ctr: print deprecation warnings on every invocation

Print deprecation warnings on any ctr command, as users won't notice the
deprecations until we actually remove the deprecated features.

The warnings can be suppressed by setting
`CONTAINERD_SUPPRESS_DEPRECATION_WARNINGS=1`.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 468bee9)
(cherry-pick was not clean)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/ctr/commands/client.go

@@ -18,11 +18,14 @@ package commands
 
 import (
 	gocontext "context"
+	"os"
+	"strconv"
 
 	"github.com/containerd/containerd"
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/namespaces"
 	"github.com/containerd/containerd/pkg/epoch"
+	ptypes "github.com/containerd/containerd/protobuf/types"
 	"github.com/urfave/cli"
 )
 
@@ -62,5 +65,22 @@ func NewClient(context *cli.Context, opts ...containerd.ClientOpt) (*containerd.
 		return nil, nil, nil, err
 	}
 	ctx, cancel := AppContext(context)
+	var suppressDeprecationWarnings bool
+	if s := os.Getenv("CONTAINERD_SUPPRESS_DEPRECATION_WARNINGS"); s != "" {
+		suppressDeprecationWarnings, err = strconv.ParseBool(s)
+		if err != nil {
+			log.L.WithError(err).Warn("Failed to parse CONTAINERD_SUPPRESS_DEPRECATION_WARNINGS=" + s)
+		}
+	}
+	if !suppressDeprecationWarnings {
+		resp, err := client.IntrospectionService().Server(ctx, &ptypes.Empty{})
+		if err != nil {
+			log.L.WithError(err).Warn("Failed to check deprecations")
+		} else {
+			for _, d := range resp.Deprecations {
+				log.L.Warn("DEPRECATION: " + d.Message)
+			}
+		}
+	}
 	return client, ctx, cancel, nil
 }

cmd/ctr/commands/deprecations/deprecations.go

@@ -47,6 +47,9 @@ var listCommand = cli.Command{
 		},
 	},
 	Action: func(context *cli.Context) error {
+		// Suppress automatic warnings, since we print the warnings by ourselves.
+		os.Setenv("CONTAINERD_SUPPRESS_DEPRECATION_WARNINGS", "1")
+
 		client, ctx, cancel, err := commands.NewClient(context)
 		if err != nil {
 			return err