config: add a new `concretizer` config section

The concretizer is going to grow to have many more configuration,
and we really need some structured config for that.

* We have the `config:concretizer` option that chooses the solver,
  but extending that is awkward (we'd need to replace a string with
  a `dict`) and the solver choice will be deprecated eventually.

* We have the `concretization` option in environments, but it's
  not a top-level config section -- it's just for environments,
  and it also only admits a string right now.

To avoid overlapping with either of these and to allow the most
extensibility in the future, this adds a new `concretizer` config
section that can be used in and outside of environments. There
is only one option right now: `reuse`.  This can expand to include
other options later.

Likely, we will soon deprecate `config:concretizer` and warn when
the user doesn't use `clingo`, and we will eventually (sometime later)
move the `together` / `separately` options from `concretization` into
the top-level `concretizer` section.

This commit just adds the new section and schema. Fully wiring it
up is TBD.

5 files changed, 65 insertions, 6 deletions

diff --git a/etc/spack/defaults/concretizer.yaml b/etc/spack/defaults/concretizer.yaml
new file mode 100644
index 0000000000..52200062c8
--- /dev/null
+++ b/etc/spack/defaults/concretizer.yaml
@@ -0,0 +1,17 @@
+# -------------------------------------------------------------------------
+# This is the default spack configuration file.
+#
+# Settings here are versioned with Spack and are intended to provide
+# sensible defaults out of the box. Spack maintainers should edit this
+# file to keep it current.
+#
+# Users can override these settings by editing
+# `$SPACK_ROOT/etc/spack/concretizer.yaml`, `~/.spack/concretizer.yaml`,
+# or by adding a `concretizer:` section to an environment.
+# -------------------------------------------------------------------------
+concretizer:
+  # Whether to consider installed packages or packages from buildcaches when
+  # concretizing specs. If `true`, we'll try to use as many installs/binaries
+  # as possible, rather than building. If `false`, we'll always give you a fresh
+  # concretization.
+  reuse: false
diff --git a/etc/spack/defaults/config.yaml b/etc/spack/defaults/config.yaml
index eb0d4fc409..d0d3468e8d 100644
--- a/etc/spack/defaults/config.yaml
+++ b/etc/spack/defaults/config.yaml
@@ -155,14 +155,17 @@ config:
 
   # The concretization algorithm to use in Spack. Options are:
   #
-  #   'original': Spack's original greedy, fixed-point concretizer. This
-  #       algorithm can make decisions too early and will not backtrack
-  #       sufficiently for many specs.
-  #
   #   'clingo': Uses a logic solver under the hood to solve DAGs with full
   #       backtracking and optimization for user preferences. Spack will
   #       try to bootstrap the logic solver, if not already available.
   #
+  #   'original': Spack's original greedy, fixed-point concretizer. This
+  #       algorithm can make decisions too early and will not backtrack
+  #       sufficiently for many specs. This will soon be deprecated in
+  #       favor of clingo.
+  #
+  # See `concretizer.yaml` for more settings you can fine-tune when
+  # using clingo.
   concretizer: clingo
 
 
diff --git a/lib/spack/spack/config.py b/lib/spack/spack/config.py
index 6062634ca5..f9e3f2125a 100644
--- a/lib/spack/spack/config.py
+++ b/lib/spack/spack/config.py
@@ -53,6 +53,7 @@ import spack.platforms
 import spack.schema
 import spack.schema.bootstrap
 import spack.schema.compilers
+import spack.schema.concretizer
 import spack.schema.config
 import spack.schema.env
 import spack.schema.mirrors
@@ -69,6 +70,7 @@ from spack.util.cpus import cpus_available
 #: Dict from section names -> schema for that section
 section_schemas = {
     'compilers': spack.schema.compilers.schema,
+    'concretizer': spack.schema.concretizer.schema,
     'mirrors': spack.schema.mirrors.schema,
     'repos': spack.schema.repos.schema,
     'packages': spack.schema.packages.schema,
@@ -101,7 +103,7 @@ config_defaults = {
         'dirty': False,
         'build_jobs': min(16, cpus_available()),
         'build_stage': '$tempdir/spack-stage',
-        'concretizer': 'original',
+        'concretizer': 'clingo',
     }
 }
 
diff --git a/lib/spack/spack/schema/concretizer.py b/lib/spack/spack/schema/concretizer.py
new file mode 100644
index 0000000000..e3bdde4adf
--- /dev/null
+++ b/lib/spack/spack/schema/concretizer.py
@@ -0,0 +1,30 @@
+# Copyright 2013-2022 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)
+
+"""Schema for concretizer.yaml configuration file.
+
+.. literalinclude:: _spack_root/lib/spack/spack/schema/concretizer.py
+   :lines: 13-
+"""
+
+properties = {
+    'concretizer': {
+        'type': 'object',
+        'additionalProperties': False,
+        'properties': {
+            'reuse': {'type': 'boolean'},
+        }
+    }
+}
+
+
+#: Full schema with metadata
+schema = {
+    '$schema': 'http://json-schema.org/draft-07/schema#',
+    'title': 'Spack concretizer configuration file schema',
+    'type': 'object',
+    'additionalProperties': False,
+    'properties': properties,
+}
diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index a87fa445f9..1403077997 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -2060,7 +2060,11 @@ class Solver(object):
         self.set_default_configuration()
 
     def set_default_configuration(self):
-        self.reuse = False
+        # These properties are settable via spack configuration. `None`
+        # means go with the configuration setting; user can override.
+        self.reuse = None
+
+        # these are concretizer settings
         self.dump = ()
         self.models = 0
         self.timers = False
@@ -2079,6 +2083,9 @@ class Solver(object):
                     continue
                 spack.spec.Spec.ensure_valid_variants(s)
 
+        if self.reuse is None:
+            self.reuse = spack.config.get("concretizer:reuse", False)
+
         setup = SpackSolverSetup(reuse=self.reuse, tests=self.tests)
         return driver.solve(
             setup, specs, self.dump, self.models, self.timers, self.stats