maybe it needs a lock?

main.go

@@ -20,7 +20,7 @@ import (
 var (
 	httpAddr       = flag.String("http-addr", ":9455", "Address to listen on")
 	inverterAddr   = flag.String("inverter-addr", "192.168.86.33:502", "Address of inverter")
-	scrapeInterval = flag.Duration("scrape-interval", 5*time.Second, "Period of modbus scraping loop")
+	scrapeInterval = flag.Duration("scrape-interval", 15*time.Second, "Period of modbus scraping loop")
 
 	registerGauges = make(map[uint16]prometheus.Gauge)
 
@@ -73,6 +73,21 @@ func statusHandler(w http.ResponseWriter, r *http.Request) {
 	http.Error(w, "not implemented", http.StatusNotFound)
 }
 
+func readRegs(client modbus.Client, start, qty uint16) {
+	data, err := client.ReadInputRegisters(5000, 50)
+	if err != nil {
+		die("Couldn't read input registers %d-%d: %v", start+1, start+qty, err)
+	}
+	for addr, reg := range sungrowInputRegs {
+		if addr <= start || addr > start+qty {
+			continue
+		}
+		val := reg.read(data[(addr-start-1)*2:])
+		//fmt.Printf("%s: %v %s\n", reg.name, val, reg.unit)
+		registerGauges[addr].Set(val)
+	}
+}
+
 func main() {
 	flag.Parse()
 	scrapeIntervalGauge.Set(float64(*scrapeInterval))
@@ -118,19 +133,8 @@ func main() {
 	for {
 		start := time.Now()
 		scrapeStart.SetToCurrentTime()
-		d0, err := client.ReadInputRegisters(5000, 100)
-		if err != nil {
-			die("Couldn't read input registers 5001-5100: %v", err)
-		}
-		d1, err := client.ReadInputRegisters(5100, 50)
-		if err != nil {
-			die("Couldn't read input registers 5101-5149: %v", err)
-		}
-		data := append(d0, d1...)
-		for addr, reg := range sungrowInputRegs {
-			//fmt.Printf("%s: %v %s\n", reg.name, val, reg.unit)
-			registerGauges[addr].Set(reg.read(data[(addr-5001)*2:]))
-		}
+		readRegs(client, 5000, 50)
+		readRegs(client, 5100, 50)
 		scrapeEnd.SetToCurrentTime()
 		scrapeDuration.Set(time.Since(start).Seconds())
 		scrapeCounter.Inc()

sungrow.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"sync"
 )
 
 const (
@@ -40,10 +41,11 @@ func getKey(conn io.ReadWriter) ([]byte, error) {
 // sungrowConn wraps a regular TCP connection with funky Sungrow encryption.
 type sungrowConn struct {
 	net.Conn
-
 	block cipher.Block
 	fifo  *bytes.Buffer
-	txid  uint16
+
+	mu   sync.Mutex
+	txid uint16
 }
 
 func (c *sungrowConn) Write(msg []byte) (int, error) {
@@ -51,7 +53,10 @@ func (c *sungrowConn) Write(msg []byte) (int, error) {
 		return c.Conn.Write(msg)
 	}
 
+	c.mu.Lock()
 	c.txid = binary.BigEndian.Uint16(msg[:2])
+	c.mu.Unlock()
+
 	c.fifo.Truncate(0)
 
 	bs := c.block.BlockSize()
@@ -76,6 +81,7 @@ func (c *sungrowConn) Read(out []byte) (int, error) {
 	if c.block == nil {
 		return c.Conn.Read(out)
 	}
+
 	if c.fifo.Len() > 0 {
 		return c.fifo.Read(out)
 	}
@@ -95,7 +101,9 @@ func (c *sungrowConn) Read(out []byte) (int, error) {
 	for cp := pkt; len(cp) > 0; cp = cp[bs:] {
 		c.block.Decrypt(cp, cp)
 	}
+	c.mu.Lock()
 	binary.BigEndian.PutUint16(pkt, c.txid)
+	c.mu.Unlock()
 	c.fifo.Write(pkt[:pktlen])
 	return c.fifo.Read(out)
 }

sungrow_map.go

@@ -36,6 +36,7 @@ var sungrowInputRegs = map[uint16]*register{
 	5035: {"power_factor", s16, 0.001, ""},
 	5036: {"frequency", u16, 0.1, "Hz"},
 	5049: {"nominal_reactive_power", s16, 0.1, "kVA"},
+
 	5113: {"daily_running_time", u16, 1, "m"},
 	5144: {"total_power_yield_2", u32, 0.1, "kWh"},
 	5148: {"frequency_2", u16, 0.01, "Hz"},