// Package otlpresource builds the OpenTelemetry resource shared between the
// tracing and logger packages so spans, metrics, and logs describe the same
// process with the same attributes (service.name, process.pid, host.*, etc.).
// Keep the detector list in sync across signals.
package otlpresource

import (
	"context"
	"log/slog"

	"go.opentelemetry.io/otel/attribute"
	"go.opentelemetry.io/otel/sdk/resource"
	semconv "go.opentelemetry.io/otel/semconv/v1.26.0"
)

// Options carries optional attributes that only some callers supply.
// Zero values are skipped.
type Options struct {
	ServiceVersion string // added as semconv.ServiceVersionKey
	Environment    string // added as attribute "environment"
}

// New builds the shared OTel resource. Resource detection is best-effort:
// any detector error is logged via the provided slog logger and a usable
// resource is still returned. The returned error is currently always nil; the
// signature is preserved so future callers can distinguish hard failures if
// the contract changes.
func New(ctx context.Context, log *slog.Logger, opts Options) (*resource.Resource, error) {
	detectors := []resource.Option{
		resource.WithFromEnv(),      // OTEL_SERVICE_NAME / OTEL_RESOURCE_ATTRIBUTES
		resource.WithTelemetrySDK(), // telemetry.sdk.*
		resource.WithProcess(),      // process.pid + process.executable.* + process.runtime.*
		resource.WithOS(),
		resource.WithContainer(),
		resource.WithHost(),
	}
	if opts.ServiceVersion != "" {
		detectors = append(detectors,
			resource.WithAttributes(semconv.ServiceVersionKey.String(opts.ServiceVersion)))
	}
	if opts.Environment != "" {
		detectors = append(detectors,
			resource.WithAttributes(attribute.String("environment", opts.Environment)))
	}

	// Detector failures (e.g. processOwnerDetector hitting user.Current() in
	// a non-cgo build with no /etc/passwd entry and unset $USER) must not be
	// fatal. Downgrade everything to a warning and fall back to an empty
	// resource if the SDK returns nil.
	res, err := resource.New(ctx, detectors...)
	if err != nil && log != nil {
		log.Warn("otel resource setup", "err", err)
	}
	if res == nil {
		res = resource.Empty()
	}
	return res, nil
}