Initial implementation: MQTT-based energy meter subscriber

Subscribes to Eastron SDM630 power meter data via MQTT broker,
decodes Modbus RTU frames, and writes readings to InfluxDB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

poller.go

@@ -0,0 +1,93 @@
+package main
+
+import (
+	"context"
+	"time"
+
+	influxdb2 "github.com/influxdata/influxdb-client-go/v2"
+	"github.com/influxdata/influxdb-client-go/v2/api"
+)
+
+const responseTimeout = 2 * time.Second
+
+// Poller periodically publishes FC4 read requests for every configured register
+// on every device, waiting for each response before sending the next request.
+// After all registers for a device are collected, one InfluxDB point is written.
+type Poller struct {
+	requester *Requester
+	handler   *handler
+	writeAPI  api.WriteAPIBlocking
+	org       string
+	bucket    string
+	devices   []DeviceConfig
+	interval  time.Duration
+}
+
+func NewPoller(r *Requester, h *handler, writeAPI api.WriteAPIBlocking, cfg Config, devices []DeviceConfig, interval time.Duration) *Poller {
+	return &Poller{
+		requester: r,
+		handler:   h,
+		writeAPI:  writeAPI,
+		org:       cfg.InfluxOrg,
+		bucket:    cfg.InfluxBucket,
+		devices:   devices,
+		interval:  interval,
+	}
+}
+
+// Run blocks until ctx is cancelled, polling immediately on start.
+func (p *Poller) Run(ctx context.Context) {
+	p.poll(ctx)
+	ticker := time.NewTicker(p.interval)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			p.poll(ctx)
+		}
+	}
+}
+
+func (p *Poller) poll(ctx context.Context) {
+	now := time.Now()
+	for _, dev := range p.devices {
+		point := influxdb2.NewPointWithMeasurement(dev.Name).SetTime(now)
+		hasFields := false
+
+		for _, reg := range dev.Registers {
+			if reg.Type != "input" {
+				logger.Printf("skipping holding register %s/%s (FC3 not implemented)", dev.Name, reg.Name)
+				continue
+			}
+
+			// Register the waiter before publishing to avoid a race with a fast response.
+			respCh := p.handler.waitForResponse(dev.SlaveAddress)
+
+			if err := p.requester.ReadInputRegisters(dev.Segment, byte(dev.SlaveAddress), reg.Address, reg.Count); err != nil {
+				logger.Printf("poll %s/%s: %v", dev.Name, reg.Name, err)
+				continue
+			}
+
+			select {
+			case readings := <-respCh:
+				for _, r := range readings {
+					point.AddField(r.Name, r.Value)
+					hasFields = true
+				}
+			case <-time.After(responseTimeout):
+				logger.Printf("timeout waiting for response: %s/%s", dev.Name, reg.Name)
+			case <-ctx.Done():
+				return
+			}
+		}
+
+		if !hasFields {
+			continue
+		}
+		if err := p.writeAPI.WritePoint(ctx, point); err != nil {
+			logger.Printf("influx write %s: %v", dev.Name, err)
+		}
+	}
+}