fermentord/pkg/temperature/ds18b20.go

package temperature

// Kernel documentation: https://www.kernel.org/doc/html/latest/w1/slaves/w1_therm.html

import (
	"context"
	"errors"
	"io/ioutil"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"git.joco.dk/sng/fermentord/internal/metrics"
	"github.com/getsentry/sentry-go"
	"github.com/rs/zerolog/log"
)

var (
	// ErrReadSensor indicates that the sensor could not be read
	ErrReadSensor = errors.New("failed to read sensor")
)

var (
	// C receives the temperature readings
	C chan TemperatureReading

	// ConfigUpdates receives the list of sensors to read
	ConfigUpdates chan []string
	sensors       []string
)

func Initialize() {
	sensors = make([]string, 0)
	C = make(chan TemperatureReading, 30)
	ConfigUpdates = make(chan []string, 1)
}

func PollSensors(ctx context.Context, wg *sync.WaitGroup, readingInterval time.Duration) {
	defer wg.Done()

	if readingInterval < 750*time.Millisecond {
		log.Fatal().
			Dur("interval", readingInterval).
			Msg("Reading interval must be at least 750 ms.")
	}

	ticker := time.NewTicker(readingInterval)
	defer ticker.Stop()

	for {
		select {
		case <-ticker.C:
			// Trigger a bulk read and wait 750ms for sensors to convert data.
			triggerBulkRead()
			time.Sleep(750 * time.Millisecond)

			readSensors()

		case c := <-ConfigUpdates:
			sensors = c

		case <-ctx.Done():
			close(C)
			return
		}
	}
}

func readSensors() {
	for _, sensor := range sensors {
		start := time.Now()
		t, err := read(sensor)
		dur := time.Since(start).Seconds()

		if err != nil {
			sentry.CaptureException(err)
			log.Error().
				Err(err).
				Str("sensor", sensor).
				Msg("Error reading temperature sensor.")
			continue
		}

		metrics.TemperatureSensorReadingDegreesCelcius.
			WithLabelValues("sensor", sensor).
			Set(float64(t))

		metrics.TemperatureSensorReadingDurationSeconds.
			WithLabelValues("sensor", sensor).
			Observe(dur)

		C <- TemperatureReading{
			Time:         time.Now(),
			Sensor:       sensor,
			MilliDegrees: t,
		}
	}
}

func triggerBulkRead() error {
	f, err := os.OpenFile("/sys/bus/w1/w1_bus_master/therm_bulk_read", os.O_APPEND, os.ModeAppend)
	if err != nil {
		return err
	}
	defer f.Close()

	_, err = f.WriteString("trigger\n")
	return err
}

// read returns the temperature of the specified sensor in millidegrees celcius.
func read(sensor string) (int64, error) {
	path := "/sys/bus/w1/devices/" + sensor + "/w1_slave"

	data, err := ioutil.ReadFile(path)
	if err != nil {
		return 0.0, err
	}

	raw := string(data)

	if !strings.Contains(raw, " YES") {
		return 0.0, ErrReadSensor
	}

	i := strings.LastIndex(raw, "t=")
	if i == -1 {
		return 0.0, ErrReadSensor
	}

	c, err := strconv.ParseInt(raw[i+2:len(raw)-1], 10, 64)
	if err != nil {
		return 0.0, err
	}

	return c, nil
}