navidrome/core/artwork/trace.go

package artwork

import (
	"context"
	"errors"
	"io"
	"slices"
	"sync"
)

const (
	outcomeHit        = "hit"
	outcomeMiss       = "miss"
	outcomeUnreadable = "unreadable"
	outcomeSkipped    = "skipped"
	outcomeWouldTry   = "would-try"
	outcomeError      = "error"
	outcomeNotReached = "not-reached"
)

// traceStep is one candidate the priority chain considered.
type traceStep struct {
	Candidate string
	Outcome   string
	Detail    string
}

// chainTrace collects the walk of a single resolution. The artwork worker never attaches
// one; only the CLI does, so resolution stays allocation-free in the hot path.
type chainTrace struct {
	mu    sync.Mutex
	steps []traceStep
}

func (t *chainTrace) add(step traceStep) {
	if t == nil {
		return
	}
	t.mu.Lock()
	defer t.mu.Unlock()
	t.steps = append(t.steps, step)
}

func (t *chainTrace) Steps() []traceStep {
	t.mu.Lock()
	defer t.mu.Unlock()
	return slices.Clone(t.steps)
}

type traceCtxKey struct{}

func withTrace(ctx context.Context, t *chainTrace) context.Context {
	return context.WithValue(ctx, traceCtxKey{}, t)
}

func traceFrom(ctx context.Context) *chainTrace {
	t, _ := ctx.Value(traceCtxKey{}).(*chainTrace)
	return t
}

var errOfflineSkipped = errors.New("artwork: external lookup skipped (offline)")

// tracingGate records each external agent's outcome without changing what the gate returns.
func tracingGate(t *chainTrace, inner gateFunc) gateFunc {
	return func(name string, f func() (io.ReadCloser, string, error)) (io.ReadCloser, string, error) {
		r, path, err := inner(name, f)
		candidate := "external:" + name
		switch {
		case r != nil:
			t.add(traceStep{Candidate: candidate, Outcome: outcomeHit, Detail: path})
		case errors.Is(err, errBreakerOpen):
			t.add(traceStep{Candidate: candidate, Outcome: outcomeSkipped, Detail: "circuit breaker open"})
		case isTransientExternal(err):
			t.add(traceStep{Candidate: candidate, Outcome: outcomeError, Detail: err.Error()})
		default:
			t.add(traceStep{Candidate: candidate, Outcome: outcomeMiss})
		}
		return r, path, err
	}
}

// offlineGate reports which agents would be asked without asking them, so a diagnostic
// command cannot add load to a provider that is already rate-limiting us.
func offlineGate(t *chainTrace) gateFunc {
	return func(name string, _ func() (io.ReadCloser, string, error)) (io.ReadCloser, string, error) {
		t.add(traceStep{Candidate: "external:" + name, Outcome: outcomeWouldTry})
		return nil, "", errOfflineSkipped
	}
}