fix: correct tags filtering on consumer-group consumers (#88)

Due to the nature of the Consumer Group API, it's not possible
to natively filter its Consumers using tags:

```
$ http :8001/consumer_groups/foo_group_1/consumers\?tags="managed-by:deck"
HTTP/1.1 400 Bad Request

{
    "code": 11,
    "message": "invalid option (tags: cannot be used with 'consumer_group_consumers')",
    "name": "invalid options",
    "options": {
        "tags": "cannot be used with 'consumer_group_consumers'"
    }
}
```

So, in order to fetch ConsumerGroup:Consumer mappings, we need to rely
on the unfiltered Consumer Group API:

```
$ http :8001/consumer_groups/foo_group_1
HTTP/1.1 200 OK

{
    "consumer_group": {
        "created_at": 1715183554,
        "id": "e0e495f2-9acd-41f7-b4da-33659033af13",
        "name": "foo_group_1",
        "tags": [
            "managed-by-deck"
        ],
        "updated_at": 1715183554
    },
    "consumers": [
        {
            "created_at": 1715183563,
            "id": "a704b13e-1a9b-409b-9611-fd2bfcea63bb",
            "tags": [
                "managed-by:api"
            ],
            "type": 0,
            "updated_at": 1715183563,
            "username": "foo",
            "username_lower": "foo"
        }
    ]
}
```

This is a problem in cases when Consumer Groups are managed by decK
while their mappings with Consumers are managed directly via the API.
In such case, since the current library implementation doesn't allow
to filter mappings by tags, decK would wipe out the API managed
mappings when running against the specific tags it handle:

```
$ cat kong.yaml
_format_version: "3.0"
_info:
  select_tags:
  - managed-by:deck
consumer_groups:
- id: c0f6c818-470c-4df7-8515-c8e904765fcc
  name: foo_group_1
  tags:
  - managed-by:deck
```

```
$ deck gateway sync kong.yaml
deleting consumer-group-consumer foo
Summary:
  Created: 0
  Updated: 0
  Deleted: 1
```

This commit makes sure the correct filtering is "manually" done,
avoiding existing Consumers to be wiped out during syncs when using
Tags:

```
$ deck gateway sync kong.yaml
Summary:
  Created: 0
  Updated: 0
  Deleted: 0
```

```
$ http :8001/consumer_groups/foo_group_1
HTTP/1.1 200 OK

{
    "consumer_group": {
        "created_at": 1716450737,
        "id": "c0f6c818-470c-4df7-8515-c8e904765fcc",
        "name": "foo_group_1",
        "tags": [
            "managed-by:deck"
        ],
        "updated_at": 1716451431
    },
    "consumers": [
        {
            "created_at": 1716450861,
            "id": "bafd4f5d-3f4e-4c0f-8d6c-6901b0b8f483",
            "tags": [
                "managed-by:api"
            ],
            "type": 0,
            "updated_at": 1716450861,
            "username": "foo"
        }
    ]
}
```

---------

Co-authored-by: Patryk Małek <[email protected]>

pkg/dump/dump.go

@@ -647,9 +647,17 @@ func GetAllConsumerGroups(ctx context.Context,
 			}
 			group := &kong.ConsumerGroupObject{
 				ConsumerGroup: r.ConsumerGroup,
-				Consumers:     r.Consumers,
 				Plugins:       r.Plugins,
 			}
+			consumers := []*kong.Consumer{}
+			for _, c := range r.Consumers {
+				// if tags are set and if the consumer is not tagged, skip it
+				if len(tags) > 0 && !utils.HasTags(c, tags) {
+					continue
+				}
+				consumers = append(consumers, c)
+			}
+			group.Consumers = consumers
 			consumerGroupObjects = append(consumerGroupObjects, group)
 		}
 		if nextopt == nil {

pkg/utils/tags.go

@@ -3,6 +3,8 @@ package utils
 import (
 	"fmt"
 	"reflect"
+
+	"github.com/kong/go-kong/kong"
 )
 
 // MustMergeTags is same as MergeTags but panics if there is an error.
@@ -82,3 +84,31 @@ func RemoveTags(obj interface{}, tags []string) error {
 	structTags.Set(res)
 	return nil
 }
+
+// HasTags checks if the given object has any of the specified tags.
+// The function returns true if at least one of the provided tags is present in the object's tags.
+func HasTags[T *kong.Consumer](obj T, tags []string) bool {
+	if len(tags) == 0 {
+		return true
+	}
+
+	m := make(map[string]struct{})
+	for _, tag := range tags {
+		m[tag] = struct{}{}
+	}
+
+	switch obj := any(obj).(type) {
+	case *kong.Consumer:
+		for _, tag := range obj.Tags {
+			if tag == nil {
+				continue
+			}
+			if _, ok := m[*tag]; ok {
+				return true
+			}
+		}
+	default:
+		return false
+	}
+	return false
+}

pkg/utils/tags_test.go

@@ -3,6 +3,7 @@ package utils
 import (
 	"testing"
 
+	"github.com/kong/go-kong/kong"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -90,3 +91,33 @@ func TestRemoveTags(t *testing.T) {
 	RemoveTags(&f, nil)
 	assert.True(equalArray([]*string{&a, &b}, f.Tags))
 }
+
+func TestHasTags(t *testing.T) {
+	assert := assert.New(t)
+
+	assert.False(HasTags(&kong.Consumer{}, []string{"tag1"}))
+
+	consumer := &kong.Consumer{
+		Tags: []*string{
+			kong.String("tag1"),
+			kong.String("tag2"),
+		},
+	}
+	assert.True(HasTags(consumer, []string{"tag1"}))
+	assert.True(HasTags(consumer, []string{"tag1", "tag2"}))
+	assert.True(HasTags(consumer, []string{"tag1", "tag2", "tag3"}))
+	assert.False(HasTags(consumer, []string{"tag3"}))
+}
+
+func BenchmarkHasTags(b *testing.B) {
+	consumer := &kong.Consumer{
+		Tags: []*string{
+			kong.String("tag1"),
+			kong.String("tag2"),
+		},
+	}
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		HasTags(consumer, []string{"tag1", "tag2", "tag3"})
+	}
+}