support kapp config within configmap

docs/config.md

@@ -283,3 +283,32 @@ Path specifies location within a resource (as used `rebaseRules` and `ownershipL
 ```
 [spec, volumeClaimTemplates, {index: 0}, metadata, labels]
 ```
+
+---
+### Config wrapped in ConfigMap
+
+Available of v0.34.0+.
+
+Config resource could be wrapped in a ConfigMap to support same deployment configuration by tools that do not understand kapp's `Config` resource directly. ConfigMap carrying kapp config must to be labeled with `kapp.k14s.io/config` and have `config.yml` data key. Such config maps will be applied to the cluster, unlike config given as `Config` resource.
+
+```yaml
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: my-kapp-config
+  labels:
+    kapp.k14s.io/config: ""
+data:
+  config.yml: |
+    apiVersion: kapp.k14s.io/v1alpha1
+    kind: Config
+    rebaseRules:
+    - path: [rules]
+      type: copy
+      sources: [existing, new]
+      resourceMatchers:
+      - notMatcher:
+          matcher:
+            emptyFieldMatcher:
+              path: [aggregationRule]
+```

pkg/kapp/config/conf.go

@@ -7,6 +7,12 @@ import (
 	"fmt"
 
 	ctlres "github.com/k14s/kapp/pkg/kapp/resources"
+	corev1 "k8s.io/api/core/v1"
+)
+
+const (
+	configLabelKey     = "kapp.k14s.io/config"
+	configMapConfigKey = "config.yml"
 )
 
 type Conf struct {
@@ -18,25 +24,64 @@ func NewConfFromResources(resources []ctlres.Resource) ([]ctlres.Resource, Conf,
 	var configs []Config
 
 	for _, res := range resources {
-		if res.APIVersion() == configAPIVersion {
-			if res.Kind() == configKind {
-				config, err := NewConfigFromResource(res)
-				if err != nil {
-					return nil, Conf{}, err
-				}
-				configs = append(configs, config)
-			} else {
-				errMsg := "Unexpected kind in resource '%s', wanted '%s'"
-				return nil, Conf{}, fmt.Errorf(errMsg, res.Description(), configKind)
+		_, isLabeledAsConfig := res.Labels()[configLabelKey]
+
+		switch {
+		case res.APIVersion() == configAPIVersion:
+			config, err := NewConfigFromResource(res)
+			if err != nil {
+				return nil, Conf{}, fmt.Errorf(
+					"Parsing resource '%s' as kapp config: %s", res.Description(), err)
 			}
-		} else {
+			configs = append(configs, config)
+
+		case isLabeledAsConfig:
+			config, err := newConfigFromConfigMapRes(res)
+			if err != nil {
+				return nil, Conf{}, fmt.Errorf(
+					"Parsing resource '%s' labeled as kapp config: %s", res.Description(), err)
+			}
+			// Make sure to add ConfigMap resource to regular resources list
+			// (our goal of allowing kapp config in ConfigMaps is to allow
+			// both kubectl and kapp to work against exactly same configuration;
+			// hence want to preserve same behaviour)
+			rsWithoutConfigs = append(rsWithoutConfigs, res)
+			configs = append(configs, config)
+
+		default:
 			rsWithoutConfigs = append(rsWithoutConfigs, res)
 		}
 	}
 
 	return rsWithoutConfigs, Conf{configs}, nil
 }
 
+func newConfigFromConfigMapRes(res ctlres.Resource) (Config, error) {
+	if res.APIVersion() != "v1" || res.Kind() != "ConfigMap" {
+		errMsg := "Expected kapp config to be within v1/ConfigMap but apiVersion or kind do not match"
+		return Config{}, fmt.Errorf(errMsg, res.Description())
+	}
+
+	configCM := corev1.ConfigMap{}
+
+	err := res.AsTypedObj(&configCM)
+	if err != nil {
+		return Config{}, fmt.Errorf("Converting resource to ConfigMap: %s", err)
+	}
+
+	configStr, found := configCM.Data[configMapConfigKey]
+	if !found {
+		return Config{}, fmt.Errorf("Expected to find field 'data.\"%s\"', but did not", configMapConfigKey)
+	}
+
+	configRes, err := ctlres.NewResourceFromBytes([]byte(configStr))
+	if err != nil {
+		return Config{}, fmt.Errorf("Parsing kapp config as resource: %s", err)
+	}
+
+	return NewConfigFromResource(configRes)
+}
+
 func (c Conf) RebaseMods() []ctlres.ResourceModWithMultiple {
 	var mods []ctlres.ResourceModWithMultiple
 	for _, config := range c.configs {

pkg/kapp/config/config.go

@@ -108,6 +108,18 @@ type ChangeRuleBinding struct {
 }
 
 func NewConfigFromResource(res ctlres.Resource) (Config, error) {
+	if res.APIVersion() != configAPIVersion {
+		return Config{}, fmt.Errorf(
+			"Expected kapp config to have apiVersion '%s', but was '%s'",
+			configAPIVersion, res.APIVersion())
+	}
+
+	if res.Kind() != configKind {
+		return Config{}, fmt.Errorf(
+			"Expected kapp config to have kind '%s', but was '%s'",
+			configKind, res.Kind())
+	}
+
 	bs, err := res.AsYAMLBytes()
 	if err != nil {
 		return Config{}, err

test/e2e/config_test.go

@@ -29,14 +29,26 @@ rebaseRules:
       kind: ConfigMap
       namespace: kapp-test
       name: first
-- path: [data, keep]
-  type: copy
-  sources: [existing, new]
-  resourceMatchers:
-  - kindNamespaceNameMatcher:
-      kind: ConfigMap
-      namespace: kapp-test
-      name: second
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: kapp-config
+  labels:
+    kapp.k14s.io/config: ""
+data:
+  config.yml: |
+    apiVersion: kapp.k14s.io/v1alpha1
+    kind: Config
+    rebaseRules:
+    - path: [data, keep]
+      type: copy
+      sources: [existing, new]
+      resourceMatchers:
+      - kindNamespaceNameMatcher:
+          kind: ConfigMap
+          namespace: kapp-test
+          name: second
 `
 
 	yaml1 := `