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package eventstreamapi
import (
"fmt"
"io"
"github.com/aws/aws-sdk-go/aws"
"github.com/aws/aws-sdk-go/private/protocol"
"github.com/aws/aws-sdk-go/private/protocol/eventstream"
)
// Unmarshaler provides the interface for unmarshaling a EventStream
// message into a SDK type.
type Unmarshaler interface {
UnmarshalEvent(protocol.PayloadUnmarshaler, eventstream.Message) error
}
// EventStream headers with specific meaning to async API functionality.
const (
MessageTypeHeader = `:message-type` // Identifies type of message.
EventMessageType = `event`
ErrorMessageType = `error`
ExceptionMessageType = `exception`
// Message Events
EventTypeHeader = `:event-type` // Identifies message event type e.g. "Stats".
// Message Error
ErrorCodeHeader = `:error-code`
ErrorMessageHeader = `:error-message`
// Message Exception
ExceptionTypeHeader = `:exception-type`
)
// EventReader provides reading from the EventStream of an reader.
type EventReader struct {
reader io.ReadCloser
decoder *eventstream.Decoder
unmarshalerForEventType func(string) (Unmarshaler, error)
payloadUnmarshaler protocol.PayloadUnmarshaler
payloadBuf []byte
}
// NewEventReader returns a EventReader built from the reader and unmarshaler
// provided. Use ReadStream method to start reading from the EventStream.
func NewEventReader(
reader io.ReadCloser,
payloadUnmarshaler protocol.PayloadUnmarshaler,
unmarshalerForEventType func(string) (Unmarshaler, error),
) *EventReader {
return &EventReader{
reader: reader,
decoder: eventstream.NewDecoder(reader),
payloadUnmarshaler: payloadUnmarshaler,
unmarshalerForEventType: unmarshalerForEventType,
payloadBuf: make([]byte, 10*1024),
}
}
// UseLogger instructs the EventReader to use the logger and log level
// specified.
func (r *EventReader) UseLogger(logger aws.Logger, logLevel aws.LogLevelType) {
if logger != nil && logLevel.Matches(aws.LogDebugWithEventStreamBody) {
r.decoder.UseLogger(logger)
}
}
// ReadEvent attempts to read a message from the EventStream and return the
// unmarshaled event value that the message is for.
//
// For EventStream API errors check if the returned error satisfies the
// awserr.Error interface to get the error's Code and Message components.
//
// EventUnmarshalers called with EventStream messages must take copies of the
// message's Payload. The payload will is reused between events read.
func (r *EventReader) ReadEvent() (event interface{}, err error) {
msg, err := r.decoder.Decode(r.payloadBuf)
if err != nil {
return nil, err
}
defer func() {
// Reclaim payload buffer for next message read.
r.payloadBuf = msg.Payload[0:0]
}()
typ, err := GetHeaderString(msg, MessageTypeHeader)
if err != nil {
return nil, err
}
switch typ {
case EventMessageType:
return r.unmarshalEventMessage(msg)
case ExceptionMessageType:
err = r.unmarshalEventException(msg)
return nil, err
case ErrorMessageType:
return nil, r.unmarshalErrorMessage(msg)
default:
return nil, fmt.Errorf("unknown eventstream message type, %v", typ)
}
}
func (r *EventReader) unmarshalEventMessage(
msg eventstream.Message,
) (event interface{}, err error) {
eventType, err := GetHeaderString(msg, EventTypeHeader)
if err != nil {
return nil, err
}
ev, err := r.unmarshalerForEventType(eventType)
if err != nil {
return nil, err
}
err = ev.UnmarshalEvent(r.payloadUnmarshaler, msg)
if err != nil {
return nil, err
}
return ev, nil
}
func (r *EventReader) unmarshalEventException(
msg eventstream.Message,
) (err error) {
eventType, err := GetHeaderString(msg, ExceptionTypeHeader)
if err != nil {
return err
}
ev, err := r.unmarshalerForEventType(eventType)
if err != nil {
return err
}
err = ev.UnmarshalEvent(r.payloadUnmarshaler, msg)
if err != nil {
return err
}
var ok bool
err, ok = ev.(error)
if !ok {
err = messageError{
code: "SerializationError",
msg: fmt.Sprintf(
"event stream exception %s mapped to non-error %T, %v",
eventType, ev, ev,
),
}
}
return err
}
func (r *EventReader) unmarshalErrorMessage(msg eventstream.Message) (err error) {
var msgErr messageError
msgErr.code, err = GetHeaderString(msg, ErrorCodeHeader)
if err != nil {
return err
}
msgErr.msg, err = GetHeaderString(msg, ErrorMessageHeader)
if err != nil {
return err
}
return msgErr
}
// Close closes the EventReader's EventStream reader.
func (r *EventReader) Close() error {
return r.reader.Close()
}
// GetHeaderString returns the value of the header as a string. If the header
// is not set or the value is not a string an error will be returned.
func GetHeaderString(msg eventstream.Message, headerName string) (string, error) {
headerVal := msg.Headers.Get(headerName)
if headerVal == nil {
return "", fmt.Errorf("error header %s not present", headerName)
}
v, ok := headerVal.Get().(string)
if !ok {
return "", fmt.Errorf("error header value is not a string, %T", headerVal)
}
return v, nil
}
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