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

GoogleChrome · devtools-bot · Jun 12, 2023 · May 23, 2023 · May 23, 2023 · Jun 9, 2023
commit f953d1a3938be471a5de052a0e793bb6378e5e88
diff --git a/core/computed/metrics/time-to-first-byte.js b/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 timing = (timestamp - timeOriginTs) / 1000;
-
+    const {processedNavigation} = data;
+    const timeOriginTs = processedNavigation.timestamps.timeOrigin;
+    const timestampMs =
+      mainResource.timing.requestTime * 1000 + mainResource.timing.receiveHeadersStart;
+    const timestamp = timestampMs * 1000;
+    const timing = timestampMs - timeOriginTs / 1000;
     return {timing, timestamp};
   }
 }

diff --git a/core/lib/network-request.js b/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'.

diff --git a/core/test/computed/metrics/time-to-first-byte-test.js b/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',

diff --git a/package.json b/package.json
@@ -191,7 +191,7 @@
     "chrome-launcher": "^0.15.2",
     "configstore": "^5.0.1",
     "csp_evaluator": "1.1.1",
-    "devtools-protocol": "0.0.1130274",
+    "devtools-protocol": "0.0.1151065",
     "enquirer": "^2.3.6",
     "http-link-header": "^1.1.1",
     "intl-messageformat": "^4.4.0",
@@ -215,8 +215,8 @@
     "yargs-parser": "^21.0.0"
   },
   "resolutions": {
-    "puppeteer/**/devtools-protocol": "0.0.1130274",
-    "puppeteer-core/**/devtools-protocol": "0.0.1130274"
+    "puppeteer/**/devtools-protocol": "0.0.1151065",
+    "puppeteer-core/**/devtools-protocol": "0.0.1151065"
   },
   "repository": "GoogleChrome/lighthouse",
   "keywords": [

diff --git a/yarn.lock b/yarn.lock
@@ -3002,10 +3002,10 @@ delayed-stream@~1.0.0:
   resolved "https://registry.yarnpkg.com/delayed-stream/-/delayed-stream-1.0.0.tgz#df3ae199acadfb7d440aaae0b29e2272b24ec619"
   integrity sha1-3zrhmayt+31ECqrgsp4icrJOxhk=
 
-devtools-protocol@0.0.1120988, devtools-protocol@0.0.1130274:
-  version "0.0.1130274"
-  resolved "https://registry.yarnpkg.com/devtools-protocol/-/devtools-protocol-0.0.1130274.tgz#1cd0c472a84fc9a09becaaed35a247a6eab9310c"
-  integrity sha512-kIozBWajgsi1g0W8yzALI4ZdCp6KG1yWaq8NN1ehQM3zX6JRegLSzfexz7XT5eFjmq1RkpMYgeKmfi3GsHrCLw==
+devtools-protocol@0.0.1120988, devtools-protocol@0.0.1151065:
+  version "0.0.1151065"
+  resolved "https://registry.yarnpkg.com/devtools-protocol/-/devtools-protocol-0.0.1151065.tgz#d7691671986617f3f3f806afe30c714254ac467a"
+  integrity sha512-2lYwxmian7ojCncbCennkq+xiymLza4MAaruCW078ORUzm+a8ZjYnYQxtk2h/3xXOM+9HGsYyWSR3UIhUfoSmQ==
 
 diff-sequences@^28.0.2:
   version "28.0.2"