Add support for custom events

Add the ability to produce and consume custom events, including
testing of the custom/conformance.json from the spec.

This does not include validation against customSchemaUri yet,
that will be implemented as a separate feature as it applies to
both regular and custom events.

Signed-off-by: Andrea Frittoli <andrea.frittoli@gmail.com>

README.md

@@ -77,6 +77,14 @@ func main() {
 
 See the [CloudEvents](https://github.com/cloudevents/sdk-go#send-your-first-cloudevent) docs as well.
 
+## Documentation
+
+More examples are available in the [docs](./docs) folder.
+Online API Reference:
+- [SDK Root](https://pkg.go.dev/github.com/cdevents/sdk-go/pkg/api)
+- [v03 Specific](https://pkg.go.dev/github.com/cdevents/sdk-go/pkg/api/v03)
+- [v04 Specific](https://pkg.go.dev/github.com/cdevents/sdk-go/pkg/api/v04)
+
 ## Contributing
 
 If you would like to contribute, see our [development](DEVELOPMENT.md) guide.

docs/README.md

@@ -0,0 +1,117 @@
+# CDEvents Go SDK Docs
+
+This folder contains example of how to use this SDK.
+
+## Create a Custom CDEvent
+
+If a tool wants to emit events that are not supported by the CDEvents specification,
+they can do so via [custom events](https://github.com/cdevents/spec/tree/main/custom).
+
+Custom events are follow the CDEvents format and can be defined via the
+`CustomTypeEvent` object, available since v0.4, as well as using the `CustomCDEventReader`
+and `CustomCDEventWriter` interfaces.
+
+Let's consider the following scenario: a tool called "MyRegistry" has a concept of "Quota"
+which can be "exceeded" by users of the system. We want to use events to notify when that
+happens, but CDEvents does not define any quota related subject.
+
+```golang
+type Quota struct {
+	User string `json:"user,omitempty"` // The use the quota applies ot
+	Limit string `json:"limit,omitempty"` // The limit enforced by the quota e.g. 100Gb
+	Current int `json:"current,omitempty"` // The current % of the quota used e.g. 90%
+	Threshold int `json:"threshold,omitempty"` // The threshold for warning event e.g. 85%
+	Level string `json:"level,omitempty"` // INFO: <threshold, WARNING: >threshold, <quota, CRITICAL: >quota
+}
+```
+For this scenario we will need a few imports:
+
+```golang
+import (
+	"context"
+	"log"
+
+	cdevents "github.com/cdevents/sdk-go/pkg/api"
+	cdeventsv04 "github.com/cdevents/sdk-go/pkg/api/v04"
+	cloudevents "github.com/cloudevents/sdk-go/v2"
+)
+```
+
+Let's define a custom event type for this scenario.
+This is our first iteration, so the event will have version "0.1.0".
+
+```golang
+eventType := cdevents.CDEventType{
+    Subject:   "quota",
+    Predicate: "exceeded",
+    Version:   "0.1.0",
+    Custom:    "myregistry",
+}
+```
+
+With a `Quota` object, let's create a CDEvent for it:
+
+```golang
+quotaRule123 := Quota{
+    User:      "heavy_user",
+    Limit:     "50Tb",
+    Current:   90,
+    Threshold: 85,
+    Level:     "WARNING",
+}
+
+// Create the base event
+event, err := cdeventsv04.NewCustomTypeEvent()
+if err != nil {
+    log.Fatalf("could not create a cdevent, %v", err)
+}
+event.SetEventType(eventType)
+
+// Set the required context fields
+event.SetSubjectId("quotaRule123")
+event.SetSource("myregistry/region/staging")
+
+// Set the required subject content
+event.SetSubjectContent(quotaRule123)
+
+// If we host a schema for the overall custom CDEvent, we can add it
+// to the event so that the receiver may validate custom fields like
+// the event type and subject content
+event.SetSchemaUri("https://myregistry.dev/schemas/cdevents/quota-exceeded/0_1_0")
+```
+
+To see the event, let's render it as JSON and log it:
+
+```golang
+// Render the event as JSON
+eventJson, err := cdevents.AsJsonString(event)
+if err != nil {
+    log.Fatalf("failed to marshal the CDEvent, %v", err)
+}
+// Print the event
+log.Printf("event: %v", eventJson)
+```
+
+To send the event, let's setup a test sink, for instance using [smee.io/](https://smee.io/).
+Then let's render the event as CloudEvent and send it to the sink:
+
+```golang
+ce, err = cdevents.AsCloudEvent(event)
+if err != nil {
+    log.Fatalf("failed to create cloudevent, %v", err)
+}
+
+// Set send options
+ctx := cloudevents.ContextWithTarget(context.Background(), "https://smee.io/<you-channel-id>")
+ctx = cloudevents.WithEncodingBinary(ctx)
+
+c, err = cloudevents.NewClientHTTP()
+if err != nil {
+    log.Fatalf("failed to create client, %v", err)
+}
+
+// Send the CloudEvent
+if result := c.Send(ctx, *ce); cloudevents.IsUndelivered(result) {
+    log.Fatalf("failed to send, %v", result)
+}
+```

pkg/api/bindings.go

@@ -30,7 +30,10 @@ import (
 	"golang.org/x/mod/semver"
 )
 
-const SCHEMA_ID_REGEX = `^https://cdevents.dev/([0-9]\.[0-9])\.[0-9]/schema/([^ ]*)$`
+const (
+	SCHEMA_ID_REGEX   = `^https://cdevents.dev/([0-9]\.[0-9])\.[0-9]/schema/([^ ]*)$`
+	CustomEventMapKey = "custom"
+)
 
 var (
 	// Validation helper as singleton
@@ -172,24 +175,29 @@ func NewFromJsonBytesContext[CDEventType CDEvent](event []byte, cdeventsMap map[
 	eventAux := &struct {
 		Context Context `json:"context"`
 	}{}
-	var nilReturn CDEventType
+	var nilReturn, receiver CDEventType
+	var ok bool
 	err := json.Unmarshal(event, eventAux)
 	if err != nil {
 		return nilReturn, err
 	}
 	eventType := eventAux.Context.GetType()
-	receiver, ok := cdeventsMap[eventType.UnversionedString()]
-	if !ok {
-		// This should not happen as unmarshalling and validate checks if the type is known to the SDK
-		return nilReturn, fmt.Errorf("unknown event type %s", eventAux.Context.GetType())
-	}
-	// Check if the receiver is compatible. It must have the same subject and predicate
-	// and share the same major version.
-	// If the minor version is different and the message received as a version that is
-	// greater than the SDK one, some fields may be lost, as newer versions may add new
-	// fields to the event specification.
-	if !eventType.IsCompatible(receiver.GetType()) {
-		return nilReturn, fmt.Errorf("sdk event version %s not compatible with %s", receiver.GetType().Version, eventType.Version)
+	if eventType.Custom != "" {
+		receiver = cdeventsMap[CustomEventMapKey] // Custom type receiver does not have a predefined type
+	} else {
+		receiver, ok = cdeventsMap[eventType.UnversionedString()]
+		if !ok {
+			// This should not happen as unmarshalling and validate checks if the type is known to the SDK
+			return nilReturn, fmt.Errorf("unknown event type %s", eventAux.Context.GetType())
+		}
+		// Check if the receiver is compatible. It must have the same subject and predicate
+		// and share the same major version.
+		// If the minor version is different and the message received as a version that is
+		// greater than the SDK one, some fields may be lost, as newer versions may add new
+		// fields to the event specification.
+		if !eventType.IsCompatible(receiver.GetType()) {
+			return nilReturn, fmt.Errorf("sdk event version %s not compatible with %s", receiver.GetType().Version, eventType.Version)
+		}
 	}
 	err = json.Unmarshal(event, receiver)
 	if err != nil {

pkg/api/bindings_test.go

@@ -346,7 +346,7 @@ func TestInvalidEvent(t *testing.T) {
 		Context: api.ContextV04{
 			Context: api.Context{
 				Type:    api.CDEventType{Subject: "not-a-valid-type"},
-				Version: api.CDEventsSpecVersion,
+				Version: testapi.SpecVersion,
 			},
 		},
 		Subject: testapi.FooSubjectBarPredicateSubject{

pkg/api/types.go

@@ -31,19 +31,22 @@ import (
 )
 
 const (
-	EventTypeRoot             = "dev.cdevents"
-	CDEventsSpecVersion       = "0.3.0"
-	CDEventsSchemaURLTemplate = "https://cdevents.dev/%s/schema/%s-%s-event"
-	CDEventsTypeRegex         = "^dev\\.cdevents\\.(?P<subject>[a-z]+)\\.(?P<predicate>[a-z]+)\\.(?P<version>.*)$"
+	EventTypeRoot                   = "dev.cdevents"
+	CustomEventTypeRoot             = "dev.cdeventsx"
+	CDEventsSchemaURLTemplate       = "https://cdevents.dev/%s/schema/%s-%s-event"
+	CDEventsCustomSchemaURLTemplate = "https://cdevents.dev/%s/schema/custom"
+	CDEventsTypeRegex               = "^dev\\.cdevents\\.(?P<subject>[a-z]+)\\.(?P<predicate>[a-z]+)\\.(?P<version>.*)$"
+	CDEventsCustomTypeRegex         = "^dev\\.cdeventsx\\.(?P<tool>[a-z]+)-(?P<subject>[a-z]+)\\.(?P<predicate>[a-z]+)\\.(?P<version>.*)$"
 
 	LinkTypePath     LinkType = "PATH"
 	LinkTypeEnd      LinkType = "END"
 	LinkTypeRelation LinkType = "RELATION"
 )
 
 var (
-	CDEventsTypeCRegex = regexp.MustCompile(CDEventsTypeRegex)
-	LinkTypes          = map[LinkType]interface{}{
+	CDEventsTypeCRegex       = regexp.MustCompile(CDEventsTypeRegex)
+	CDEventsCustomTypeCRegex = regexp.MustCompile(CDEventsCustomTypeRegex)
+	LinkTypes                = map[LinkType]interface{}{
 		LinkTypePath:     "",
 		LinkTypeEnd:      "",
 		LinkTypeRelation: "",
@@ -380,18 +383,59 @@ type CDEventType struct {
 
 	// Version is a semantic version in the form <major>.<minor>.<patch>
 	Version string
+
+	// Custom holds the tool name in case of custom events
+	Custom string
+}
+
+func (t CDEventType) Root() string {
+	root := EventTypeRoot
+	if t.Custom != "" {
+		root = CustomEventTypeRoot
+	}
+	return root
+}
+
+// FQSubject returns the fully qualified subject, which includes
+// the tool name from t.Custom in case of custom events
+func (t CDEventType) FQSubject() string {
+	s := t.Subject
+	if s == "" {
+		s = "<undefined-subject>"
+	}
+	if t.Custom != "" {
+		s = t.Custom + "-" + s
+	}
+	return s
 }
 
 func (t CDEventType) String() string {
-	return EventTypeRoot + "." + t.Subject + "." + t.Predicate + "." + t.Version
+	predicate := t.Predicate
+	if predicate == "" {
+		predicate = "<undefined-predicate>"
+	}
+	version := t.Version
+	if version == "" {
+		version = "<undefined-version>"
+	}
+	return t.Root() + "." + t.FQSubject() + "." + predicate + "." + version
 }
 
 func (t CDEventType) UnversionedString() string {
-	return EventTypeRoot + "." + t.Subject + "." + t.Predicate
+	predicate := t.Predicate
+	if predicate == "" {
+		predicate = "<undefined-predicate>"
+	}
+	return t.Root() + "." + t.FQSubject() + "." + predicate
 }
 
 func (t CDEventType) Short() string {
-	return t.Subject + "_" + t.Predicate
+	s := t.FQSubject()
+	p := t.Predicate
+	if s == "" || p == "" {
+		return ""
+	}
+	return t.FQSubject() + "_" + t.Predicate
 }
 
 // Two CDEventTypes are compatible if the subject and predicates
@@ -420,15 +464,32 @@ func (t CDEventType) MarshalJSON() ([]byte, error) {
 }
 
 func CDEventTypeFromString(cdeventType string) (*CDEventType, error) {
+	names := CDEventsTypeCRegex.SubexpNames()
 	parts := CDEventsTypeCRegex.FindStringSubmatch(cdeventType)
 	if len(parts) != 4 {
-		return nil, fmt.Errorf("cannot parse event type %s", cdeventType)
+		names = CDEventsCustomTypeCRegex.SubexpNames()
+		parts = CDEventsCustomTypeCRegex.FindStringSubmatch(cdeventType)
+		if len(parts) != 5 {
+			return nil, fmt.Errorf("cannot parse event type %s", cdeventType)
+		}
+	}
+	returnType := CDEventType{}
+	for i, matchName := range names {
+		if i == 0 {
+			continue
+		}
+		switch matchName {
+		case "subject":
+			returnType.Subject = parts[i]
+		case "predicate":
+			returnType.Predicate = parts[i]
+		case "version":
+			returnType.Version = parts[i]
+		case "tool":
+			returnType.Custom = parts[i]
+		}
 	}
-	return &CDEventType{
-		Subject:   parts[1],
-		Predicate: parts[2],
-		Version:   parts[3],
-	}, nil
+	return &returnType, nil
 }
 
 type CDEventReader interface {
@@ -459,6 +520,11 @@ type CDEventReader interface {
 	// for direct access to the content fields
 	GetSubject() Subject
 
+	// The event specific subject. It is possible to use a type assertion with
+	// the generic Subject to obtain an event specific implementation of Subject
+	// for direct access to the content fields
+	GetSubjectContent() interface{}
+
 	// The URL and content of the schema file associated to the event type
 	GetSchema() (string, *jsonschema.Schema, error)
 
@@ -531,6 +597,20 @@ type CDEventWriterV04 interface {
 	SetSchemaUri(schema string)
 }
 
+type CustomCDEventReader interface {
+	CDEventReaderV04
+}
+
+type CustomCDEventWriter interface {
+	CDEventWriterV04
+
+	// CustomCDEvent can represent different event types
+	SetEventType(eventType CDEventType)
+
+	// CustomCDEvent types can have different subject fields
+	SetSubjectContent(subjectContent interface{})
+}
+
 type CDEventCustomDataEncoding string
 
 func (t CDEventCustomDataEncoding) String() string {