implement subscription in corrosion client

internal/corrosion/client.go

@@ -39,7 +39,8 @@ func NewAPIClient(addr netip.AddrPort) (*APIClient, error) {
 	return &APIClient{
 		baseURL: baseURL,
 		client: &http.Client{
-			Timeout: http2Timeout,
+			// TODO: use timeout for non-subscription requests?
+			//Timeout: http2Timeout,
 			Transport: &RetryRoundTripper{
 				Base: &http2.Transport{
 					AllowHTTP: true,
@@ -184,8 +185,7 @@ type QueryEvent struct {
 	Columns []string     `json:"columns"`
 	Row     *RowEvent    `json:"row"`
 	EOQ     *EndOfQuery  `json:"eoq"`
-	// TODO: implement event type Change to support subscriptions.
+	Change  *ChangeEvent `json:"change"`
-	//Change []any       `json:"change"`
 	// Error is a server-side error that occurred during query execution. It's considered fatal for the client
 	// as it cannot be recovered from server-side.
 	Error *string `json:"error"`
@@ -256,7 +256,7 @@ func (c *APIClient) QueryContext(ctx context.Context, query string, args ...any)
 		return nil, fmt.Errorf("unexpected status code %d: %s", resp.StatusCode, respBody)
 	}
 
-	rows, err := newRows(ctx, resp.Body)
+	rows, err := newRows(ctx, resp.Body, true)
 	if err != nil {
 		resp.Body.Close()
 		return nil, fmt.Errorf("parse query response: %w", err)
@@ -270,14 +270,15 @@ type Rows struct {
 	ctx        context.Context
 	body       io.ReadCloser
 	decoder    *json.Decoder
+	eoq        *EndOfQuery
+	closeOnEOQ bool
 
 	columns []string
 	row     RowEvent
-	time    float64
 	err     error
 }
 
-func newRows(ctx context.Context, body io.ReadCloser) (*Rows, error) {
+func newRows(ctx context.Context, body io.ReadCloser, closeOnEOQ bool) (*Rows, error) {
 	select {
 	case <-ctx.Done():
 		return nil, ctx.Err()
@@ -297,6 +298,7 @@ func newRows(ctx context.Context, body io.ReadCloser) (*Rows, error) {
 		ctx:        ctx,
 		body:       body,
 		decoder:    decoder,
+		closeOnEOQ: closeOnEOQ,
 		columns:    e.Columns,
 	}, nil
 }
@@ -343,8 +345,11 @@ func (rs *Rows) Next() bool {
 		return true
 	}
 	if e.EOQ != nil {
-		rs.time = e.EOQ.Time
+		rs.eoq = e.EOQ
+		// Rows could be used as part of a subscription, so don't close the body if so.
+		if rs.closeOnEOQ {
 			_ = rs.Close()
+		}
 		return false
 	}
 
@@ -381,13 +386,13 @@ func (rs *Rows) Scan(dest ...any) error {
 // Time returns the time taken to execute the query in seconds. It's only available after all rows have been consumed.
 // It doesn't include the time to send the query, receive the response, or iterate over the rows.
 func (rs *Rows) Time() (float64, error) {
-	if rs.time == 0 {
+	if rs.eoq == nil {
 		if rs.Err() != nil {
 			return 0, fmt.Errorf("time is not available: %w", rs.Err())
 		}
 		return 0, errors.New("time is not available until all rows are consumed")
 	}
-	return rs.time, nil
+	return rs.eoq.Time, nil
 }
 
 // Close closes the [Rows], preventing further enumeration. If [Rows.Next] is called and returns false,
@@ -396,3 +401,242 @@ func (rs *Rows) Time() (float64, error) {
 func (rs *Rows) Close() error {
 	return rs.body.Close()
 }
+
+// SubscribeContext creates a subscription to receive updates for a desired SQL query. If skipRows is false,
+// Subscription.Rows must be consumed before Subscription.Changes can be called. If skipRows is true, Subscription.Rows
+// will be nil.
+func (c *APIClient) SubscribeContext(
+	ctx context.Context, query string, args []any, skipRows bool,
+) (*Subscription, error) {
+	statement := Statement{
+		Query:  query,
+		Params: args,
+	}
+	body, err := json.Marshal(statement)
+	if err != nil {
+		return nil, fmt.Errorf("marshal query: %w", err)
+	}
+
+	subURL := c.baseURL.JoinPath("/v1/subscriptions")
+	if skipRows {
+		q := subURL.Query()
+		q.Set("skip_rows", "true")
+		subURL.RawQuery = q.Encode()
+	}
+
+	req, err := http.NewRequestWithContext(ctx, "POST", subURL.String(), bytes.NewReader(body))
+	if err != nil {
+		return nil, fmt.Errorf("create request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+
+	resp, err := c.client.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("send request: %w", err)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		respBody, err := io.ReadAll(resp.Body)
+		resp.Body.Close()
+		if err != nil {
+			return nil, fmt.Errorf("read response body: %w", err)
+		}
+		return nil, fmt.Errorf("unexpected status code %d: %s", resp.StatusCode, respBody)
+	}
+
+	id := resp.Header.Get("corro-query-id")
+	if id == "" {
+		resp.Body.Close()
+		return nil, errors.New("missing corro-query-id header in response")
+	}
+
+	if skipRows {
+		return newSubscription(ctx, id, nil, resp.Body, nil), nil
+	}
+
+	rows, err := newRows(ctx, resp.Body, false)
+	if err != nil {
+		resp.Body.Close()
+		return nil, fmt.Errorf("parse query response: %w", err)
+	}
+	return newSubscription(ctx, id, rows, rows.body, rows.decoder), nil
+}
+
+func (c *APIClient) ResubscribeContext(
+	ctx context.Context, id string, skipRows bool, fromChange uint64,
+) (*Subscription, error) {
+	// TODO
+	return nil, nil
+}
+
+type ChangeType string
+
+var (
+	ChangeTypeInsert ChangeType = "insert"
+	ChangeTypeUpdate ChangeType = "update"
+	ChangeTypeDelete ChangeType = "delete"
+)
+
+type ChangeEvent struct {
+	Type     ChangeType
+	RowID    uint64
+	Values   []json.RawMessage
+	ChangeID uint64
+}
+
+func (ce *ChangeEvent) UnmarshalJSON(data []byte) error {
+	var raw []json.RawMessage
+	if err := json.Unmarshal(data, &raw); err != nil {
+		return fmt.Errorf("invalid change event: %w", err)
+	}
+	if len(raw) != 4 {
+		return fmt.Errorf("invalid change event: expected an array of 4 elements")
+	}
+	if err := json.Unmarshal(raw[0], &ce.Type); err != nil {
+		return fmt.Errorf("invalid change event type: %w", err)
+	}
+	if err := json.Unmarshal(raw[1], &ce.RowID); err != nil {
+		return fmt.Errorf("invalid change event row ID: %w", err)
+	}
+	if err := json.Unmarshal(raw[2], &ce.Values); err != nil {
+		return fmt.Errorf("invalid change event values: %w", err)
+	}
+	if err := json.Unmarshal(raw[3], &ce.ChangeID); err != nil {
+		return fmt.Errorf("invalid change event change ID: %w", err)
+	}
+	return nil
+}
+
+func (ce *ChangeEvent) MarshalJSON() ([]byte, error) {
+	return json.Marshal([]any{ce.Type, ce.RowID, ce.Values, ce.ChangeID})
+}
+
+// Scan copies the column values in the change event into the values pointed at by dest.
+// The number of values in dest must be the same as the number of columns in the change.
+// Scan converts JSON-encoded column values to the provided Go types using [json.Unmarshal].
+func (ce *ChangeEvent) Scan(dest ...any) error {
+	if len(dest) != len(ce.Values) {
+		return fmt.Errorf("expected %d values, got %d", len(ce.Values), len(dest))
+	}
+
+	for i, v := range ce.Values {
+		if err := json.Unmarshal(v, dest[i]); err != nil {
+			return fmt.Errorf("unmarshal column value #%d: %w", i, err)
+		}
+	}
+	return nil
+}
+
+// Subscription receives updates from the Corrosion database for a desired SQL query.
+type Subscription struct {
+	ID   string
+	Rows *Rows
+
+	ctx          context.Context
+	cancel       context.CancelFunc
+	body         io.ReadCloser
+	decoder      *json.Decoder
+	changes      chan *ChangeEvent
+	lastChangeID uint64
+	err          error
+}
+
+func newSubscription(
+	ctx context.Context, id string, rows *Rows, body io.ReadCloser, decoder *json.Decoder,
+) *Subscription {
+	ctx, cancel := context.WithCancel(ctx)
+	if decoder == nil {
+		decoder = json.NewDecoder(body)
+	}
+	return &Subscription{
+		ID:      id,
+		Rows:    rows,
+		ctx:     ctx,
+		cancel:  cancel,
+		body:    body,
+		decoder: decoder,
+	}
+}
+
+// Changes returns a channel that receives change events for the query. Changes are not available until all rows
+// are consumed. The channel is closed when the context is done, or an error occurs while reading the changes,
+// or when the subscription is closed explicitly. If it's closed due to an error, [Subscription.Err] will return
+// the error.
+func (s *Subscription) Changes() (<-chan *ChangeEvent, error) {
+	if s.changes != nil {
+		return s.changes, nil
+	}
+
+	if s.Rows != nil {
+		if s.Rows.eoq == nil {
+			return nil, errors.New("changes are not available until all rows are consumed")
+		}
+		s.lastChangeID = *s.Rows.eoq.ChangeID
+	}
+	s.changes = make(chan *ChangeEvent)
+
+	go func() {
+		// Close the body when the context is done to unblock the decoder in the following goroutine.
+		<-s.ctx.Done()
+		s.body.Close()
+	}()
+
+	go func() {
+		defer s.cancel()
+		defer close(s.changes)
+
+		for {
+			select {
+			case <-s.ctx.Done():
+				return
+			default:
+			}
+
+			var e QueryEvent
+			if err := s.decoder.Decode(&e); err != nil {
+				if s.ctx.Err() != nil {
+					s.err = s.ctx.Err()
+				} else {
+					s.err = fmt.Errorf("decode query event: %w", err)
+				}
+				return
+			}
+
+			if e.Error != nil {
+				s.err = fmt.Errorf("query error: %s", *e.Error)
+				return
+			}
+			if e.Change == nil {
+				s.err = fmt.Errorf("expected change event, got: %+v", e)
+				return
+			}
+			// If skipRows is true, the last change ID is unknown.
+			if s.lastChangeID != 0 && e.Change.ChangeID != s.lastChangeID+1 {
+				s.err = fmt.Errorf("missed a change: expected change ID %d, got %d",
+					s.lastChangeID+1, e.Change.ChangeID)
+				return
+			}
+
+			s.lastChangeID = e.Change.ChangeID
+			select {
+			case s.changes <- e.Change:
+			case <-s.ctx.Done():
+				return
+			}
+		}
+	}()
+
+	return s.changes, nil
+}
+
+// Err returns the error, if any, that was encountered during fetching changes.
+// Err may be called after an explicit or implicit [Subscription.Close].
+func (s *Subscription) Err() error {
+	return s.err
+}
+
+func (s *Subscription) Close() error {
+	s.cancel()
+	return s.body.Close()
+}