Bug 1631822 - Return a random TRR from getFastestTRR if requested. r=johannh

Differential Revision: https://phabricator.services.mozilla.com/D73359

browser/components/doh/TRRPerformance.jsm

@@ -295,22 +295,27 @@ class TRRRacer {
     }
   }
 
-  getFastestTRR() {
+  getFastestTRR(returnRandomDefault = false) {
     if (!this._complete) {
       throw new Error("getFastestTRR: Measurement still running.");
     }
 
-    return this._getFastestTRRFromResults(this._aggregator.results);
+    return this._getFastestTRRFromResults(
+      this._aggregator.results,
+      returnRandomDefault
+    );
   }
 
   /*
    * Given an array of { trr, time }, returns the trr with smallest mean time.
    * Separate from _getFastestTRR for easy unit-testing.
    *
-   * @returns The TRR with the fastest average time, or undefined if no result
-   *          had a valid measured time (!= -1)
+   * @returns The TRR with the fastest average time.
+   *          If returnRandomDefault is false-y, returns undefined if no valid
+   *          times were present in the results. Otherwise, returns one of the
+   *          present TRRs at random.
    */
-  _getFastestTRRFromResults(results) {
+  _getFastestTRRFromResults(results, returnRandomDefault = false) {
     // First, organize the results into a map of TRR -> array of times
     let TRRTimingMap = new Map();
     for (let { trr, time } of results) {
@@ -346,6 +351,10 @@ class TRRRacer {
       }
     }
 
+    if (returnRandomDefault && !fastestTRR) {
+      fastestTRR = trrs[Math.floor(Math.random() * trrs.length)];
+    }
+
     return fastestTRR;
   }
 

browser/components/doh/test/unit/test_TRRRacer.js

@@ -175,4 +175,23 @@ add_task(async function test_TRRRacer_getFastestTRRFromResults() {
   let racer = new TRRRacer();
   let fastest = racer._getFastestTRRFromResults(results);
   Assert.equal(fastest, "trr2");
+
+  // When no valid entries are available, undefined is the default output.
+  results = [
+    { trr: "trr1", time: -1 },
+    { trr: "trr2", time: -1 },
+  ];
+
+  fastest = racer._getFastestTRRFromResults(results);
+  Assert.equal(fastest, undefined);
+
+  // When passing `returnRandomDefault = true`, verify that both TRRs are
+  // possible outputs. The probability that the randomization is working
+  // correctly and we consistently get the same output after 50 iterations is
+  // 0.5^50 ~= 8.9*10^-16.
+  let firstResult = racer._getFastestTRRFromResults(results, true);
+  while (racer._getFastestTRRFromResults(results, true) == firstResult) {
+    continue;
+  }
+  Assert.ok(true, "Both TRRs were possible outputs when all results invalid.");
 });

browser/extensions/doh-rollout/experiments/trrselect/api.js

@@ -44,7 +44,7 @@ this.trrselect = class trrselect extends ExtensionAPI {
 
             await new Promise(resolve => {
               let racer = new TRRRacer(() => {
-                setDryRunResultAndRecordTelemetry(racer.getFastestTRR());
+                setDryRunResultAndRecordTelemetry(racer.getFastestTRR(true));
                 resolve();
               });
               racer.run();