mesh-controller/internal/link/build.go

package link

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

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

// Asking a machine to build a module, and hearing what came out.
//
// **A build is work, not state.** Everything else the control plane sends a node is a declaration
// — *this is what you should be* — and the node reconciles toward it forever. A build happens
// once, produces something, and is finished. Putting it in a declaration would mean a machine
// rebuilding on every reconcile, or the declaration carrying "and I already did this", which is
// state about an event rather than about the machine.
//
// So it travels on its own queue, and the reply comes back correlated. That is also why the
// builder is a **separate consumer** rather than the host: the host applies declarations and
// holds no opinion about what they contain, and a host that also built things would be a host
// with a container runtime requirement and a git dependency (novox/hq ADR 0005).

// BuildQueue is where build requests wait. One queue, so several build machines can share the
// work and each request is done exactly once — which is what a queue is for and what a
// per-machine routing key would not give.
const BuildQueue = "builds"

// KeyBuilt is what a builder publishes when it has finished, successfully or not.
const KeyBuilt = "built"

// ReplyQueue is where the answer to one request goes.
//
// **Named here rather than left to the broker**, so it can be scoped and reasoned about. A broker
// generates its own name for an unnamed queue, and a builder permitted to write to whatever that
// convention happens to produce works on one broker and silently cannot answer on another.
func ReplyQueue(id string) string { return BuildQueue + ".reply." + id }

// BuildRequest is one module to build.
type BuildRequest struct {
	// ID correlates the answer with the asking. Not the module name: two builds of one module can
	// be in flight, and the second answer is not the first one's.
	ID string `json:"id"`
	// Repository is where the source is, as git would clone it.
	Repository string `json:"repository"`
	// Ref is the branch, tag or commit. Empty means whatever the repository's default is, which
	// is the only case where the mesh does not know what it built until it has built it.
	Ref string `json:"ref,omitempty"`
}

// BuildResult is what a builder says back.
//
// **Failure is a result, not an absence.** A build that fails and says nothing is
// indistinguishable from a builder that is not running, and those want completely different
// responses — the same rule the host follows about a service that does not exist.
type BuildResult struct {
	ID         string `json:"id"`
	Repository string `json:"repository"`
	Ref        string `json:"ref,omitempty"`

	// On is the machine that did it, so a failure that is about one machine can be told from one
	// about the source.
	On string `json:"on"`

	// Commit is what was actually built. The mesh records it, which is what makes "is this
	// current?" answerable without building again.
	Commit string `json:"commit,omitempty"`

	// Manifest is the module as the mesh should hold it, artifacts resolved to digests. Raw,
	// because the control plane parses it with the same parser it uses for one handed over by
	// hand — a second path would be a second thing to disagree.
	Manifest json.RawMessage `json:"manifest,omitempty"`

	// Made is each artifact, for reporting.
	Made []MadeArtifact `json:"made,omitempty"`

	// Failed is why, when it did.
	Failed string `json:"failed,omitempty"`
}

// MadeArtifact is one thing a build produced, as a person would want it reported.
type MadeArtifact struct {
	Name      string `json:"name"`
	Kind      string `json:"kind"`
	Reference string `json:"reference"`
}

// RequestBuild asks for a module to be built and waits for the answer.
//
// Waiting rather than returning immediately, because the thing a person wants after asking for a
// build is to know whether it worked. A build that is dispatched and forgotten needs somewhere to
// look afterwards, and there is nowhere yet.
func RequestBuild(ctx context.Context, channel *amqp.Channel, request BuildRequest,
	timeout time.Duration) (BuildResult, error) {

	// Its own queue for the answer, declared before the ask. Consuming from the shared exchange
	// would mean competing with the control plane's own consumer for a message meant for this
	// caller — which is the fault this package's own doc comment records having had.
	replies, err := channel.QueueDeclare(ReplyQueue(request.ID), false, true, true, false, nil)
	if err != nil {
		return BuildResult{}, err
	}
	// Bound to the exchange, and the answer comes back through it.
	//
	// **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. Found by
	// running it: the builder built, could not answer, and the connection closed saying only
	// "not allowed to publish to exchange ''".
	//
	// The cost is that every asker sees every result, which is why the correlation is checked
	// below rather than assumed.
	if err := channel.QueueBind(replies.Name, KeyBuilt, Exchange, false, nil); err != nil {
		return BuildResult{}, err
	}
	answers, err := 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 := 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, timeout)
	defer cancel()
	for {
		select {
		case <-waiting.Done():
			return BuildResult{}, fmt.Errorf(
				"no builder answered within %s. Something must be consuming %q, and nothing is "+
					"— or it is building something that takes longer than this",
				timeout, BuildQueue)
		case delivery, ok := <-answers:
			if !ok {
				return BuildResult{}, fmt.Errorf("the connection closed while waiting for a build")
			}
			var result BuildResult
			if err := json.Unmarshal(delivery.Body, &result); err != nil {
				return BuildResult{}, fmt.Errorf("a builder answered with something unreadable: %w", err)
			}
			if result.ID != request.ID {
				// Somebody else's answer on this queue. Ignored rather than returned, because
				// returning it would attribute one build's outcome to another's.
				continue
			}
			return result, nil
		}
	}
}