ASP-based solver: avoid cycles in clingo using hidden directive (#40720)

The code should be functonally equivalent to what it was before,
but now to avoid cycles by design we are using a "hidden"
feature of clingo

lib/spack/spack/solver/asp.py

@@ -8,7 +8,6 @@
 import enum
 import itertools
 import os
-import pathlib
 import pprint
 import re
 import types
@@ -889,14 +888,6 @@ def on_model(model):
 
         timer.start("solve")
         solve_result = self.control.solve(**solve_kwargs)
-
-        if solve_result.satisfiable and self._model_has_cycles(models):
-            tty.debug(f"cycles detected, falling back to slower algorithm [specs={specs}]")
-            self.control.load(os.path.join(parent_dir, "cycle_detection.lp"))
-            self.control.ground([("no_cycle", [])])
-            models.clear()
-            solve_result = self.control.solve(**solve_kwargs)
-
         timer.stop("solve")
 
         # once done, construct the solve result
@@ -950,26 +941,6 @@ def on_model(model):
 
         return result, timer, self.control.statistics
 
-    def _model_has_cycles(self, models):
-        """Returns true if the best model has cycles in it"""
-        cycle_detection = clingo.Control()
-        parent_dir = pathlib.Path(__file__).parent
-        lp_file = parent_dir / "cycle_detection.lp"
-
-        min_cost, best_model = min(models)
-        with cycle_detection.backend() as backend:
-            for atom in best_model:
-                if atom.name == "attr" and str(atom.arguments[0]) == '"depends_on"':
-                    symbol = fn.depends_on(atom.arguments[1], atom.arguments[2])
-                    atom_id = backend.add_atom(symbol.symbol())
-                    backend.add_rule([atom_id], [], choice=False)
-
-            cycle_detection.load(str(lp_file))
-            cycle_detection.ground([("base", []), ("no_cycle", [])])
-            cycle_result = cycle_detection.solve()
-
-        return cycle_result.unsatisfiable
-
 
 class ConcreteSpecsByHash(collections.abc.Mapping):
     """Mapping containing concrete specs keyed by DAG hash.

lib/spack/spack/solver/concretize.lp

@@ -1325,6 +1325,10 @@ build_priority(PackageNode, 0)   :- not build(PackageNode), attr("node", Package
 
 #defined installed_hash/2.
 
+% This statement, which is a hidden feature of clingo, let us avoid cycles in the DAG
+#edge (A, B) : depends_on(A, B).
+
+
 %-----------------------------------------------------------------
 % Optimization to avoid errors
 %-----------------------------------------------------------------

lib/spack/spack/solver/cycle_detection.lp

@@ -1,21 +0,0 @@
-% Copyright 2013-2023 Lawrence Livermore National Security, LLC and other
-% Spack Project Developers. See the top-level COPYRIGHT file for details.
-%
-% SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-%=============================================================================
-% Avoid cycles in the DAG
-%
-% Some combinations of conditional dependencies can result in cycles;
-% this ensures that we solve around them. Note that these rules are quite
-% demanding on both grounding and solving, since they need to compute and
-% consider all possible paths between pair of nodes.
-%=============================================================================
-
-
-#program no_cycle.
-path(Parent, Child) :- depends_on(Parent, Child).
-path(Parent, Descendant) :- path(Parent, A), depends_on(A, Descendant).
-:- path(A, A).
-
-#defined depends_on/2.