deprecation: new package for deprecations

This package enumerates the known deprecations in the current version of
containerd.  New deprecations should be added here, and old ones
removed.

Signed-off-by: Samuel Karp <samuelkarp@google.com>

pkg/deprecation/deprecation.go

@@ -0,0 +1,46 @@
+/*
+   Copyright The containerd Authors.
+
+   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 deprecation
+
+type Warning string
+
+const (
+	// Prefix is a standard prefix for all Warnings, used for filtering plugin Exports
+	Prefix = "io.containerd.deprecation/"
+	// PullSchema1Image is a warning for the use of schema 1 images
+	PullSchema1Image Warning = Prefix + "pull-schema-1-image"
+	// GoPluginLibrary is a warning for the use of dynamic library Go plugins
+	GoPluginLibrary Warning = Prefix + "go-plugin-library"
+)
+
+var messages = map[Warning]string{
+	PullSchema1Image: "Schema 1 images are deprecated since containerd v1.7 and removed in containerd v2.0. " +
+		`Since containerd v1.7.8, schema 1 images are identified by the "io.containerd.image/converted-docker-schema1" label.`,
+	GoPluginLibrary: "Dynamically-linked Go plugins as containerd runtimes are deprecated since containerd v2.0 and removed in containerd v2.1.",
+}
+
+// Valid checks whether a given Warning is valid
+func Valid(id Warning) bool {
+	_, ok := messages[id]
+	return ok
+}
+
+// Message returns the human-readable message for a given Warning
+func Message(id Warning) (string, bool) {
+	msg, ok := messages[id]
+	return msg, ok
+}