Don't accidentally try to read from the same Modbus device simultaneo…

…usly

Shouldn't really be a problem with Modbus TCP, but better safe than sorry

package.json

@@ -32,6 +32,7 @@
   },
   "dependencies": {
     "@influxdata/influxdb-client": "^1.33.2",
+    "async-mutex": "^0.5.0",
     "modbus-serial": "^8.0.16",
     "mqtt": "^5.1.2",
     "set-interval-async": "^3.0.3",

src/sensor/modbus.ts

@@ -5,11 +5,13 @@ import { createLogger } from '../logger'
 import { getClient, requestTimeout } from '../modbus/client'
 import { getRegisterLength, ModbusRegister, RegisterType, stringify } from '../modbus/register'
 import ModbusRTU from 'modbus-serial'
+import { Mutex } from 'async-mutex'
 
 export const DEFAULT_PORT = 502
 export const DEFAULT_UNIT = 1
 
 const logger = createLogger('sensor.modbus')
+const mutex = new Mutex()
 
 export const getSensorData: PowerSensorPollFunction = async (
   timestamp: number,
@@ -58,16 +60,19 @@ const readRegisters = async (
   const address = register.address
   const length = getRegisterLength(register)
 
-  switch (register.registerType) {
-    case RegisterType.HOLDING_REGISTER:
-      return client.readHoldingRegisters(address, length)
-    case RegisterType.INPUT_REGISTER:
-      return client.readInputRegisters(address, length)
-    case RegisterType.COIL:
-      return client.readCoils(address, length)
-    case RegisterType.DISCRETE_INPUT:
-      return client.readDiscreteInputs(address, length)
-  }
+  // Serialize access to the underlying Modbus client
+  return mutex.runExclusive(async () => {
+    switch (register.registerType) {
+      case RegisterType.HOLDING_REGISTER:
+        return client.readHoldingRegisters(address, length)
+      case RegisterType.INPUT_REGISTER:
+        return client.readInputRegisters(address, length)
+      case RegisterType.COIL:
+        return client.readCoils(address, length)
+      case RegisterType.DISCRETE_INPUT:
+        return client.readDiscreteInputs(address, length)
+    }
+  })
 }
 
 const parseReadRegisterResult = (result: ReadRegisterResult | ReadCoilResult, register: ModbusRegister): number => {