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diff --git a/services/common/hawkclient.js b/services/common/hawkclient.js
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+/* 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";
+
+/*
+ * HAWK is an HTTP authentication scheme using a message authentication code
+ * (MAC) algorithm to provide partial HTTP request cryptographic verification.
+ *
+ * For details, see: https://github.com/hueniverse/hawk
+ *
+ * With HAWK, it is essential that the clocks on clients and server not have an
+ * absolute delta of greater than one minute, as the HAWK protocol uses
+ * timestamps to reduce the possibility of replay attacks.  However, it is
+ * likely that some clients' clocks will be more than a little off, especially
+ * in mobile devices, which would break HAWK-based services (like sync and
+ * firefox accounts) for those clients.
+ *
+ * This library provides a stateful HAWK client that calculates (roughly) the
+ * clock delta on the client vs the server.  The library provides an interface
+ * for deriving HAWK credentials and making HAWK-authenticated REST requests to
+ * a single remote server.  Therefore, callers who want to interact with
+ * multiple HAWK services should instantiate one HawkClient per service.
+ */
+
+var EXPORTED_SYMBOLS = ["HawkClient"];
+
+const { HAWKAuthenticatedRESTRequest } = ChromeUtils.import(
+  "resource://services-common/hawkrequest.js"
+);
+const { Observers } = ChromeUtils.import(
+  "resource://services-common/observers.js"
+);
+const { Log } = ChromeUtils.import("resource://gre/modules/Log.jsm");
+const { XPCOMUtils } = ChromeUtils.import(
+  "resource://gre/modules/XPCOMUtils.jsm"
+);
+const { Services } = ChromeUtils.import("resource://gre/modules/Services.jsm");
+
+// log.appender.dump should be one of "Fatal", "Error", "Warn", "Info", "Config",
+// "Debug", "Trace" or "All". If none is specified, "Error" will be used by
+// default.
+// Note however that Sync will also add this log to *its* DumpAppender, so
+// in a Sync context it shouldn't be necessary to adjust this - however, that
+// also means error logs are likely to be dump'd twice but that's OK.
+const PREF_LOG_LEVEL = "services.common.hawk.log.appender.dump";
+
+// A pref that can be set so "sensitive" information (eg, personally
+// identifiable info, credentials, etc) will be logged.
+const PREF_LOG_SENSITIVE_DETAILS = "services.common.hawk.log.sensitive";
+
+XPCOMUtils.defineLazyGetter(this, "log", function() {
+  let log = Log.repository.getLogger("Hawk");
+  // We set the log itself to "debug" and set the level from the preference to
+  // the appender.  This allows other things to send the logs to different
+  // appenders, while still allowing the pref to control what is seen via dump()
+  log.level = Log.Level.Debug;
+  let appender = new Log.DumpAppender();
+  log.addAppender(appender);
+  appender.level = Log.Level.Error;
+  try {
+    let level =
+      Services.prefs.getPrefType(PREF_LOG_LEVEL) ==
+        Ci.nsIPrefBranch.PREF_STRING &&
+      Services.prefs.getCharPref(PREF_LOG_LEVEL);
+    appender.level = Log.Level[level] || Log.Level.Error;
+  } catch (e) {
+    log.error(e);
+  }
+
+  return log;
+});
+
+// A boolean to indicate if personally identifiable information (or anything
+// else sensitive, such as credentials) should be logged.
+XPCOMUtils.defineLazyGetter(this, "logPII", function() {
+  try {
+    return Services.prefs.getBoolPref(PREF_LOG_SENSITIVE_DETAILS);
+  } catch (_) {
+    return false;
+  }
+});
+
+/*
+ * A general purpose client for making HAWK authenticated requests to a single
+ * host.  Keeps track of the clock offset between the client and the host for
+ * computation of the timestamp in the HAWK Authorization header.
+ *
+ * Clients should create one HawkClient object per each server they wish to
+ * interact with.
+ *
+ * @param host
+ *        The url of the host
+ */
+var HawkClient = function(host) {
+  this.host = host;
+
+  // Clock offset in milliseconds between our client's clock and the date
+  // reported in responses from our host.
+  this._localtimeOffsetMsec = 0;
+};
+
+this.HawkClient.prototype = {
+  /*
+   * Construct an error message for a response.  Private.
+   *
+   * @param restResponse
+   *        A RESTResponse object from a RESTRequest
+   *
+   * @param error
+   *        A string or object describing the error
+   */
+  _constructError(restResponse, error) {
+    let errorObj = {
+      error,
+      // This object is likely to be JSON.stringify'd, but neither Error()
+      // objects nor Components.Exception objects do the right thing there,
+      // so we add a new element which is simply the .toString() version of
+      // the error object, so it does appear in JSON'd values.
+      errorString: error.toString(),
+      message: restResponse.statusText,
+      code: restResponse.status,
+      errno: restResponse.status,
+      toString() {
+        return this.code + ": " + this.message;
+      },
+    };
+    let retryAfter =
+      restResponse.headers && restResponse.headers["retry-after"];
+    retryAfter = retryAfter ? parseInt(retryAfter) : retryAfter;
+    if (retryAfter) {
+      errorObj.retryAfter = retryAfter;
+      // and notify observers of the retry interval
+      if (this.observerPrefix) {
+        Observers.notify(this.observerPrefix + ":backoff:interval", retryAfter);
+      }
+    }
+    return errorObj;
+  },
+
+  /*
+   *
+   * Update clock offset by determining difference from date gives in the (RFC
+   * 1123) Date header of a server response.  Because HAWK tolerates a window
+   * of one minute of clock skew (so two minutes total since the skew can be
+   * positive or negative), the simple method of calculating offset here is
+   * probably good enough.  We keep the value in milliseconds to make life
+   * easier, even though the value will not have millisecond accuracy.
+   *
+   * @param dateString
+   *        An RFC 1123 date string (e.g., "Mon, 13 Jan 2014 21:45:06 GMT")
+   *
+   * For HAWK clock skew and replay protection, see
+   * https://github.com/hueniverse/hawk#replay-protection
+   */
+  _updateClockOffset(dateString) {
+    try {
+      let serverDateMsec = Date.parse(dateString);
+      this._localtimeOffsetMsec = serverDateMsec - this.now();
+      log.debug(
+        "Clock offset vs " + this.host + ": " + this._localtimeOffsetMsec
+      );
+    } catch (err) {
+      log.warn("Bad date header in server response: " + dateString);
+    }
+  },
+
+  /*
+   * Get the current clock offset in milliseconds.
+   *
+   * The offset is the number of milliseconds that must be added to the client
+   * clock to make it equal to the server clock.  For example, if the client is
+   * five minutes ahead of the server, the localtimeOffsetMsec will be -300000.
+   */
+  get localtimeOffsetMsec() {
+    return this._localtimeOffsetMsec;
+  },
+
+  /*
+   * return current time in milliseconds
+   */
+  now() {
+    return Date.now();
+  },
+
+  /* A general method for sending raw RESTRequest calls authorized using HAWK
+   *
+   * @param path
+   *        API endpoint path
+   * @param method
+   *        The HTTP request method
+   * @param credentials
+   *        Hawk credentials
+   * @param payloadObj
+   *        An object that can be encodable as JSON as the payload of the
+   *        request
+   * @param extraHeaders
+   *        An object with header/value pairs to send with the request.
+   * @return Promise
+   *        Returns a promise that resolves to the response of the API call,
+   *        or is rejected with an error.  If the server response can be parsed
+   *        as JSON and contains an 'error' property, the promise will be
+   *        rejected with this JSON-parsed response.
+   */
+  async request(
+    path,
+    method,
+    credentials = null,
+    payloadObj = {},
+    extraHeaders = {},
+    retryOK = true
+  ) {
+    method = method.toLowerCase();
+
+    let uri = this.host + path;
+
+    let extra = {
+      now: this.now(),
+      localtimeOffsetMsec: this.localtimeOffsetMsec,
+      headers: extraHeaders,
+    };
+
+    let request = this.newHAWKAuthenticatedRESTRequest(uri, credentials, extra);
+    let error;
+    let restResponse = await request[method](payloadObj).catch(e => {
+      // Keep a reference to the error, log a message about it, and return the
+      // response anyway.
+      error = e;
+      log.warn("hawk request error", error);
+      return request.response;
+    });
+
+    // This shouldn't happen anymore, but it's not exactly difficult to handle.
+    if (!restResponse) {
+      throw error;
+    }
+
+    let status = restResponse.status;
+
+    log.debug(
+      "(Response) " +
+        path +
+        ": code: " +
+        status +
+        " - Status text: " +
+        restResponse.statusText
+    );
+    if (logPII) {
+      log.debug("Response text", restResponse.body);
+    }
+
+    // All responses may have backoff headers, which are a server-side safety
+    // valve to allow slowing down clients without hurting performance.
+    this._maybeNotifyBackoff(restResponse, "x-weave-backoff");
+    this._maybeNotifyBackoff(restResponse, "x-backoff");
+
+    if (error) {
+      // When things really blow up, reconstruct an error object that follows
+      // the general format of the server on error responses.
+      throw this._constructError(restResponse, error);
+    }
+
+    this._updateClockOffset(restResponse.headers.date);
+
+    if (status === 401 && retryOK && !("retry-after" in restResponse.headers)) {
+      // Retry once if we were rejected due to a bad timestamp.
+      // Clock offset is adjusted already in the top of this function.
+      log.debug("Received 401 for " + path + ": retrying");
+      return this.request(
+        path,
+        method,
+        credentials,
+        payloadObj,
+        extraHeaders,
+        false
+      );
+    }
+
+    // If the server returned a json error message, use it in the rejection
+    // of the promise.
+    //
+    // In the case of a 401, in which we are probably being rejected for a
+    // bad timestamp, retry exactly once, during which time clock offset will
+    // be adjusted.
+
+    let jsonResponse = {};
+    try {
+      jsonResponse = JSON.parse(restResponse.body);
+    } catch (notJSON) {}
+
+    let okResponse = 200 <= status && status < 300;
+    if (!okResponse || jsonResponse.error) {
+      if (jsonResponse.error) {
+        throw jsonResponse;
+      }
+      throw this._constructError(restResponse, "Request failed");
+    }
+
+    // It's up to the caller to know how to decode the response.
+    // We just return the whole response.
+    return restResponse;
+  },
+
+  /*
+   * The prefix used for all notifications sent by this module.  This
+   * allows the handler of notifications to be sure they are handling
+   * notifications for the service they expect.
+   *
+   * If not set, no notifications will be sent.
+   */
+  observerPrefix: null,
+
+  // Given an optional header value, notify that a backoff has been requested.
+  _maybeNotifyBackoff(response, headerName) {
+    if (!this.observerPrefix || !response.headers) {
+      return;
+    }
+    let headerVal = response.headers[headerName];
+    if (!headerVal) {
+      return;
+    }
+    let backoffInterval;
+    try {
+      backoffInterval = parseInt(headerVal, 10);
+    } catch (ex) {
+      log.error(
+        "hawkclient response had invalid backoff value in '" +
+          headerName +
+          "' header: " +
+          headerVal
+      );
+      return;
+    }
+    Observers.notify(
+      this.observerPrefix + ":backoff:interval",
+      backoffInterval
+    );
+  },
+
+  // override points for testing.
+  newHAWKAuthenticatedRESTRequest(uri, credentials, extra) {
+    return new HAWKAuthenticatedRESTRequest(uri, credentials, extra);
+  },
+};