Allow sections of Plugins to be merged, and not overwritten as entire sections.

See for rationale the Pull Request description. Added unit test to
demonstrate the difference of this change.

Signed-off-by: Ray Burgemeestre <rayb@nvidia.com>

cmd/containerd/server/config/config.go

@@ -412,10 +412,6 @@ func mergeConfig(to, from *Config) error {
 	}
 
 	// Replace entire sections instead of merging map's values.
-	for k, v := range from.Plugins {
-		to.Plugins[k] = v
-	}
-
 	for k, v := range from.StreamProcessors {
 		to.StreamProcessors[k] = v
 	}

cmd/containerd/server/config/config_test.go

@@ -17,12 +17,17 @@
 package config
 
 import (
+	"bytes"
 	"context"
+	"fmt"
 	"os"
 	"path/filepath"
 	"sort"
+	"strings"
 	"testing"
 
+	"github.com/pelletier/go-toml/v2"
+
 	"github.com/stretchr/testify/assert"
 
 	"github.com/containerd/containerd/v2/version"
@@ -260,3 +265,71 @@ func TestDecodePluginInV1Config(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, true, pluginConfig["shim_debug"])
 }
+
+func TestMergingPluginsWithTwoCriDropInConfigs(t *testing.T) {
+	data1 := `
+[plugins."io.containerd.grpc.v1.cri".cni]
+    bin_dir = "/cm/local/apps/kubernetes/current/bin/cni"
+`
+	data2 := `
+[plugins."io.containerd.grpc.v1.cri".registry]
+    config_path = "/cm/local/apps/containerd/var/etc/certs.d"
+`
+	expected := `
+[cni]
+  bin_dir = '/cm/local/apps/kubernetes/current/bin/cni'
+
+[registry]
+  config_path = '/cm/local/apps/containerd/var/etc/certs.d'
+`
+
+	testMergeConfig(t, []string{data1, data2}, expected, "io.containerd.grpc.v1.cri")
+	testMergeConfig(t, []string{data2, data1}, expected, "io.containerd.grpc.v1.cri")
+}
+
+func TestMergingPluginsWithTwoCriCniDropInConfigs(t *testing.T) {
+	data1 := `
+[plugins."io.containerd.grpc.v1.cri".cni]
+    bin_dir = "/cm/local/apps/kubernetes/current/bin/cni"
+`
+	data2 := `
+[plugins."io.containerd.grpc.v1.cri".cni]
+    conf_dir = "/tmp"
+`
+	expected := `
+[cni]
+  bin_dir = '/cm/local/apps/kubernetes/current/bin/cni'
+  conf_dir = '/tmp'
+`
+	testMergeConfig(t, []string{data1, data2}, expected, "io.containerd.grpc.v1.cri")
+}
+
+func testMergeConfig(t *testing.T, inputs []string, expected string, comparePlugin string) {
+	tempDir := t.TempDir()
+	var result Config
+
+	for i, data := range inputs {
+		filename := fmt.Sprintf("data%d.toml", i+1)
+		filepath := filepath.Join(tempDir, filename)
+		err := os.WriteFile(filepath, []byte(data), 0600)
+		assert.NoError(t, err)
+
+		var tempOut Config
+		err = LoadConfig(context.Background(), filepath, &tempOut)
+		assert.NoError(t, err)
+
+		if i == 0 {
+			result = tempOut
+		} else {
+			err = mergeConfig(&result, &tempOut)
+			assert.NoError(t, err)
+		}
+	}
+
+	criPlugin := result.Plugins[comparePlugin]
+	var buf bytes.Buffer
+	if err := toml.NewEncoder(&buf).SetIndentTables(true).Encode(criPlugin); err != nil {
+		panic(err)
+	}
+	assert.Equal(t, strings.TrimLeft(expected, "\n"), buf.String())
+}