From 6d69d23aa5574df501ac8bdccdeaaf0ecc550f1b Mon Sep 17 00:00:00 2001
From: Massimiliano Culpo <massimiliano.culpo@gmail.com>
Date: Thu, 21 Oct 2021 17:44:53 +0200
Subject: Add a unit test to prevent regression

---
 lib/spack/spack/test/cmd/dependencies.py               |  4 +++-
 lib/spack/spack/test/concretize.py                     | 14 ++++++++++++++
 lib/spack/spack/test/data/config/packages.yaml         |  3 ++-
 lib/spack/spack/test/package_class.py                  |  1 +
 .../packages/low-priority-provider/package.py          | 18 ++++++++++++++++++
 .../packages/many-virtual-consumer/package.py          | 18 ++++++++++++++++++
 6 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 var/spack/repos/builtin.mock/packages/low-priority-provider/package.py
 create mode 100644 var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py

diff --git a/lib/spack/spack/test/cmd/dependencies.py b/lib/spack/spack/test/cmd/dependencies.py
index 7ca1eae2e5..9d2ebccd9d 100644
--- a/lib/spack/spack/test/cmd/dependencies.py
+++ b/lib/spack/spack/test/cmd/dependencies.py
@@ -14,7 +14,9 @@ from spack.main import SpackCommand
 
 dependencies = SpackCommand('dependencies')
 
-mpis = ['mpich', 'mpich2', 'multi-provider-mpi', 'zmpi']
+mpis = [
+    'low-priority-provider', 'mpich', 'mpich2', 'multi-provider-mpi', 'zmpi'
+]
 mpi_deps = ['fake']
 
 
diff --git a/lib/spack/spack/test/concretize.py b/lib/spack/spack/test/concretize.py
index d30191f4f2..b2ac1b6446 100644
--- a/lib/spack/spack/test/concretize.py
+++ b/lib/spack/spack/test/concretize.py
@@ -1274,3 +1274,17 @@ class TestConcretize(object):
         # package doesn't end up using a later implementation
         s = spack.spec.Spec('hpcviewer@2019.02').concretized()
         assert s['java'].satisfies('virtual-with-versions@1.8.0')
+
+    @pytest.mark.regression('26866')
+    def test_non_default_provider_of_multiple_virtuals(self):
+        s = spack.spec.Spec(
+            'many-virtual-consumer ^low-priority-provider'
+        ).concretized()
+        assert s['mpi'].name == 'low-priority-provider'
+        assert s['lapack'].name == 'low-priority-provider'
+
+        for virtual_pkg in ('mpi', 'lapack'):
+            for pkg in spack.repo.path.providers_for(virtual_pkg):
+                if pkg.name == 'low-priority-provider':
+                    continue
+                assert pkg not in s
diff --git a/lib/spack/spack/test/data/config/packages.yaml b/lib/spack/spack/test/data/config/packages.yaml
index 84f470d208..7d5e06a74c 100644
--- a/lib/spack/spack/test/data/config/packages.yaml
+++ b/lib/spack/spack/test/data/config/packages.yaml
@@ -1,7 +1,8 @@
 packages:
   all:
     providers:
-      mpi: [openmpi, mpich]
+      mpi: [openmpi, mpich, zmpi]
+      lapack: [openblas-with-lapack]
       blas: [openblas]
   externaltool:
     buildable: False
diff --git a/lib/spack/spack/test/package_class.py b/lib/spack/spack/test/package_class.py
index 22939105a4..8c55b47ba4 100644
--- a/lib/spack/spack/test/package_class.py
+++ b/lib/spack/spack/test/package_class.py
@@ -30,6 +30,7 @@ def mpi_names(mock_repo_path):
 def mpileaks_possible_deps(mock_packages, mpi_names):
     possible = {
         'callpath': set(['dyninst'] + mpi_names),
+        'low-priority-provider': set(),
         'dyninst': set(['libdwarf', 'libelf']),
         'fake': set(),
         'libdwarf': set(['libelf']),
diff --git a/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py b/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py
new file mode 100644
index 0000000000..d0b768637f
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/low-priority-provider/package.py
@@ -0,0 +1,18 @@
+# Copyright 2013-2021 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)
+
+from spack import *
+
+
+class LowPriorityProvider(Package):
+    """Provides multiple virtuals but is low in the priority of clingo"""
+
+    homepage = "http://www.example.com"
+    url = "http://www.example.com/a-1.0.tar.gz"
+
+    version('1.0', '0123456789abcdef0123456789abcdef')
+
+    provides('lapack')
+    provides('mpi')
diff --git a/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py b/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py
new file mode 100644
index 0000000000..40da0f40e0
--- /dev/null
+++ b/var/spack/repos/builtin.mock/packages/many-virtual-consumer/package.py
@@ -0,0 +1,18 @@
+# Copyright 2013-2021 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)
+class ManyVirtualConsumer(Package):
+    """PAckage that depends on many virtual packages"""
+    url = "http://www.example.com/"
+    url = "http://www.example.com/2.0.tar.gz"
+
+    version('1.0', 'abcdef1234567890abcdef1234567890')
+
+    depends_on('mpi')
+    depends_on('lapack')
+
+    # This directive is an example of imposing a constraint on a
+    # dependency is that dependency is in the DAG. This pattern
+    # is mainly used with virtual providers.
+    depends_on('low-priority-provider@1.0', when='^low-priority-provider')
-- 
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