From cd55fd4bd3af0dcb1c92954ef36eb2a3e530b505 Mon Sep 17 00:00:00 2001
From: Todd Gamblin <tgamblin@llnl.gov>
Date: Sun, 19 Apr 2020 12:26:47 -0700
Subject: concretizer: allow non-default OS, inherit OS along dependencies

---
 lib/spack/spack/solver/asp.py        | 20 +++++++++++++++++---
 lib/spack/spack/solver/concretize.lp | 24 +++++++++++++-----------
 2 files changed, 30 insertions(+), 14 deletions(-)

diff --git a/lib/spack/spack/solver/asp.py b/lib/spack/spack/solver/asp.py
index 8d08245812..146f816c5a 100644
--- a/lib/spack/spack/solver/asp.py
+++ b/lib/spack/spack/solver/asp.py
@@ -19,6 +19,7 @@ import llnl.util.tty as tty
 import llnl.util.tty.color as color
 
 import spack
+import spack.architecture
 import spack.cmd
 import spack.config
 import spack.dependency
@@ -617,9 +618,22 @@ class AspGenerator(object):
         self.fact(fn.node_platform_default(default.platform))
 
     def os_defaults(self, specs):
-        self.h2('Default operating system')
-        default = default_arch()
-        self.fact(fn.node_os_default(default.os))
+        self.h2('Possible operating systems')
+        platform = spack.architecture.platform()
+
+        # create set of OS's to consider
+        possible = set([
+            platform.front_os, platform.back_os, platform.default_os])
+        for spec in specs:
+            if spec.architecture and spec.architecture.os:
+                possible.add(spec.architecture.os)
+
+        # make directives for possible OS's
+        for os in sorted(possible):
+            self.fact(fn.os(os))
+
+        # mark this one as default
+        self.fact(fn.node_os_default(platform.default_os))
 
     def target_defaults(self, specs):
         """Add facts about targets and target compatibility."""
diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index f0f706c83a..daf0c23ac7 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -169,22 +169,24 @@ node_platform_set(Package) :- node_platform_set(Package, _).
 % OS semantics
 %-----------------------------------------------------------------------------
 % one os per node
-1 { node_os(Package, OS) : node_os(Package, OS) } 1 :- node(Package).
+1 { node_os(Package, OS) : os(OS) } 1 :- node(Package).
 
-% node_os_set implies that the node has that os
+% node_os_set implies that the node must have that os
 node_os(Package, OS) :- node(Package), node_os_set(Package, OS).
 node_os_set(Package) :- node_os_set(Package, _).
 
-% fall back to default if not set
+% inherit OS along dependencies
+node_os_inherit(Package, OS) :- node_os_set(Package, OS).
+node_os_inherit(Dependency, OS)
+  :- node_os_inherit(Package, OS), depends_on(Package, Dependency),
+     not node_os_set(Dependency).
+node_os_inherit(Package) :- node_os_inherit(Package, _).
+
+% fall back to default if not set or inherited
 node_os(Package, OS)
-  :- node(Package), not node_os_set(Package), node_os_default(OS).
-
-% same OS throughout DAG (can currently only set one OS)
-% TODO make this smarter; build env may have a different os from run env
-node_os_set(Package, OS)
-  :- depends_on(Package, Dependency), node_os_set(Dependency, OS).
-node_os_set(Dependency, OS)
-  :- depends_on(Package, Dependency), node_os_set(Package, OS).
+  :- node(Package),
+     not node_os_set(Package), not node_os_inherit(Package),
+     node_os_default(OS).
 
 #defined node_os_set/2.
 
-- 
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