Fix marker-only compare profiles (#5130)

Author: gregtatum

src/profile-logic/merge-compare.js

@@ -217,7 +217,19 @@ export function mergeProfilesForDiffing(
 
     // We adjust the various times so that the 2 profiles are aligned at the
     // start and the data is consistent.
-    const startTimeAdjustment = -thread.samples.time[0];
+    let startTimeAdjustment = 0;
+    if (thread.samples.length) {
+      startTimeAdjustment = -thread.samples.time[0];
+    } else if (thread.markers.length) {
+      for (const startTime of thread.markers.startTime) {
+        // Find the first marker startTime.
+        if (startTime !== null) {
+          startTimeAdjustment = -startTime;
+          break;
+        }
+      }
+    }
+
     thread.samples = adjustTableTimestamps(thread.samples, startTimeAdjustment);
     thread.markers = adjustMarkerTimestamps(
       thread.markers,

src/test/unit/merge-compare.test.js

@@ -14,7 +14,7 @@ import {
 } from '../fixtures/profiles/processed-profile';
 import { markerSchemaForTests } from '../fixtures/profiles/marker-schema';
 import { ensureExists } from 'firefox-profiler/utils/flow';
-
+import { getTimeRangeIncludingAllThreads } from 'firefox-profiler/profile-logic/profile-data';
 import type { Thread } from 'firefox-profiler/types';
 
 describe('mergeProfilesForDiffing function', function () {
@@ -233,6 +233,38 @@ describe('mergeProfilesForDiffing function', function () {
     expect(mergedProfileThreadB.nativeSymbols.libIndex).toEqual([1, 1]);
     expect(mergedThread.nativeSymbols.libIndex).toEqual([0, 0, 1, 1]);
   });
+
+  it('should use marker timing if there are no samples', () => {
+    const profiles = [
+      getProfileWithMarkers([
+        ['Thread1 Marker1', 2],
+        ['Thread1 Marker2', 3, 5],
+        ['Thread1 Marker3', 6, 7],
+      ]),
+
+      getProfileWithMarkers([
+        ['Thread1 Marker1', 5],
+        ['Thread1 Marker2', 6, 8],
+        ['Thread1 Marker3', 10, 15],
+      ]),
+    ];
+
+    const profileState = stateFromLocation({
+      pathname: '/public/fakehash1/',
+      search: '?thread=0&v=3',
+      hash: '',
+    });
+
+    const { profile } = mergeProfilesForDiffing(profiles, [
+      profileState,
+      profileState,
+    ]);
+
+    expect(getTimeRangeIncludingAllThreads(profile)).toEqual({
+      start: 0,
+      end: 11,
+    });
+  });
 });
 
 describe('mergeThreads function', function () {