path: root/toolkit/components/nimbus/lib/ExperimentManager.jsm

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

const EXPORTED_SYMBOLS = ["ExperimentManager", "_ExperimentManager"];

const { XPCOMUtils } = ChromeUtils.importESModule(
  "resource://gre/modules/XPCOMUtils.sys.mjs"
);

const lazy = {};

ChromeUtils.defineESModuleGetters(lazy, {
  FirstStartup: "resource://gre/modules/FirstStartup.sys.mjs",
  TelemetryEnvironment: "resource://gre/modules/TelemetryEnvironment.sys.mjs",
});

XPCOMUtils.defineLazyModuleGetters(lazy, {
  ClientEnvironment: "resource://normandy/lib/ClientEnvironment.jsm",
  ExperimentStore: "resource://nimbus/lib/ExperimentStore.jsm",
  NimbusFeatures: "resource://nimbus/ExperimentAPI.jsm",
  NormandyUtils: "resource://normandy/lib/NormandyUtils.jsm",
  Sampling: "resource://gre/modules/components-utils/Sampling.jsm",
  TelemetryEvents: "resource://normandy/lib/TelemetryEvents.jsm",
  PrefUtils: "resource://normandy/lib/PrefUtils.jsm",
});

XPCOMUtils.defineLazyGetter(lazy, "log", () => {
  const { Logger } = ChromeUtils.import(
    "resource://messaging-system/lib/Logger.jsm"
  );
  return new Logger("ExperimentManager");
});

const TELEMETRY_EVENT_OBJECT = "nimbus_experiment";
const TELEMETRY_EXPERIMENT_ACTIVE_PREFIX = "nimbus-";
const TELEMETRY_DEFAULT_EXPERIMENT_TYPE = "nimbus";

const UPLOAD_ENABLED_PREF = "datareporting.healthreport.uploadEnabled";
const STUDIES_OPT_OUT_PREF = "app.shield.optoutstudies.enabled";

const STUDIES_ENABLED_CHANGED = "nimbus:studies-enabled-changed";

function featuresCompat(branch) {
  if (!branch || (!branch.feature && !branch.features)) {
    return [];
  }
  let { features } = branch;
  // In <=v1.5.0 of the Nimbus API, experiments had single feature
  if (!features) {
    features = [branch.feature];
  }

  return features;
}

function getFeatureFromBranch(branch, featureId) {
  return featuresCompat(branch).find(
    featureConfig => featureConfig.featureId === featureId
  );
}

/**
 * A module for processes Experiment recipes, choosing and storing enrollment state,
 * and sending experiment-related Telemetry.
 */
class _ExperimentManager {
  constructor({ id = "experimentmanager", store } = {}) {
    this.id = id;
    this.store = store || new lazy.ExperimentStore();
    this.sessions = new Map();
    // By default, no extra context.
    this.extraContext = {};
    Services.prefs.addObserver(UPLOAD_ENABLED_PREF, this);
    Services.prefs.addObserver(STUDIES_OPT_OUT_PREF, this);

    // A Map from pref names to pref observers and metadata. See
    // `_updatePrefObservers` for the full structure.
    this._prefs = new Map();
    // A Map from enrollment slugs to a Set of prefs that enrollment is setting
    // or would set (e.g., if the enrollment is a rollout and there wasn't an
    // active experiment already setting it).
    this._prefsBySlug = new Map();
  }

  get studiesEnabled() {
    return (
      Services.prefs.getBoolPref(UPLOAD_ENABLED_PREF) &&
      Services.prefs.getBoolPref(STUDIES_OPT_OUT_PREF)
    );
  }

  /**
   * Creates a targeting context with following filters:
   *
   *   * `activeExperiments`: an array of slugs of all the active experiments
   *   * `isFirstStartup`: a boolean indicating whether or not the current enrollment
   *      is performed during the first startup
   *
   * @returns {Object} A context object
   * @memberof _ExperimentManager
   */
  createTargetingContext() {
    let context = {
      isFirstStartup: lazy.FirstStartup.state === lazy.FirstStartup.IN_PROGRESS,
      ...this.extraContext,
    };
    Object.defineProperty(context, "activeExperiments", {
      get: async () => {
        await this.store.ready();
        return this.store.getAllActive().map(exp => exp.slug);
      },
    });
    Object.defineProperty(context, "activeRollouts", {
      get: async () => {
        await this.store.ready();
        return this.store.getAllRollouts().map(rollout => rollout.slug);
      },
    });
    return context;
  }

  /**
   * Runs on startup, including before first run.
   *
   * @param {object} extraContext extra targeting context provided by the
   * ambient environment.
   */
  async onStartup(extraContext = {}) {
    await this.store.init();
    this.extraContext = extraContext;

    const restoredExperiments = this.store.getAllActive();
    const restoredRollouts = this.store.getAllRollouts();

    for (const experiment of restoredExperiments) {
      this.setExperimentActive(experiment);
      if (this._restoreEnrollmentPrefs(experiment)) {
        this._updatePrefObservers(experiment);
      }
    }
    for (const rollout of restoredRollouts) {
      this.setExperimentActive(rollout);
      if (this._restoreEnrollmentPrefs(rollout)) {
        this._updatePrefObservers(rollout);
      }
    }

    this.observe();
  }

  /**
   * Runs every time a Recipe is updated or seen for the first time.
   * @param {RecipeArgs} recipe
   * @param {string} source
   */
  async onRecipe(recipe, source) {
    const { slug, isEnrollmentPaused } = recipe;

    if (!source) {
      throw new Error("When calling onRecipe, you must specify a source.");
    }

    if (!this.sessions.has(source)) {
      this.sessions.set(source, new Set());
    }
    this.sessions.get(source).add(slug);

    if (this.store.has(slug)) {
      this.updateEnrollment(recipe);
    } else if (isEnrollmentPaused) {
      lazy.log.debug(`Enrollment is paused for "${slug}"`);
    } else if (!(await this.isInBucketAllocation(recipe.bucketConfig))) {
      lazy.log.debug("Client was not enrolled because of the bucket sampling");
    } else {
      await this.enroll(recipe, source);
    }
  }

  _checkUnseenEnrollments(
    enrollments,
    sourceToCheck,
    recipeMismatches,
    invalidRecipes,
    invalidBranches,
    invalidFeatures
  ) {
    for (const enrollment of enrollments) {
      const { slug, source } = enrollment;
      if (sourceToCheck !== source) {
        continue;
      }
      if (!this.sessions.get(source)?.has(slug)) {
        lazy.log.debug(`Stopping study for recipe ${slug}`);
        try {
          let reason;
          if (recipeMismatches.includes(slug)) {
            reason = "targeting-mismatch";
          } else if (invalidRecipes.includes(slug)) {
            reason = "invalid-recipe";
          } else if (invalidBranches.has(slug) || invalidFeatures.has(slug)) {
            reason = "invalid-branch";
          } else {
            reason = "recipe-not-seen";
          }
          this.unenroll(slug, reason);
        } catch (err) {
          Cu.reportError(err);
        }
      }
    }
  }

  /**
   * Removes stored enrollments that were not seen after syncing with Remote Settings
   * Runs when the all recipes been processed during an update, including at first run.
   * @param {string} sourceToCheck
   * @param {object} options Extra context used in telemetry reporting
   * @param {string[]} options.recipeMismatches
   *         The list of experiments that do not match targeting.
   * @param {string[]} options.invalidRecipes
   *         The list of recipes that do not match
   * @param {Map<string, string[]>} options.invalidBranches
   *         A mapping of experiment slugs to a list of branches that failed
   *         feature validation.
   * @param {Map<string, string[]>} options.invalidFeatures
   *        The mapping of experiment slugs to a list of invalid feature IDs.
   */
  onFinalize(
    sourceToCheck,
    {
      recipeMismatches = [],
      invalidRecipes = [],
      invalidBranches = new Map(),
      invalidFeatures = new Map(),
      validationEnabled = true,
    } = {}
  ) {
    if (!sourceToCheck) {
      throw new Error("When calling onFinalize, you must specify a source.");
    }
    const activeExperiments = this.store.getAllActive();
    const activeRollouts = this.store.getAllRollouts();
    this._checkUnseenEnrollments(
      activeExperiments,
      sourceToCheck,
      recipeMismatches,
      invalidRecipes,
      invalidBranches,
      invalidFeatures
    );
    this._checkUnseenEnrollments(
      activeRollouts,
      sourceToCheck,
      recipeMismatches,
      invalidRecipes,
      invalidBranches,
      invalidFeatures
    );

    // If validation is disabled, then we will never send validation failed
    // telemetry.
    if (validationEnabled) {
      for (const slug of invalidRecipes) {
        this.sendValidationFailedTelemetry(slug, "invalid-recipe");
      }
      for (const [slug, branches] of invalidBranches.entries()) {
        for (const branch of branches) {
          this.sendValidationFailedTelemetry(slug, "invalid-branch", {
            branch,
          });
        }
      }
      for (const [slug, featureIds] of invalidFeatures.entries()) {
        for (const featureId of featureIds) {
          this.sendValidationFailedTelemetry(slug, "invalid-feature", {
            feature: featureId,
          });
        }
      }
    }

    this.sessions.delete(sourceToCheck);
  }

  /**
   * Bucket configuration specifies a specific percentage of clients that can
   * be enrolled.
   * @param {BucketConfig} bucketConfig
   * @returns {Promise<boolean>}
   */
  isInBucketAllocation(bucketConfig) {
    if (!bucketConfig) {
      lazy.log.debug("Cannot enroll if recipe bucketConfig is not set.");
      return false;
    }

    let id;
    if (bucketConfig.randomizationUnit === "normandy_id") {
      id = lazy.ClientEnvironment.userId;
    } else {
      // Others not currently supported.
      lazy.log.debug(
        `Invalid randomizationUnit: ${bucketConfig.randomizationUnit}`
      );
      return false;
    }

    return lazy.Sampling.bucketSample(
      [id, bucketConfig.namespace],
      bucketConfig.start,
      bucketConfig.count,
      bucketConfig.total
    );
  }

  /**
   * Start a new experiment by enrolling the users
   *
   * @param {RecipeArgs} recipe
   * @param {string} source
   * @returns {Promise<Enrollment>} The experiment object stored in the data store
   * @rejects {Error}
   * @memberof _ExperimentManager
   */
  async enroll(recipe, source) {
    let { slug, branches } = recipe;
    if (this.store.has(slug)) {
      this.sendFailureTelemetry("enrollFailed", slug, "name-conflict");
      throw new Error(`An experiment with the slug "${slug}" already exists.`);
    }

    let storeLookupByFeature = recipe.isRollout
      ? this.store.getRolloutForFeature.bind(this.store)
      : this.store.hasExperimentForFeature.bind(this.store);
    const branch = await this.chooseBranch(slug, branches);
    const features = featuresCompat(branch);
    for (let feature of features) {
      if (storeLookupByFeature(feature?.featureId)) {
        lazy.log.debug(
          `Skipping enrollment for "${slug}" because there is an existing ${
            recipe.isRollout ? "rollout" : "experiment"
          } for this feature.`
        );
        this.sendFailureTelemetry("enrollFailed", slug, "feature-conflict");

        return null;
      }
    }

    return this._enroll(recipe, branch, source);
  }

  _enroll(
    {
      slug,
      experimentType = TELEMETRY_DEFAULT_EXPERIMENT_TYPE,
      userFacingName,
      userFacingDescription,
      featureIds,
      isRollout,
    },
    branch,
    source,
    options = {}
  ) {
    const { prefs, prefsToSet } = this._getPrefsForBranch(branch, isRollout);

    /** @type {Enrollment} */
    const experiment = {
      slug,
      branch,
      active: true,
      enrollmentId: lazy.NormandyUtils.generateUuid(),
      experimentType,
      source,
      userFacingName,
      userFacingDescription,
      lastSeen: new Date().toJSON(),
      featureIds,
      prefs,
    };

    if (typeof isRollout !== "undefined") {
      experiment.isRollout = isRollout;
    }

    // Tag this as a forced enrollment. This prevents all unenrolling unless
    // manually triggered from about:studies
    if (options.force) {
      experiment.force = true;
    }

    if (isRollout) {
      experiment.experimentType = "rollout";
      this.store.addEnrollment(experiment);
      this.setExperimentActive(experiment);
    } else {
      this.store.addEnrollment(experiment);
      this.setExperimentActive(experiment);
    }
    this.sendEnrollmentTelemetry(experiment);

    this._setEnrollmentPrefs(prefsToSet);
    this._updatePrefObservers(experiment);

    lazy.log.debug(
      `New ${isRollout ? "rollout" : "experiment"} started: ${slug}, ${
        branch.slug
      }`
    );

    return experiment;
  }

  forceEnroll(recipe, branch, source = "force-enrollment") {
    /**
     * If we happen to be enrolled in an experiment for the same feature
     * we need to unenroll from that experiment.
     * If the experiment has the same slug after unenrollment adding it to the
     * store will overwrite the initial experiment.
     */
    const features = featuresCompat(branch);
    for (let feature of features) {
      const isRollout = recipe.isRollout ?? false;
      let enrollment = isRollout
        ? this.store.getRolloutForFeature(feature?.featureId)
        : this.store.getExperimentForFeature(feature?.featureId);
      if (enrollment) {
        lazy.log.debug(
          `Existing ${
            isRollout ? "rollout" : "experiment"
          } found for the same feature ${feature.featureId}, unenrolling.`
        );

        this.unenroll(enrollment.slug, source);
      }
    }

    recipe.userFacingName = `${recipe.userFacingName} - Forced enrollment`;

    const slug = `optin-${recipe.slug}`;
    const enrollment = this._enroll(
      {
        ...recipe,
        slug,
      },
      branch,
      source,
      { force: true }
    );

    Services.obs.notifyObservers(null, "nimbus:force-enroll", slug);

    return enrollment;
  }

  /**
   * Update an enrollment that was already set
   *
   * @param {RecipeArgs} recipe
   */
  updateEnrollment(recipe) {
    /** @type Enrollment */
    const enrollment = this.store.get(recipe.slug);

    // Don't update experiments that were already unenrolled.
    if (enrollment.active === false) {
      lazy.log.debug(`Enrollment ${recipe.slug} has expired, aborting.`);
      return false;
    }

    // Stay in the same branch, don't re-sample every time.
    const branch = recipe.branches.find(
      branch => branch.slug === enrollment.branch.slug
    );

    if (!branch) {
      // Our branch has been removed. Unenroll.
      this.unenroll(recipe.slug, "branch-removed");
    }

    return true;
  }

  /**
   * Stop an enrollment that is currently active
   *
   * @param {string} slug
   *        The slug of the enrollment to stop.
   * @param {string} reason
   *        An optional reason for the unenrollment.
   *
   *        This will be reported in telemetry.
   */
  unenroll(slug, reason = "unknown") {
    const enrollment = this.store.get(slug);
    if (!enrollment) {
      this.sendFailureTelemetry("unenrollFailed", slug, "does-not-exist");
      throw new Error(`Could not find an experiment with the slug "${slug}"`);
    }

    this._unenroll(enrollment, { reason });
  }

  /**
   * Stop an enrollment that is currently active.
   *
   * @param {Enrollment} enrollment
   *        The enrollment to end.
   *
   * @param {object} options
   * @param {string} options.reason
   *        An optional reason for the unenrollment.
   *
   *        This will be reported in telemetry.
   *
   * @param {object?} options.changedPref
   *        If the unenrollment was due to pref change, this will contain the
   *        information about the pref that changed.
   *
   * @param {string} options.changedPref.name
   *        The name of the pref that caused the unenrollment.
   *
   * @param {string} options.changedPref.branch
   *        The branch that was changed ("user" or "default").
   */
  _unenroll(
    enrollment,
    { reason = "unknown", changedPref = undefined, duringRestore = false } = {}
  ) {
    const { slug } = enrollment;

    if (!enrollment.active) {
      this.sendFailureTelemetry("unenrollFailed", slug, "already-unenrolled");
      throw new Error(
        `Cannot stop experiment "${slug}" because it is already expired`
      );
    }

    lazy.TelemetryEnvironment.setExperimentInactive(slug);
    // We also need to set the experiment inactive in the Glean Experiment API
    Services.fog.setExperimentInactive(slug);
    this.store.updateExperiment(slug, { active: false });

    lazy.TelemetryEvents.sendEvent("unenroll", TELEMETRY_EVENT_OBJECT, slug, {
      reason,
      branch: enrollment.branch.slug,
      enrollmentId:
        enrollment.enrollmentId || lazy.TelemetryEvents.NO_ENROLLMENT_ID_MARKER,
    });
    // Sent Glean event equivalent
    Glean.nimbusEvents.unenrollment.record({
      experiment: slug,
      branch: enrollment.branch.slug,
      enrollment_id:
        enrollment.enrollmentId || lazy.TelemetryEvents.NO_ENROLLMENT_ID_MARKER,
      reason,
    });

    this._unsetEnrollmentPrefs(enrollment, { changedPref, duringRestore });

    lazy.log.debug(`Recipe unenrolled: ${slug}`);
  }

  /**
   * Unenroll from all active studies if user opts out.
   */
  observe(aSubject, aTopic, aPrefName) {
    if (!this.studiesEnabled) {
      for (const { slug } of this.store.getAllActive()) {
        this.unenroll(slug, "studies-opt-out");
      }
      for (const { slug } of this.store.getAllRollouts()) {
        this.unenroll(slug, "studies-opt-out");
      }
    }

    Services.obs.notifyObservers(null, STUDIES_ENABLED_CHANGED);
  }

  /**
   * Send Telemetry for undesired event
   *
   * @param {string} eventName
   * @param {string} slug
   * @param {string} reason
   */
  sendFailureTelemetry(eventName, slug, reason) {
    lazy.TelemetryEvents.sendEvent(eventName, TELEMETRY_EVENT_OBJECT, slug, {
      reason,
    });
    if (eventName == "enrollFailed") {
      Glean.nimbusEvents.enrollFailed.record({
        experiment: slug,
        reason,
      });
    } else if (eventName == "unenrollFailed") {
      Glean.nimbusEvents.unenrollFailed.record({
        experiment: slug,
        reason,
      });
    }
  }

  sendValidationFailedTelemetry(slug, reason, extra) {
    lazy.TelemetryEvents.sendEvent(
      "validationFailed",
      TELEMETRY_EVENT_OBJECT,
      slug,
      {
        reason,
        ...extra,
      }
    );
    Glean.nimbusEvents.validationFailed.record({
      experiment: slug,
      reason,
      ...extra,
    });
  }

  /**
   *
   * @param {Enrollment} experiment
   */
  sendEnrollmentTelemetry({ slug, branch, experimentType, enrollmentId }) {
    lazy.TelemetryEvents.sendEvent("enroll", TELEMETRY_EVENT_OBJECT, slug, {
      experimentType,
      branch: branch.slug,
      enrollmentId:
        enrollmentId || lazy.TelemetryEvents.NO_ENROLLMENT_ID_MARKER,
    });
    Glean.nimbusEvents.enrollment.record({
      experiment: slug,
      branch: branch.slug,
      enrollment_id:
        enrollmentId || lazy.TelemetryEvents.NO_ENROLLMENT_ID_MARKER,
      experiment_type: experimentType,
    });
  }

  /**
   * Sets Telemetry when activating an experiment.
   *
   * @param {Enrollment} experiment
   */
  setExperimentActive(experiment) {
    lazy.TelemetryEnvironment.setExperimentActive(
      experiment.slug,
      experiment.branch.slug,
      {
        type: `${TELEMETRY_EXPERIMENT_ACTIVE_PREFIX}${experiment.experimentType}`,
        enrollmentId:
          experiment.enrollmentId ||
          lazy.TelemetryEvents.NO_ENROLLMENT_ID_MARKER,
      }
    );
    // Report the experiment to the Glean Experiment API
    Services.fog.setExperimentActive(experiment.slug, experiment.branch.slug, {
      type: `${TELEMETRY_EXPERIMENT_ACTIVE_PREFIX}${experiment.experimentType}`,
      enrollmentId:
        experiment.enrollmentId || lazy.TelemetryEvents.NO_ENROLLMENT_ID_MARKER,
    });
  }

  /**
   * Generate Normandy UserId respective to a branch
   * for a given experiment.
   *
   * @param {string} slug
   * @param {Array<{slug: string; ratio: number}>} branches
   * @param {string} namespace
   * @param {number} start
   * @param {number} count
   * @param {number} total
   * @returns {Promise<{[branchName: string]: string}>} An object where
   * the keys are branch names and the values are user IDs that will enroll
   * a user for that particular branch. Also includes a `notInExperiment` value
   * that will not enroll the user in the experiment
   */
  async generateTestIds({ slug, branches, namespace, start, count, total }) {
    const branchValues = {};

    if (!slug || !namespace) {
      throw new Error(`slug, namespace not in expected format`);
    }

    if (!(start < total && count < total)) {
      throw new Error("Must include start, count, and total as integers");
    }

    if (
      !Array.isArray(branches) ||
      branches.filter(branch => branch.slug && branch.ratio).length !==
        branches.length
    ) {
      throw new Error("branches parameter not in expected format");
    }

    while (Object.keys(branchValues).length < branches.length + 1) {
      const id = lazy.NormandyUtils.generateUuid();
      const enrolls = await lazy.Sampling.bucketSample(
        [id, namespace],
        start,
        count,
        total
      );
      // Does this id enroll the user in the experiment
      if (enrolls) {
        // Choose a random branch
        const { slug: pickedBranch } = await this.chooseBranch(
          slug,
          branches,
          id
        );

        if (!Object.keys(branchValues).includes(pickedBranch)) {
          branchValues[pickedBranch] = id;
          lazy.log.debug(`Found a value for "${pickedBranch}"`);
        }
      } else if (!branchValues.notInExperiment) {
        branchValues.notInExperiment = id;
      }
    }
    return branchValues;
  }

  /**
   * Choose a branch randomly.
   *
   * @param {string} slug
   * @param {Branch[]} branches
   * @returns {Promise<Branch>}
   * @memberof _ExperimentManager
   */
  async chooseBranch(slug, branches, userId = lazy.ClientEnvironment.userId) {
    const ratios = branches.map(({ ratio = 1 }) => ratio);

    // It's important that the input be:
    // - Unique per-user (no one is bucketed alike)
    // - Unique per-experiment (bucketing differs across multiple experiments)
    // - Differs from the input used for sampling the recipe (otherwise only
    //   branches that contain the same buckets as the recipe sampling will
    //   receive users)
    const input = `${this.id}-${userId}-${slug}-branch`;

    const index = await lazy.Sampling.ratioSample(input, ratios);
    return branches[index];
  }

  /**
   * Generate the list of prefs a recipe will set.
   *
   * @params {object} branch The recipe branch that will be enrolled.
   * @params {boolean} isRollout Whether or not this recipe is a rollout.
   *
   * @returns {object} An object with the following keys:
   *
   *                   `prefs`:
   *                        The full list of prefs that this recipe would set,
   *                        if there are no conflicts. This will include prefs
   *                        that, for example, will not be set because this
   *                        enrollment is a rollout and there is an active
   *                        experiment that set the same pref.
   *
   *                   `prefsToSet`:
   *                        Prefs that should be set once enrollment is
   *                        complete.
   */
  _getPrefsForBranch(branch, isRollout = false) {
    const prefs = [];
    const prefsToSet = [];

    const getConflictingEnrollment = this._makeEnrollmentCache(isRollout);

    for (const { featureId, value: featureValue } of featuresCompat(branch)) {
      const feature = lazy.NimbusFeatures[featureId];

      if (!feature) {
        continue;
      }

      // It is possible to enroll in both an experiment and a rollout, so we
      // need to check if we have another enrollment for the same feature.
      const conflictingEnrollment = getConflictingEnrollment(featureId);

      const prefBranch =
        feature.manifest.isEarlyStartup ?? false ? "user" : "default";

      for (const [variable, value] of Object.entries(featureValue)) {
        const prefName = feature.getSetPrefName(variable);

        if (prefName) {
          let originalValue;
          const conflictingPref = conflictingEnrollment?.prefs?.find(
            p => p.name === prefName
          );

          if (conflictingPref) {
            // If there is another enrollment that has already set the pref we
            // care about, we use its stored originalValue.
            originalValue = conflictingPref.originalValue;
          } else if (
            prefBranch === "user" &&
            !Services.prefs.prefHasUserValue(prefName)
          ) {
            // If there is a default value set, then attempting to read the user
            // branch would result in returning the default branch value.
            originalValue = null;
          } else {
            originalValue = lazy.PrefUtils.getPref(prefName, {
              branch: prefBranch,
            });
          }

          prefs.push({
            name: prefName,
            branch: prefBranch,
            featureId,
            variable,
            originalValue,
          });

          // An experiment takes precedence if there is already a pref set.
          if (!isRollout || !conflictingPref) {
            prefsToSet.push({ name: prefName, value, prefBranch });
          }
        }
      }
    }

    return { prefs, prefsToSet };
  }

  /**
   * Set a list of prefs from enrolling in an experiment or rollout.
   *
   * The ExperimentManager's pref observers will be disabled while setting each
   * pref so as not to accidentally unenroll an existing rollout that an
   * experiment would override.
   *
   * @param {object[]} prefsToSet
   *                   A list of objects containing the prefs to set.
   *
   *                   Each object has the following properties:
   *
   *                   * `name`: The name of the pref.
   *                   * `value`: The value of the pref.
   *                   * `prefBranch`: The branch to set the pref on (either "user" or "default").
   */
  _setEnrollmentPrefs(prefsToSet) {
    for (const { name, value, prefBranch } of prefsToSet) {
      const entry = this._prefs.get(name);

      // If another enrollment exists that has set this pref, temporarily
      // disable the pref observer so as not to cause unenrollment.
      if (entry) {
        entry.enrollmentChanging = true;
      }

      lazy.PrefUtils.setPref(name, value, { branch: prefBranch });

      if (entry) {
        entry.enrollmentChanging = false;
      }
    }
  }

  /**
   * Unset prefs set during this enrollment.
   *
   * If this enrollment is an experiment and there is an existing rollout that
   * would set a pref that was covered by this enrollment, the pref will be
   * updated to that rollout's value.
   *
   * Otherwise, it will be set to the original value from before the enrollment
   * began.
   *
   * @param {Enrollment} enrollment
   *        The enrollment that has ended.
   *
   * @param {object} options
   *
   * @param {object?} options.changedPref
   *        If provided, a changed pref that caused the unenrollment that
   *        triggered unsetting these prefs. This is provided as to not
   *        overwrite a changed pref with an original value.
   *
   * @param {string} options.changedPref.name
   *        The name of the changed pref.
   *
   * @param {string} options.changedPref.branch
   *        The branch that was changed ("user" or "default").
   *
   * @param {boolean} options.duringRestore
   *        The unenrollment was caused during restore.
   */
  _unsetEnrollmentPrefs(enrollment, { changedPref, duringRestore } = {}) {
    if (!enrollment.prefs?.length) {
      return;
    }

    const getConflictingEnrollment = this._makeEnrollmentCache(
      enrollment.isRollout
    );

    for (const pref of enrollment.prefs) {
      this._removePrefObserver(pref.name, enrollment.slug);

      if (
        changedPref?.name == pref.name &&
        changedPref.branch === pref.branch
      ) {
        // Resetting the original value would overwite the pref the user just
        // set. Skip it.
        continue;
      }

      let newValue = pref.originalValue;

      // If we are unenrolling from an experiment during a restore, we must
      // ignore any potential conflicting rollout in the store, because its
      // hasn't gone through `_restoreEnrollmentPrefs`, which might also cause
      // it to unenroll.
      //
      // Both enrollments will have the same `originalValue` stored, so it will
      // always be restored.
      if (!duringRestore || enrollment.isRollout) {
        const conflictingEnrollment = getConflictingEnrollment(pref.featureId);
        const conflictingPref = conflictingEnrollment?.prefs?.find(
          p => p.name === pref.name
        );

        if (conflictingPref) {
          if (enrollment.isRollout) {
            // If we are unenrolling from a rollout, we have an experiment that
            // has set the pref. Since experiments take priority, we do not unset
            // it.
            continue;
          } else {
            // If we are an unenrolling from an experiment, we have a rollout that would
            // set the same pref, so we update the pref to that value instead of
            // the original value.
            newValue = getFeatureFromBranch(
              conflictingEnrollment.branch,
              pref.featureId
            ).value[pref.variable];
          }
        }
      }

      // If another enrollment exists that has set this pref, temporarily
      // disable the pref observer so as not to cause unenrollment when we
      // update the pref to its value.
      const entry = this._prefs.get(pref.name);
      if (entry) {
        entry.enrollmentChanging = true;
      }

      lazy.PrefUtils.setPref(pref.name, newValue, {
        branch: pref.branch,
      });

      if (entry) {
        entry.enrollmentChanging = false;
      }
    }
  }

  /**
   * Restore the prefs set by an enrollment.
   *
   * @param {object} enrollment The enrollment.
   * @param {object} enrollment.branch The branch that was enrolled.
   * @param {object[]} enrollment.prefs The prefs that are set by the enrollment.
   * @param {object[]} enrollment.isRollout The prefs that are set by the enrollment.
   *
   * @returns {boolean} Whether the restore was successful. If false, the
   *                    enrollment has ended.
   */
  _restoreEnrollmentPrefs(enrollment) {
    const { branch, prefs = [], isRollout } = enrollment;

    if (!prefs?.length) {
      return false;
    }

    const featuresById = Object.assign(
      ...featuresCompat(branch).map(f => ({ [f.featureId]: f }))
    );

    for (const { name, featureId, variable } of prefs) {
      // If the feature no longer exists, unenroll.
      if (!Object.hasOwn(lazy.NimbusFeatures, featureId)) {
        this._unenroll(enrollment, {
          reason: "invalid-feature",
          duringRestore: true,
        });
        return false;
      }

      const variables = lazy.NimbusFeatures[featureId].manifest.variables;

      // If the feature is missing a variable that set a pref, unenroll.
      if (!Object.hasOwn(variables, variable)) {
        this._unenroll(enrollment, {
          reason: "pref-variable-missing",
          duringRestore: true,
        });
        return false;
      }

      const variableDef = variables[variable];

      // If the variable is no longer a pref-setting variable, unenroll.
      if (!Object.hasOwn(variableDef, "setPref")) {
        this._unenroll(enrollment, {
          reason: "pref-variable-no-longer",
          duringRestore: true,
        });
        return false;
      }

      // If the variable is setting a different preference, unenroll.
      if (variableDef.setPref !== name) {
        this._unenroll(enrollment, {
          reason: "pref-variable-changed",
          duringRestore: true,
        });
        return false;
      }
    }

    for (const { name, branch: prefBranch, featureId, variable } of prefs) {
      // User prefs are already persisted.
      if (prefBranch === "user") {
        continue;
      }

      // If we are a rollout, we need to check for an existing experiment that
      // has set the same pref. If so, we do not need to set the pref because
      // experiments take priority.
      if (isRollout) {
        const conflictingEnrollment = this.store.getExperimentForFeature(
          featureId
        );
        const conflictingPref = conflictingEnrollment?.prefs?.find(
          p => p.name === name
        );

        if (conflictingPref) {
          continue;
        }
      }

      const value = featuresById[featureId].value[variable];

      if (prefBranch !== "user") {
        lazy.PrefUtils.setPref(name, value, { branch: prefBranch });
      }
    }

    return true;
  }

  /**
   * Make a cache to look up enrollments of the oppposite kind by feature ID.
   *
   * @param {boolean} isRollout Whether or not the current enrollment is a
   *                            rollout. If true, the cache will return
   *                            experiments. If false, the cache will return
   *                            rollouts.
   *
   * @returns {function} The cache, as a callable function.
   */
  _makeEnrollmentCache(isRollout) {
    const getOtherEnrollment = (isRollout
      ? this.store.getExperimentForFeature
      : this.store.getRolloutForFeature
    ).bind(this.store);

    const conflictingEnrollments = {};
    return featureId => {
      if (!Object.hasOwn(conflictingEnrollments, featureId)) {
        conflictingEnrollments[featureId] = getOtherEnrollment(featureId);
      }

      return conflictingEnrollments[featureId];
    };
  }

  /**
   * Update the set of observers with prefs set by the given enrollment.
   *
   * @param {Enrollment} enrollment
   *        The enrollment that is setting prefs.
   */
  _updatePrefObservers({ slug, prefs }) {
    if (!prefs?.length) {
      return;
    }

    for (const pref of prefs) {
      const { name } = pref;

      if (!this._prefs.has(name)) {
        const observer = () => this._onExperimentPrefChanged(pref);
        const entry = {
          slugs: new Set([slug]),
          enrollmentChanging: false,
          observer,
        };

        Services.prefs.addObserver(name, observer);

        this._prefs.set(name, entry);
      } else {
        this._prefs.get(name).slugs.add(slug);
      }

      if (!this._prefsBySlug.has(slug)) {
        this._prefsBySlug.set(slug, new Set([name]));
      } else {
        this._prefsBySlug.get(slug).add(name);
      }
    }
  }

  /**
   * Remove an entry for the pref observer for the given pref and slug.
   *
   * If there are no more enrollments listening to a pref, the observer will be removed.
   *
   * This is called when an enrollment is ending.
   *
   * @param {string} name The name of the pref.
   * @param {string} slug The slug of the enrollment that is being unenrolled.
   */
  _removePrefObserver(name, slug) {
    // Update the pref observer that the current enrollment is no longer
    // involved in the pref.
    //
    // If no enrollments have a variable setting the pref, then we can remove
    // the observers.
    const entry = this._prefs.get(name);

    // If this is happening due to a pref change, the observers will already be removed.
    if (entry) {
      entry.slugs.delete(slug);
      if (entry.slugs.size == 0) {
        Services.prefs.removeObserver(name, entry.observer);
        this._prefs.delete(name);
      }
    }

    const bySlug = this._prefsBySlug.get(slug);
    if (bySlug) {
      bySlug.delete(name);
      if (bySlug.size == 0) {
        this._prefsBySlug.delete(slug);
      }
    }
  }

  /**
   * Handle a change to a pref set by enrollments by ending those enrollments.
   *
   * @param {object} pref
   *        Information about the pref that was changed.
   *
   * @param {string} pref.name
   *        The name of the pref that was changed.
   *
   * @param {string} pref.branch
   *        The branch enrollments set the pref on.
   *
   * @param {string} pref.featureId
   *        The feature ID of the feature containing the variable that set the
   *        pref.
   *
   * @param {string} pref.variable
   *        The variable in the given feature whose value determined the pref's
   *        value.
   */
  _onExperimentPrefChanged(pref) {
    const entry = this._prefs.get(pref.name);
    // If this was triggered while we are enrolling or unenrolling from an
    // experiment, then we don't want to unenroll from the rollout because the
    // experiment's value is taking precendence.
    //
    // Otherwise, all enrollments that set the variable corresponding to this
    // pref must be unenrolled.
    if (entry.enrollmentChanging) {
      return;
    }

    // Copy the `Set` into an `Array` because we modify the set later in
    // `_removePrefObserver` and we need to iterate over it multiple times.
    const slugs = Array.from(entry.slugs);

    // Remove all pref observers set by enrollments. We are potentially about
    // to set these prefs during unenrollment, so we don't want to trigger
    // them and cause nested unenrollments.
    for (const slug of slugs) {
      const toRemove = Array.from(this._prefsBySlug.get(slug) ?? []);
      for (const name of toRemove) {
        this._removePrefObserver(name, slug);
      }
    }

    // Unenroll from the rollout first to save calls to setPref.
    const enrollments = Array.from(slugs).map(slug => this.store.get(slug));

    // There is a maximum of two enrollments (one experiment and one rollout).
    if (enrollments.length == 2) {
      // Order enrollments so that we unenroll from the rollout first.
      if (!enrollments[0].isRollout) {
        enrollments.reverse();
      }
    }

    // We want to know what branch was changed so we can know if we should
    // restore prefs. (e.g., if we have a pref set on the user branch and the
    // user branch changed, we do not want to then overwrite the user's choice).

    // This is not complicated if a pref simply changed. However, we also must
    // detect `nsIPrefBranch::clearUserPref()`, which wipes out the user branch
    // and leaves the default branch untouched. That is where this gets
    // complicated:

    let branch;
    if (Services.prefs.prefHasUserValue(pref.name)) {
      // If there is a user branch value, then the user branch changed.
      branch = "user";
    } else if (!Services.prefs.prefHasDefaultValue(pref.name)) {
      // If there is not default branch value, then the user branch must have
      // been cleared becuase you cannot clear the default branch.
      branch = "user";
    } else if (pref.branch === "default") {
      const feature = getFeatureFromBranch(
        enrollments.at(-1).branch,
        pref.featureId
      );
      const expectedValue = feature.value[pref.variable];
      const value = lazy.PrefUtils.getPref(pref.name, { branch: pref.branch });

      if (value === expectedValue) {
        // If the pref was set on the default branch and still matches the
        // expected value, then the user branch must have been cleared.
        branch = "user";
      } else {
        branch = "default";
      }
    } else {
      // If the pref was set on the user branch and we don't have a user branch
      // value, then the user branch must have been cleared.
      branch = "user";
    }

    const changedPref = {
      name: pref.name,
      branch,
    };

    for (const enrollment of enrollments) {
      this._unenroll(enrollment, { reason: "changed-pref", changedPref });
    }
  }
}

const ExperimentManager = new _ExperimentManager();