lookup
+        skipnames = set(_get_direct_parametrize_args(node))
+
+        info2 = FuncFixtureInfo2(argnames=argnames, initialnames=initialnames)
+        self.buildfixtureclosure(info2, node.nodeid, skipnames)
+        return info2
+
     def pytest_plugin_registered(self, plugin: _PluggyPlugin) -> None:
         nodeid = None
         try:
@@ -1570,6 +1842,91 @@ class FixtureManager:
         fixturenames_closure.sort(key=sort_by_scope, reverse=True)
         return fixturenames_closure, arg2fixturedefs
 
+    def buildfixtureclosure(
+        self,
+        info: FuncFixtureInfo2,
+        nodeid: str,
+        ignore: Collection[str],
+    ) -> None:
+        # TODO:
+        # - reword last sentence
+        # - rethink/get advice for error handling and reporting
+
+        # Walk the fixture definitions starting from the initalnames given by the
+        # info object to build the dependency DAG (directed acyclic graph).
+        # The reversed topological sort of the DAG is equilvalent to the previous
+        # fixture_closure but is not ordered by scope. But, because the algorithm
+        # enforces the invariant that a fixtures can only depend on fixtures of
+        # the same or higher scope, sorting by scope produces another topological
+        # sort.
+        # Note that the insertion algortihm is not stable concerning the scope ordering
+        cache: Dict[str, Sequence[FixtureDef[Any]]] = info.name2fixturedefs
+        iter_stack: List[Iterator[str]] = [iter(info.initialnames)]
+        parent_stack: List[Tuple[str, int]] = []
+
+        def retrieve(name: str) -> Sequence[FixtureDef[Any]] | None:
+            try:
+                return cache.get(name, None)
+            except KeyError:
+                defs = self.getfixturedefs(name, nodeid)
+                if defs:
+                    cache[name] = defs
+                    return defs
+                else:
+                    return None
+
+        while iter_stack:
+            arg = next(iter_stack[-1], None)
+            if arg:
+                if arg in ignore:
+                    continue
+                defs = retrieve(arg)
+                if not defs:
+                    # for now continue if no definitions are found
+                    # this should be guarded against by giving appropriate ignores
+                    continue
+                if parent_stack:
+                    parent = parent_stack[-1]
+                    # entry unconditionally exists at this point
+                    parent_scope = cast(Sequence[FixtureDef[Any]], retrieve(parent[0]))[
+                        parent[1]
+                    ]._scope
+                    for i in range(-1, -len(defs) - 1, -1):
+                        try:
+                            info.add_dependency(parent, (arg, i))
+                        except CyclicDependency:
+                            continue
+                        except DuplicateDependency:
+                            break
+                        else:
+                            if defs[i]._scope < parent_scope:
+                                info.remove_dependency(parent, (arg, i))
+                                raise Exception("Scope violation: ...")
+                            # check if the fixture is already in the stack to bound
+                            # the stack growth.
+                            # this check is not a sufficient condition for termination
+                            # of the algorithm.
+                            if (arg, i) not in parent_stack:
+                                parent_stack.append((arg, i))
+                                iter_stack.append(iter(defs[i].argnames))
+                            break
+                    else:
+                        raise Exception("Cycle Detected: ...")
+                else:
+                    parent_stack.append((arg, -1))
+                    iter_stack.append(iter(defs[-1].argnames))
+                    # make sure the initalnames are actually inserted even if they
+                    # have no dependencies
+                    info.add_node((arg, -1))
+            else:
+                # remove the exhausted iterator and backtrack
+                iter_stack.pop()
+                if parent_stack:
+                    parent_stack.pop()
+        # remove potenially orphened nodes from failed insertions
+        info.prune()
+        info._compress()
+
     def pytest_generate_tests(self, metafunc: "Metafunc") -> None:
         """Generate new tests based on parametrized fixtures used by the given metafunc"""