fix(rum-core): unsampled page load transaction start must be zero (#1435

)

* fix(rum-core): unsampled page load transaction start must be zero

* chore: change comment multiline format

packages/rum-core/src/performance-monitoring/navigation/capture-navigation.js

@@ -42,6 +42,14 @@ function captureNavigation(transaction) {
    * transactions set this flag to true
    */
   if (!transaction.captureTimings) {
+    /**
+     * Make sure page load transactions always starts at 0 irrespective if we capture navigation metrics or not
+     * otherwise we would be reporting different page load durations for sampled and unsampled transactions
+     */
+    if (transaction.type === PAGE_LOAD) {
+      transaction._start = 0
+    }
+
     return
   }
 

packages/rum-core/test/performance-monitoring/capture-navigation.spec.js

@@ -53,7 +53,7 @@ function mapSpan(s) {
 
 describe('Capture hard navigation', function () {
   /**
-   * Arbitrary value considering the transcation end would be called
+   * Arbitrary value considering the transaction end would be called
    * after load event has finished
    */
   const transactionEnd = timings.loadEventEnd + 100
@@ -398,6 +398,25 @@ describe('Capture hard navigation', function () {
     expect(tr.marks.custom.testMark).toEqual(start + markValue)
   })
 
+  // relevant for sampled and unsampled PAGE LOAD transactions:
+  ;[
+    {
+      value: true,
+      description: 'enabled'
+    },
+    {
+      value: false,
+      description: 'disabled'
+    }
+  ].forEach(({ value, description }) => {
+    it(`should set transaction._start to 0 for PAGE_LOAD when captureTimings flag is ${description}`, function () {
+      const tr = new Transaction('test', PAGE_LOAD)
+      tr.captureTimings = value
+      captureNavigation(tr)
+      expect(tr._start).toBe(0)
+    })
+  })
+
   it('should not add API calls as resource timing spans', function () {
     const unMock = mockGetEntriesByType()
     const tr = new Transaction('test', PAGE_LOAD, {