feat: reinitialize agents when a prebuilt workspace is claimed (#17475)

This pull request allows coder workspace agents to be reinitialized when
a prebuilt workspace is claimed by a user. This facilitates the transfer
of ownership between the anonymous prebuilds system user and the new
owner of the workspace.

Only a single agent per prebuilt workspace is supported for now, but
plumbing has already been done to facilitate the seamless transition to
multi-agent support.

---------

Signed-off-by: Danny Kopping <dannykopping@gmail.com>
Co-authored-by: Danny Kopping <dannykopping@gmail.com>

codersdk/agentsdk/agentsdk.go

@@ -19,12 +19,15 @@ import (
 	"tailscale.com/tailcfg"
 
 	"cdr.dev/slog"
+	"github.com/coder/retry"
+	"github.com/coder/websocket"
+
 	"github.com/coder/coder/v2/agent/proto"
 	"github.com/coder/coder/v2/apiversion"
+	"github.com/coder/coder/v2/coderd/httpapi"
 	"github.com/coder/coder/v2/codersdk"
 	"github.com/coder/coder/v2/codersdk/drpcsdk"
 	tailnetproto "github.com/coder/coder/v2/tailnet/proto"
-	"github.com/coder/websocket"
 )
 
 // ExternalLogSourceID is the statically-defined ID of a log-source that
@@ -686,3 +689,188 @@ func LogsNotifyChannel(agentID uuid.UUID) string {
 type LogsNotifyMessage struct {
 	CreatedAfter int64 `json:"created_after"`
 }
+
+type ReinitializationReason string
+
+const (
+	ReinitializeReasonPrebuildClaimed ReinitializationReason = "prebuild_claimed"
+)
+
+type ReinitializationEvent struct {
+	WorkspaceID uuid.UUID
+	Reason      ReinitializationReason `json:"reason"`
+}
+
+func PrebuildClaimedChannel(id uuid.UUID) string {
+	return fmt.Sprintf("prebuild_claimed_%s", id)
+}
+
+// WaitForReinit polls a SSE endpoint, and receives an event back under the following conditions:
+// - ping: ignored, keepalive
+// - prebuild claimed: a prebuilt workspace is claimed, so the agent must reinitialize.
+func (c *Client) WaitForReinit(ctx context.Context) (*ReinitializationEvent, error) {
+	rpcURL, err := c.SDK.URL.Parse("/api/v2/workspaceagents/me/reinit")
+	if err != nil {
+		return nil, xerrors.Errorf("parse url: %w", err)
+	}
+
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		return nil, xerrors.Errorf("create cookie jar: %w", err)
+	}
+	jar.SetCookies(rpcURL, []*http.Cookie{{
+		Name:  codersdk.SessionTokenCookie,
+		Value: c.SDK.SessionToken(),
+	}})
+	httpClient := &http.Client{
+		Jar:       jar,
+		Transport: c.SDK.HTTPClient.Transport,
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, rpcURL.String(), nil)
+	if err != nil {
+		return nil, xerrors.Errorf("build request: %w", err)
+	}
+
+	res, err := httpClient.Do(req)
+	if err != nil {
+		return nil, xerrors.Errorf("execute request: %w", err)
+	}
+	defer res.Body.Close()
+
+	if res.StatusCode != http.StatusOK {
+		return nil, codersdk.ReadBodyAsError(res)
+	}
+
+	reinitEvent, err := NewSSEAgentReinitReceiver(res.Body).Receive(ctx)
+	if err != nil {
+		return nil, xerrors.Errorf("listening for reinitialization events: %w", err)
+	}
+	return reinitEvent, nil
+}
+
+func WaitForReinitLoop(ctx context.Context, logger slog.Logger, client *Client) <-chan ReinitializationEvent {
+	reinitEvents := make(chan ReinitializationEvent)
+
+	go func() {
+		for retrier := retry.New(100*time.Millisecond, 10*time.Second); retrier.Wait(ctx); {
+			logger.Debug(ctx, "waiting for agent reinitialization instructions")
+			reinitEvent, err := client.WaitForReinit(ctx)
+			if err != nil {
+				logger.Error(ctx, "failed to wait for agent reinitialization instructions", slog.Error(err))
+				continue
+			}
+			retrier.Reset()
+			select {
+			case <-ctx.Done():
+				close(reinitEvents)
+				return
+			case reinitEvents <- *reinitEvent:
+			}
+		}
+	}()
+
+	return reinitEvents
+}
+
+func NewSSEAgentReinitTransmitter(logger slog.Logger, rw http.ResponseWriter, r *http.Request) *SSEAgentReinitTransmitter {
+	return &SSEAgentReinitTransmitter{logger: logger, rw: rw, r: r}
+}
+
+type SSEAgentReinitTransmitter struct {
+	rw     http.ResponseWriter
+	r      *http.Request
+	logger slog.Logger
+}
+
+var (
+	ErrTransmissionSourceClosed = xerrors.New("transmission source closed")
+	ErrTransmissionTargetClosed = xerrors.New("transmission target closed")
+)
+
+// Transmit will read from the given chan and send events for as long as:
+// * the chan remains open
+// * the context has not been canceled
+// * not timed out
+// * the connection to the receiver remains open
+func (s *SSEAgentReinitTransmitter) Transmit(ctx context.Context, reinitEvents <-chan ReinitializationEvent) error {
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	default:
+	}
+
+	sseSendEvent, sseSenderClosed, err := httpapi.ServerSentEventSender(s.rw, s.r)
+	if err != nil {
+		return xerrors.Errorf("failed to create sse transmitter: %w", err)
+	}
+
+	defer func() {
+		// Block returning until the ServerSentEventSender is closed
+		// to avoid a race condition where we might write or flush to rw after the handler returns.
+		<-sseSenderClosed
+	}()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-sseSenderClosed:
+			return ErrTransmissionTargetClosed
+		case reinitEvent, ok := <-reinitEvents:
+			if !ok {
+				return ErrTransmissionSourceClosed
+			}
+			err := sseSendEvent(codersdk.ServerSentEvent{
+				Type: codersdk.ServerSentEventTypeData,
+				Data: reinitEvent,
+			})
+			if err != nil {
+				return err
+			}
+		}
+	}
+}
+
+func NewSSEAgentReinitReceiver(r io.ReadCloser) *SSEAgentReinitReceiver {
+	return &SSEAgentReinitReceiver{r: r}
+}
+
+type SSEAgentReinitReceiver struct {
+	r io.ReadCloser
+}
+
+func (s *SSEAgentReinitReceiver) Receive(ctx context.Context) (*ReinitializationEvent, error) {
+	nextEvent := codersdk.ServerSentEventReader(ctx, s.r)
+	for {
+		select {
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		default:
+		}
+
+		sse, err := nextEvent()
+		switch {
+		case err != nil:
+			return nil, xerrors.Errorf("failed to read server-sent event: %w", err)
+		case sse.Type == codersdk.ServerSentEventTypeError:
+			return nil, xerrors.Errorf("unexpected server sent event type error")
+		case sse.Type == codersdk.ServerSentEventTypePing:
+			continue
+		case sse.Type != codersdk.ServerSentEventTypeData:
+			return nil, xerrors.Errorf("unexpected server sent event type: %s", sse.Type)
+		}
+
+		// At this point we know that the sent event is of type codersdk.ServerSentEventTypeData
+		var reinitEvent ReinitializationEvent
+		b, ok := sse.Data.([]byte)
+		if !ok {
+			return nil, xerrors.Errorf("expected data as []byte, got %T", sse.Data)
+		}
+		err = json.Unmarshal(b, &reinitEvent)
+		if err != nil {
+			return nil, xerrors.Errorf("unmarshal reinit response: %w", err)
+		}
+		return &reinitEvent, nil
+	}
+}