mesh-controller/internal/link/builds_current.go

package link

import (
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"time"

	amqp "github.com/rabbitmq/amqp091-go"
)

// The build flow on the bus the mesh runs on today.
//
// Moved behind the seam rather than changed. The queue, the reply binding and the correlation are
// what they were, because the mesh is running on this.

// currentBuilds asks for builds over a channel.
type currentBuilds struct{ channel *amqp.Channel }

// BuildsOverCurrent is the asking side on the bus the mesh has.
func BuildsOverCurrent(channel *amqp.Channel) Builders { return currentBuilds{channel: channel} }

func (b currentBuilds) Close() {}

func (b currentBuilds) Submit(ctx context.Context, request BuildRequest,
	wait time.Duration) (BuildResult, error) {

	// Its own queue for the answer, declared before the ask. Consuming from the shared exchange
	// would mean competing with the controller's own consumer for a message meant for this caller.
	replies, err := b.channel.QueueDeclare(ReplyQueue(request.ID), false, true, true, false, nil)
	if err != nil {
		return BuildResult{}, err
	}
	// **A builder never publishes to the default exchange**, because permission there is per
	// exchange and not per queue — a builder allowed to use it could publish into any node's queue,
	// which is the privilege a build machine most obviously should not have. The cost is that every
	// asker sees every result, which is why the correlation is checked below rather than assumed.
	if err := b.channel.QueueBind(replies.Name, KeyBuilt, Exchange, false, nil); err != nil {
		return BuildResult{}, err
	}
	answers, err := b.channel.ConsumeWithContext(ctx, replies.Name, "", true, true, false, false, nil)
	if err != nil {
		return BuildResult{}, err
	}

	body, err := json.Marshal(request)
	if err != nil {
		return BuildResult{}, err
	}
	if err := b.channel.PublishWithContext(ctx, "", BuildQueue, false, false, amqp.Publishing{
		ContentType:   "application/json",
		DeliveryMode:  amqp.Persistent,
		CorrelationId: request.ID,
		ReplyTo:       replies.Name,
		Body:          body,
	}); err != nil {
		return BuildResult{}, err
	}

	waiting, cancel := context.WithTimeout(ctx, wait)
	defer cancel()
	for {
		select {
		case <-waiting.Done():
			return BuildResult{}, waitingFor(wait)
		case delivery, ok := <-answers:
			if !ok {
				return BuildResult{}, errors.New("the connection closed while waiting for a build")
			}
			result, mine, err := theOutcomeOf(delivery.Body, request.ID)
			if err != nil {
				return BuildResult{}, err
			}
			if mine {
				return result, nil
			}
		}
	}
}

// --- the machine's side ---------------------------------------------------------------------

type currentMachine struct {
	conn    *amqp.Connection
	channel *amqp.Channel
	on      string
}

// MachineOverCurrent takes build work over a channel.
func MachineOverCurrent(conn *amqp.Connection, channel *amqp.Channel, on string) BuildMachine {
	return &currentMachine{conn: conn, channel: channel, on: on}
}

func (m *currentMachine) Close() {}

func (m *currentMachine) Take(ctx context.Context, do func(context.Context, Build)) error {
	if _, err := m.channel.QueueDeclare(BuildQueue, true, false, false, false, nil); err != nil {
		return err
	}
	// One at a time. A machine that took five requests at once would run five container builds
	// against one runtime and finish all of them slower than it would have finished the first — and
	// the queue is what shares work between machines, so nothing is lost by it.
	if err := m.channel.Qos(1, 0, false); err != nil {
		return err
	}
	// Not auto-acknowledged: a request acknowledged on arrival is a build that vanishes if this
	// process dies mid-way, with nobody waiting on it ever hearing why.
	requests, err := m.channel.ConsumeWithContext(ctx, BuildQueue, "mesh-builder",
		false, false, false, false, nil)
	if err != nil {
		return err
	}
	for {
		select {
		case <-ctx.Done():
			return nil
		case delivery, ok := <-requests:
			if !ok {
				return errors.New("the broker closed the connection")
			}
			var request BuildRequest
			if err := json.Unmarshal(delivery.Body, &request); err != nil {
				// Unreadable: rejected rather than retried, because the next attempt reads the same
				// bytes. Nobody waiting hears an answer, which is correct — there was no request.
				_ = delivery.Reject(false)
				continue
			}
			do(ctx, &currentBuild{request: request, delivery: delivery, on: m.on, channel: m.channel})
		}
	}
}

type currentBuild struct {
	request  BuildRequest
	delivery amqp.Delivery
	on       string
	channel  *amqp.Channel
}

func (b *currentBuild) Request() BuildRequest { return b.request }

// Announce answers and announces, which on this bus are two publishes to two exchanges.
//
// The reply goes to whoever asked, correlated to their request; the announcement says to the whole
// mesh that a module now exists at a commit (novox/hq ADR 0072). Only a successful build is
// announced: a failed one produced no module version, and announcing one would put something in the
// graph that was never made.
func (b *currentBuild) Announce(ctx context.Context, result BuildResult) error {
	body, err := json.Marshal(result)
	if err != nil {
		return err
	}
	if err := b.channel.PublishWithContext(ctx, Exchange, KeyBuilt, false, false, amqp.Publishing{
		ContentType:   "application/json",
		CorrelationId: result.ID,
		Body:          body,
	}); err != nil {
		return fmt.Errorf("cannot answer a build request: %w", err)
	}
	if result.Failed != "" || result.Commit == "" {
		return nil
	}
	return EmitEvent(ctx, OverCurrent{Channel: b.channel}, KeyModuleBuilt, "builder", b.on,
		announcementOf(result))
}

func (b *currentBuild) Done() error { return b.delivery.Ack(false) }

func (b *currentBuild) Hold(time.Duration) error {
	// No delayed redelivery on this bus: handed back at once, which is what it has always done.
	return b.delivery.Nack(false, true)
}

// announcementOf is what the mesh is told about a finished build. One function, so the two
// transports cannot describe the same build differently.
func announcementOf(result BuildResult) map[string]any {
	return map[string]any{
		"module": ModuleOf(result.Manifest), "commit": result.Commit,
		"repository": result.Repository, "path": result.Path, "ref": result.Ref,
		"manifest": json.RawMessage(result.Manifest), "against": result.Against,
		"made": result.Made,
	}
}