Add lists of modifiers to allow for modifier compositioning

Signed-off-by: Evan Lezar <elezar@nvidia.com>

internal/modifier/discover.go

@@ -0,0 +1,53 @@
+/**
+# Copyright (c) 2022, NVIDIA CORPORATION.  All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+**/
+
+package modifier
+
+import (
+	"fmt"
+
+	"github.com/NVIDIA/nvidia-container-toolkit/internal/discover"
+	"github.com/NVIDIA/nvidia-container-toolkit/internal/edits"
+	"github.com/NVIDIA/nvidia-container-toolkit/internal/oci"
+	"github.com/opencontainers/runtime-spec/specs-go"
+	"github.com/sirupsen/logrus"
+)
+
+type discoverModifier struct {
+	logger     *logrus.Logger
+	discoverer discover.Discover
+}
+
+// NewModifierFromDiscoverer creates a modifier that applies the discovered
+// modifications to an OCI spec if required by the runtime wrapper.
+func NewModifierFromDiscoverer(logger *logrus.Logger, d discover.Discover) (oci.SpecModifier, error) {
+	m := discoverModifier{
+		logger:     logger,
+		discoverer: d,
+	}
+	return &m, nil
+}
+
+// Modify applies the modifications required by discoverer to the incomming OCI spec.
+// These modifications are applied in-place.
+func (m discoverModifier) Modify(spec *specs.Spec) error {
+	specEdits, err := edits.NewSpecEdits(m.logger, m.discoverer)
+	if err != nil {
+		return fmt.Errorf("failed to get required container edits: %v", err)
+	}
+
+	return specEdits.Modify(spec)
+}

internal/modifier/list.go

@@ -0,0 +1,54 @@
+/**
+# Copyright (c) 2022, NVIDIA CORPORATION.  All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+**/
+
+package modifier
+
+import (
+	"github.com/NVIDIA/nvidia-container-toolkit/internal/oci"
+	"github.com/opencontainers/runtime-spec/specs-go"
+)
+
+type list struct {
+	modifiers []oci.SpecModifier
+}
+
+// Merge merges a set of OCI specification modifiers as a list.
+// This can be used to compose modifiers.
+func Merge(modifiers ...oci.SpecModifier) oci.SpecModifier {
+	var filteredModifiers []oci.SpecModifier
+	for _, m := range modifiers {
+		if m == nil {
+			continue
+		}
+		filteredModifiers = append(filteredModifiers, m)
+	}
+
+	return list{
+		modifiers: filteredModifiers,
+	}
+}
+
+// Modify applies a list of modifiers in sequence and returns on any errors encountered.
+func (m list) Modify(spec *specs.Spec) error {
+	for _, mm := range m.modifiers {
+		err := mm.Modify(spec)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
+}