Bug 1864809 - Keep a reference to DNS workers while they are active. r=mkmelin

The worker is getting garbage collected before it has a chance to return with results.

Depends on D193866

Differential Revision: https://phabricator.services.mozilla.com/D193867

--HG--
extra : moz-landing-system : lando

mail/modules/DNS.sys.mjs

@@ -14,6 +14,9 @@ const NS_T_TXT = 16; // DNS_TYPE_TXT
 const NS_T_SRV = 33; // DNS_TYPE_SRV
 const NS_T_MX = 15; // DNS_TYPE_MX
 
+// References to all active workers, so they don't get GC'ed while busy.
+const workers = new Set();
+
 export const DNS = {
   /**
    * Constants for use with the lookup function.
@@ -33,13 +36,20 @@ export const DNS = {
    * @param aTypeID         The RR type to look up as a constant.
    * @returns A promise resolved when completed.
    */
-  lookup(aName, aTypeID) {
+  async lookup(aName, aTypeID) {
     const worker = new BasePromiseWorker("resource:///modules/dnsWorker.js");
-    return worker.post("execute", [
+    workers.add(worker);
+    let result;
+    try {
+      result = await worker.post("execute", [
         Services.appinfo.OS,
         "lookup",
         [...arguments],
       ]);
+    } finally {
+      workers.delete(worker);
+    }
+    return result;
   },
 
   /** Convenience functions */