core(time-to-first-byte): use receiveHeadersStart

core/computed/metrics/time-to-first-byte.js

@@ -41,15 +41,17 @@ class TimeToFirstByte extends NavigationMetric {
    * @return {Promise<LH.Artifacts.Metric>}
    */
   static async computeObservedMetric(data, context) {
-    const {processedNavigation} = data;
-    const timeOriginTs = processedNavigation.timestamps.timeOrigin;
     const mainResource = await MainResource.request(data, context);
+    if (!mainResource.timing) {
+      throw new Error('missing timing for main resource');
+    }
 
-    // Technically TTFB is the start of the response headers not the end.
-    // That signal isn't available to us so we use header end time as a best guess.
-    const timestamp = mainResource.responseHeadersEndTime * 1000;
+    const {processedNavigation} = data;
+    const timeOriginTs = processedNavigation.timestamps.timeOrigin;
+    const timestampMs =
+      mainResource.timing.requestTime * 1000 + mainResource.timing.receiveHeadersStart;
+    const timestamp = timestampMs * 1000;
     const timing = (timestamp - timeOriginTs) / 1000;
-
     return {timing, timestamp};
   }
 }

core/lib/network-request.js

@@ -274,6 +274,7 @@ class NetworkRequest {
     }
 
     this._updateResponseHeadersEndTimeIfNecessary();
+    this._backfillReceiveHeaderStartTiming();
     this._updateTransferSizeForLightrider();
     this._updateTimingsForLightrider();
   }
@@ -293,6 +294,7 @@ class NetworkRequest {
     this.localizedFailDescription = data.errorText;
 
     this._updateResponseHeadersEndTimeIfNecessary();
+    this._backfillReceiveHeaderStartTiming();
     this._updateTransferSizeForLightrider();
     this._updateTimingsForLightrider();
   }
@@ -315,6 +317,7 @@ class NetworkRequest {
     this.networkEndTime = data.timestamp * 1000;
 
     this._updateResponseHeadersEndTimeIfNecessary();
+    this._backfillReceiveHeaderStartTiming();
   }
 
   /**
@@ -449,6 +452,19 @@ class NetworkRequest {
     }
   }
 
+  /**
+   * TODO(compat): remove M116.
+   * `timing.receiveHeadersStart` was added recently, and will be in M116. Until then,
+   * set it to receiveHeadersEnd, which is close enough, to allow consumers of NetworkRequest
+   * to use the new field without accounting for this backcompat.
+   */
+  _backfillReceiveHeaderStartTiming() {
+    // Do nothing if a value is already present!
+    if (!this.timing || this.timing.receiveHeadersStart !== undefined) return;
+
+    this.timing.receiveHeadersStart = this.timing.receiveHeadersEnd;
+  }
+
   /**
    * LR gets additional, accurate timing information from its underlying fetch infrastructure.  This
    * is passed in via X-Headers similar to 'X-TotalFetchedSize'.

core/test/computed/metrics/time-to-first-byte-test.js

@@ -53,7 +53,7 @@ function mockNetworkRecords() {
     networkRequestTime: 300,
     responseHeadersEndTime: 400,
     networkEndTime: 500,
-    timing: {sendEnd: 0, receiveHeadersEnd: 100},
+    timing: {sendEnd: 0, receiveHeadersStart: 100},
     transferSize: 16_000,
     url: mainDocumentUrl,
     frameId: 'ROOT_FRAME',