Bug 1975980 - report pref changes as expected failures, upload a modifiedPrefs.json artifacts with all the pref changes of the job, r=jmaher.

Differential Revision: https://phabricator.services.mozilla.com/D256279

Author: fqueze

testing/mochitest/browser-test.js

@@ -725,15 +725,6 @@ Tester.prototype = {
   },
 
   async checkPreferencesAfterTest() {
-    // This supports the --compare-preferences flag for browser tests.
-    // The implementation for plain mochitests is at
-    // https://searchfox.org/mozilla-central/rev/c25dbe453ff9ca10f2c6bdfb873893c515a29826/testing/mochitest/tests/SimpleTest/TestRunner.js#990-1012
-    if (!gConfig.comparePrefs) {
-      // Although the plain mochitest version of this logic resets preferences
-      // unconditionally, we do not, to minimize impact on the many existing
-      // tests. We only report failures when --compare-preferences is set.
-      return;
-    }
     if (!this._ignorePrefs) {
       const ignorePrefsFile = `chrome://mochikit/content/${gConfig.ignorePrefsFile}`;
       try {
@@ -751,10 +742,38 @@ Tester.prototype = {
     const failures = await window.SpecialPowers.comparePrefsToBaseline(
       this._ignorePrefs
     );
+
+    let testPath = this.currentTest.path;
+    if (testPath.startsWith("chrome://mochitests/content/browser/")) {
+      testPath = testPath.replace("chrome://mochitests/content/browser/", "");
+    }
+    let changedPrefs = [];
     for (let p of failures) {
       this.structuredLogger.error(
-        `TEST-UNEXPECTED-FAIL | ${this.currentTest.path} | changed preference: ${p}`
+        // We only report unexpected failures when --compare-preferences is set.
+        `TEST-${gConfig.comparePrefs ? "UN" : ""}EXPECTED-FAIL | ${testPath} | changed preference: ${p}`
+      );
+      changedPrefs.push(p);
+    }
+
+    if (changedPrefs.length && Services.env.exists("MOZ_UPLOAD_DIR")) {
+      let modifiedPrefsPath = PathUtils.join(
+        Services.env.get("MOZ_UPLOAD_DIR"),
+        "modifiedPrefs.json"
       );
+
+      if (!this._modifiedPrefs) {
+        try {
+          this._modifiedPrefs = JSON.parse(
+            await IOUtils.readUTF8(modifiedPrefsPath)
+          );
+        } catch (e) {
+          this._modifiedPrefs = {};
+        }
+      }
+
+      this._modifiedPrefs[testPath] = changedPrefs;
+      await IOUtils.writeJSON(modifiedPrefsPath, this._modifiedPrefs);
     }
   },