NodeJS Async/Await or Promise for net-snmp module - node.js

Question

please someone help, I just cant get it.
Can you please help on how to make async/await or promise (doneCb), so script waits for first vlc_snmp(..) to finish and then to call next?
Example:
function doneCb(error) {
console.log(final_result);
final_result = [];
if (error)
console.error(error.toString());
}
function feedCb(varbinds) {
for (var i = 0; i < varbinds.length; i++) {
if (snmp.isVarbindError(varbinds[i]))
console.error(snmp.varbindError(varbinds[i]));
else {
var snmp_rez = {
oid: (varbinds[i].oid).toString()
value: (varbinds[i].value).toString()
};
final_result.push(snmp_rez);
}
}
}
var session = snmp.createSession(VLC_IP, "public", options);
var maxRepetitions = 20;
function vlc_snmp(OID) {
session.subtree(OID_, maxRepetitions, feedCb, doneCb);
}
vlc_snmp(OID_SERIAL_NUMBER);
//wait OID_SERIAL_NUMBER to finish and then call next
vlc_snmp(OID_DEVICE_NAME);

Answer 1

You should be able to use the async / await statements to wait for your done callback to be called. We wrap this callback in the vlc_snmp function, and return a Promise. This allows us to use the await statement.
I've mocked out some of the code that I don't have access to, it should behave somewhat similarly to the real code.
The key point here is, when a function returns a Promise we can await the result in an async function, that will give you the behaviour you wish.
final_result = [];
const VLC_IP = "";
const options = {};
const OID_ = "OID_";
const OID_SERIAL_NUMBER = "OID_SERIAL_NUMBER"
const OID_DEVICE_NAME = "OID_DEVICE_NAME"
// Mock out snmp to call feedcb and donecb
const snmp = {
createSession(...args) {
return {
subtree(oid, maxRepetitions, feedCb, doneCb) {
setTimeout(feedCb, 500, [{ oid, value: 42}])
setTimeout(doneCb, 1000);
}
}
},
isVarbindError(input) {
return false;
}
}
function doneCb(error) {
console.log("doneCb: final_result:", final_result);
final_result = [];
if (error)
console.error("doneCb: Error:", error.toString());
}
function feedCb(varbinds) {
for (var i = 0; i < varbinds.length; i++) {
if (snmp.isVarbindError(varbinds[i]))
console.error(snmp.varbindError(varbinds[i]));
else {
var snmp_rez = {
oid: (varbinds[i].oid).toString(),
value: (varbinds[i].value).toString()
};
final_result.push(snmp_rez);
}
}
}
var session = snmp.createSession(VLC_IP, "public", options);
var maxRepetitions = 20;
function vlc_snmp(OID) {
return new Promise((resolve,reject) => {
session.subtree(OID, maxRepetitions, feedCb, (error) => {
// This is a wrapper callback on doneCb
// Always call the doneCb
doneCb(error);
if (error) {
reject(error);
} else {
resolve();
}
});
});
}
async function run_vls_snmp() {
console.log("run_vls_snmp: Calling vlc_snmp(OID_SERIAL_NUMBER)...");
await vlc_snmp(OID_SERIAL_NUMBER);
console.log("run_vls_snmp: Calling vlc_snmp(OID_DEVICE_NAME)...");
//wait OID_SERIAL_NUMBER to finish and then call next
await vlc_snmp(OID_DEVICE_NAME);
}
run_vls_snmp();