Add m-esr52 at 52.6.0

1 files changed, 969 insertions, 0 deletions

diff --git a/toolkit/modules/Log.jsm b/toolkit/modules/Log.jsm
new file mode 100644
index 000000000..6b741ff9e
--- /dev/null
+++ b/toolkit/modules/Log.jsm
@@ -0,0 +1,969 @@
+/* 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";
+
+this.EXPORTED_SYMBOLS = ["Log"];
+
+const {classes: Cc, interfaces: Ci, results: Cr, utils: Cu} = Components;
+
+const ONE_BYTE = 1;
+const ONE_KILOBYTE = 1024 * ONE_BYTE;
+const ONE_MEGABYTE = 1024 * ONE_KILOBYTE;
+
+const STREAM_SEGMENT_SIZE = 4096;
+const PR_UINT32_MAX = 0xffffffff;
+
+Cu.import("resource://gre/modules/XPCOMUtils.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "OS",
+                                  "resource://gre/modules/osfile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "Task",
+                                  "resource://gre/modules/Task.jsm");
+const INTERNAL_FIELDS = new Set(["_level", "_message", "_time", "_namespace"]);
+
+
+/*
+ * Dump a message everywhere we can if we have a failure.
+ */
+function dumpError(text) {
+  dump(text + "\n");
+  Cu.reportError(text);
+}
+
+this.Log = {
+  Level: {
+    Fatal:  70,
+    Error:  60,
+    Warn:   50,
+    Info:   40,
+    Config: 30,
+    Debug:  20,
+    Trace:  10,
+    All:    -1, // We don't want All to be falsy.
+    Desc: {
+      70: "FATAL",
+      60: "ERROR",
+      50: "WARN",
+      40: "INFO",
+      30: "CONFIG",
+      20: "DEBUG",
+      10: "TRACE",
+      "-1":  "ALL",
+    },
+    Numbers: {
+      "FATAL": 70,
+      "ERROR": 60,
+      "WARN": 50,
+      "INFO": 40,
+      "CONFIG": 30,
+      "DEBUG": 20,
+      "TRACE": 10,
+      "ALL": -1,
+    }
+  },
+
+  get repository() {
+    delete Log.repository;
+    Log.repository = new LoggerRepository();
+    return Log.repository;
+  },
+  set repository(value) {
+    delete Log.repository;
+    Log.repository = value;
+  },
+
+  LogMessage: LogMessage,
+  Logger: Logger,
+  LoggerRepository: LoggerRepository,
+
+  Formatter: Formatter,
+  BasicFormatter: BasicFormatter,
+  MessageOnlyFormatter: MessageOnlyFormatter,
+  StructuredFormatter: StructuredFormatter,
+
+  Appender: Appender,
+  DumpAppender: DumpAppender,
+  ConsoleAppender: ConsoleAppender,
+  StorageStreamAppender: StorageStreamAppender,
+
+  FileAppender: FileAppender,
+  BoundedFileAppender: BoundedFileAppender,
+
+  ParameterFormatter: ParameterFormatter,
+  // Logging helper:
+  // let logger = Log.repository.getLogger("foo");
+  // logger.info(Log.enumerateInterfaces(someObject).join(","));
+  enumerateInterfaces: function Log_enumerateInterfaces(aObject) {
+    let interfaces = [];
+
+    for (i in Ci) {
+      try {
+        aObject.QueryInterface(Ci[i]);
+        interfaces.push(i);
+      }
+      catch (ex) {}
+    }
+
+    return interfaces;
+  },
+
+  // Logging helper:
+  // let logger = Log.repository.getLogger("foo");
+  // logger.info(Log.enumerateProperties(someObject).join(","));
+  enumerateProperties: function (aObject, aExcludeComplexTypes) {
+    let properties = [];
+
+    for (p in aObject) {
+      try {
+        if (aExcludeComplexTypes &&
+            (typeof(aObject[p]) == "object" || typeof(aObject[p]) == "function"))
+          continue;
+        properties.push(p + " = " + aObject[p]);
+      }
+      catch (ex) {
+        properties.push(p + " = " + ex);
+      }
+    }
+
+    return properties;
+  },
+
+  _formatError: function _formatError(e) {
+    let result = e.toString();
+    if (e.fileName) {
+      result +=  " (" + e.fileName;
+      if (e.lineNumber) {
+        result += ":" + e.lineNumber;
+      }
+      if (e.columnNumber) {
+        result += ":" + e.columnNumber;
+      }
+      result += ")";
+    }
+    return result + " " + Log.stackTrace(e);
+  },
+
+  // This is for back compatibility with services/common/utils.js; we duplicate
+  // some of the logic in ParameterFormatter
+  exceptionStr: function exceptionStr(e) {
+    if (!e) {
+      return "" + e;
+    }
+    if (e instanceof Ci.nsIException) {
+      return e.toString() + " " + Log.stackTrace(e);
+    }
+    else if (isError(e)) {
+      return Log._formatError(e);
+    }
+    // else
+    let message = e.message ? e.message : e;
+    return message + " " + Log.stackTrace(e);
+  },
+
+  stackTrace: function stackTrace(e) {
+    // Wrapped nsIException
+    if (e.location) {
+      let frame = e.location;
+      let output = [];
+      while (frame) {
+        // Works on frames or exceptions, munges file:// URIs to shorten the paths
+        // FIXME: filename munging is sort of hackish, might be confusing if
+        // there are multiple extensions with similar filenames
+        let str = "<file:unknown>";
+
+        let file = frame.filename || frame.fileName;
+        if (file) {
+          str = file.replace(/^(?:chrome|file):.*?([^\/\.]+\.\w+)$/, "$1");
+        }
+
+        if (frame.lineNumber) {
+          str += ":" + frame.lineNumber;
+        }
+
+        if (frame.name) {
+          str = frame.name + "()@" + str;
+        }
+
+        if (str) {
+          output.push(str);
+        }
+        frame = frame.caller;
+      }
+      return "Stack trace: " + output.join(" < ");
+    }
+    // Standard JS exception
+    if (e.stack) {
+      return "JS Stack trace: " + Task.Debugging.generateReadableStack(e.stack).trim()
+        .replace(/\n/g, " < ").replace(/@[^@]*?([^\/\.]+\.\w+:)/g, "@$1");
+    }
+
+    return "No traceback available";
+  }
+};
+
+/*
+ * LogMessage
+ * Encapsulates a single log event's data
+ */
+function LogMessage(loggerName, level, message, params) {
+  this.loggerName = loggerName;
+  this.level = level;
+  /*
+   * Special case to handle "log./level/(object)", for example logging a caught exception
+   * without providing text or params like: catch(e) { logger.warn(e) }
+   * Treating this as an empty text with the object in the 'params' field causes the
+   * object to be formatted properly by BasicFormatter.
+   */
+  if (!params && message && (typeof(message) == "object") &&
+      (typeof(message.valueOf()) != "string")) {
+    this.message = null;
+    this.params = message;
+  } else {
+    // If the message text is empty, or a string, or a String object, normal handling
+    this.message = message;
+    this.params = params;
+  }
+
+  // The _structured field will correspond to whether this message is to
+  // be interpreted as a structured message.
+  this._structured = this.params && this.params.action;
+  this.time = Date.now();
+}
+LogMessage.prototype = {
+  get levelDesc() {
+    if (this.level in Log.Level.Desc)
+      return Log.Level.Desc[this.level];
+    return "UNKNOWN";
+  },
+
+  toString: function LogMsg_toString() {
+    let msg = "LogMessage [" + this.time + " " + this.level + " " +
+      this.message;
+    if (this.params) {
+      msg += " " + JSON.stringify(this.params);
+    }
+    return msg + "]"
+  }
+};
+
+/*
+ * Logger
+ * Hierarchical version.  Logs to all appenders, assigned or inherited
+ */
+
+function Logger(name, repository) {
+  if (!repository)
+    repository = Log.repository;
+  this._name = name;
+  this.children = [];
+  this.ownAppenders = [];
+  this.appenders = [];
+  this._repository = repository;
+}
+Logger.prototype = {
+  get name() {
+    return this._name;
+  },
+
+  _level: null,
+  get level() {
+    if (this._level != null)
+      return this._level;
+    if (this.parent)
+      return this.parent.level;
+    dumpError("Log warning: root logger configuration error: no level defined");
+    return Log.Level.All;
+  },
+  set level(level) {
+    this._level = level;
+  },
+
+  _parent: null,
+  get parent() {
+    return this._parent;
+  },
+  set parent(parent) {
+    if (this._parent == parent) {
+      return;
+    }
+    // Remove ourselves from parent's children
+    if (this._parent) {
+      let index = this._parent.children.indexOf(this);
+      if (index != -1) {
+        this._parent.children.splice(index, 1);
+      }
+    }
+    this._parent = parent;
+    parent.children.push(this);
+    this.updateAppenders();
+  },
+
+  updateAppenders: function updateAppenders() {
+    if (this._parent) {
+      let notOwnAppenders = this._parent.appenders.filter(function(appender) {
+        return this.ownAppenders.indexOf(appender) == -1;
+      }, this);
+      this.appenders = notOwnAppenders.concat(this.ownAppenders);
+    } else {
+      this.appenders = this.ownAppenders.slice();
+    }
+
+    // Update children's appenders.
+    for (let i = 0; i < this.children.length; i++) {
+      this.children[i].updateAppenders();
+    }
+  },
+
+  addAppender: function Logger_addAppender(appender) {
+    if (this.ownAppenders.indexOf(appender) != -1) {
+      return;
+    }
+    this.ownAppenders.push(appender);
+    this.updateAppenders();
+  },
+
+  removeAppender: function Logger_removeAppender(appender) {
+    let index = this.ownAppenders.indexOf(appender);
+    if (index == -1) {
+      return;
+    }
+    this.ownAppenders.splice(index, 1);
+    this.updateAppenders();
+  },
+
+  /**
+   * Logs a structured message object.
+   *
+   * @param action
+   *        (string) A message action, one of a set of actions known to the
+   *          log consumer.
+   * @param params
+   *        (object) Parameters to be included in the message.
+   *          If _level is included as a key and the corresponding value
+   *          is a number or known level name, the message will be logged
+   *          at the indicated level. If _message is included as a key, the
+   *          value is used as the descriptive text for the message.
+   */
+  logStructured: function (action, params) {
+    if (!action) {
+      throw "An action is required when logging a structured message.";
+    }
+    if (!params) {
+      this.log(this.level, undefined, {"action": action});
+      return;
+    }
+    if (typeof(params) != "object") {
+      throw "The params argument is required to be an object.";
+    }
+
+    let level = params._level;
+    if (level) {
+      let ulevel = level.toUpperCase();
+      if (ulevel in Log.Level.Numbers) {
+        level = Log.Level.Numbers[ulevel];
+      }
+    } else {
+      level = this.level;
+    }
+
+    params.action = action;
+    this.log(level, params._message, params);
+  },
+
+  log: function (level, string, params) {
+    if (this.level > level)
+      return;
+
+    // Hold off on creating the message object until we actually have
+    // an appender that's responsible.
+    let message;
+    let appenders = this.appenders;
+    for (let appender of appenders) {
+      if (appender.level > level) {
+        continue;
+      }
+      if (!message) {
+        message = new LogMessage(this._name, level, string, params);
+      }
+      appender.append(message);
+    }
+  },
+
+  fatal: function (string, params) {
+    this.log(Log.Level.Fatal, string, params);
+  },
+  error: function (string, params) {
+    this.log(Log.Level.Error, string, params);
+  },
+  warn: function (string, params) {
+    this.log(Log.Level.Warn, string, params);
+  },
+  info: function (string, params) {
+    this.log(Log.Level.Info, string, params);
+  },
+  config: function (string, params) {
+    this.log(Log.Level.Config, string, params);
+  },
+  debug: function (string, params) {
+    this.log(Log.Level.Debug, string, params);
+  },
+  trace: function (string, params) {
+    this.log(Log.Level.Trace, string, params);
+  }
+};
+
+/*
+ * LoggerRepository
+ * Implements a hierarchy of Loggers
+ */
+
+function LoggerRepository() {}
+LoggerRepository.prototype = {
+  _loggers: {},
+
+  _rootLogger: null,
+  get rootLogger() {
+    if (!this._rootLogger) {
+      this._rootLogger = new Logger("root", this);
+      this._rootLogger.level = Log.Level.All;
+    }
+    return this._rootLogger;
+  },
+  set rootLogger(logger) {
+    throw "Cannot change the root logger";
+  },
+
+  _updateParents: function LogRep__updateParents(name) {
+    let pieces = name.split('.');
+    let cur, parent;
+
+    // find the closest parent
+    // don't test for the logger name itself, as there's a chance it's already
+    // there in this._loggers
+    for (let i = 0; i < pieces.length - 1; i++) {
+      if (cur)
+        cur += '.' + pieces[i];
+      else
+        cur = pieces[i];
+      if (cur in this._loggers)
+        parent = cur;
+    }
+
+    // if we didn't assign a parent above, there is no parent
+    if (!parent)
+      this._loggers[name].parent = this.rootLogger;
+    else
+      this._loggers[name].parent = this._loggers[parent];
+
+    // trigger updates for any possible descendants of this logger
+    for (let logger in this._loggers) {
+      if (logger != name && logger.indexOf(name) == 0)
+        this._updateParents(logger);
+    }
+  },
+
+  /**
+   * Obtain a named Logger.
+   *
+   * The returned Logger instance for a particular name is shared among
+   * all callers. In other words, if two consumers call getLogger("foo"),
+   * they will both have a reference to the same object.
+   *
+   * @return Logger
+   */
+  getLogger: function (name) {
+    if (name in this._loggers)
+      return this._loggers[name];
+    this._loggers[name] = new Logger(name, this);
+    this._updateParents(name);
+    return this._loggers[name];
+  },
+
+  /**
+   * Obtain a Logger that logs all string messages with a prefix.
+   *
+   * A common pattern is to have separate Logger instances for each instance
+   * of an object. But, you still want to distinguish between each instance.
+   * Since Log.repository.getLogger() returns shared Logger objects,
+   * monkeypatching one Logger modifies them all.
+   *
+   * This function returns a new object with a prototype chain that chains
+   * up to the original Logger instance. The new prototype has log functions
+   * that prefix content to each message.
+   *
+   * @param name
+   *        (string) The Logger to retrieve.
+   * @param prefix
+   *        (string) The string to prefix each logged message with.
+   */
+  getLoggerWithMessagePrefix: function (name, prefix) {
+    let log = this.getLogger(name);
+
+    let proxy = Object.create(log);
+    proxy.log = (level, string, params) => log.log(level, prefix + string, params);
+    return proxy;
+  },
+};
+
+/*
+ * Formatters
+ * These massage a LogMessage into whatever output is desired.
+ * BasicFormatter and StructuredFormatter are implemented here.
+ */
+
+// Abstract formatter
+function Formatter() {}
+Formatter.prototype = {
+  format: function Formatter_format(message) {}
+};
+
+// Basic formatter that doesn't do anything fancy.
+function BasicFormatter(dateFormat) {
+  if (dateFormat) {
+    this.dateFormat = dateFormat;
+  }
+  this.parameterFormatter = new ParameterFormatter();
+}
+BasicFormatter.prototype = {
+  __proto__: Formatter.prototype,
+
+  /**
+   * Format the text of a message with optional parameters.
+   * If the text contains ${identifier}, replace that with
+   * the value of params[identifier]; if ${}, replace that with
+   * the entire params object. If no params have been substituted
+   * into the text, format the entire object and append that
+   * to the message.
+   */
+  formatText: function (message) {
+    let params = message.params;
+    if (typeof(params) == "undefined") {
+      return message.message || "";
+    }
+    // Defensive handling of non-object params
+    // We could add a special case for NSRESULT values here...
+    let pIsObject = (typeof(params) == 'object' || typeof(params) == 'function');
+
+    // if we have params, try and find substitutions.
+    if (this.parameterFormatter) {
+      // have we successfully substituted any parameters into the message?
+      // in the log message
+      let subDone = false;
+      let regex = /\$\{(\S*)\}/g;
+      let textParts = [];
+      if (message.message) {
+        textParts.push(message.message.replace(regex, (_, sub) => {
+          // ${foo} means use the params['foo']
+          if (sub) {
+            if (pIsObject && sub in message.params) {
+              subDone = true;
+              return this.parameterFormatter.format(message.params[sub]);
+            }
+            return '${' + sub + '}';
+          }
+          // ${} means use the entire params object.
+          subDone = true;
+          return this.parameterFormatter.format(message.params);
+        }));
+      }
+      if (!subDone) {
+        // There were no substitutions in the text, so format the entire params object
+        let rest = this.parameterFormatter.format(message.params);
+        if (rest !== null && rest != "{}") {
+          textParts.push(rest);
+        }
+      }
+      return textParts.join(': ');
+    }
+    return undefined;
+  },
+
+  format: function BF_format(message) {
+    return message.time + "\t" +
+      message.loggerName + "\t" +
+      message.levelDesc + "\t" +
+      this.formatText(message);
+  }
+};
+
+/**
+ * A formatter that only formats the string message component.
+ */
+function MessageOnlyFormatter() {
+}
+MessageOnlyFormatter.prototype = Object.freeze({
+  __proto__: Formatter.prototype,
+
+  format: function (message) {
+    return message.message;
+  },
+});
+
+// Structured formatter that outputs JSON based on message data.
+// This formatter will format unstructured messages by supplying
+// default values.
+function StructuredFormatter() { }
+StructuredFormatter.prototype = {
+  __proto__: Formatter.prototype,
+
+  format: function (logMessage) {
+    let output = {
+      _time: logMessage.time,
+      _namespace: logMessage.loggerName,
+      _level: logMessage.levelDesc
+    };
+
+    for (let key in logMessage.params) {
+      output[key] = logMessage.params[key];
+    }
+
+    if (!output.action) {
+      output.action = "UNKNOWN";
+    }
+
+    if (!output._message && logMessage.message) {
+      output._message = logMessage.message;
+    }
+
+    return JSON.stringify(output);
+  }
+}
+
+/**
+ * Test an object to see if it is a Mozilla JS Error.
+ */
+function isError(aObj) {
+  return (aObj && typeof(aObj) == 'object' && "name" in aObj && "message" in aObj &&
+          "fileName" in aObj && "lineNumber" in aObj && "stack" in aObj);
+}
+
+/*
+ * Parameter Formatters
+ * These massage an object used as a parameter for a LogMessage into
+ * a string representation of the object.
+ */
+
+function ParameterFormatter() {
+  this._name = "ParameterFormatter"
+}
+ParameterFormatter.prototype = {
+  format: function(ob) {
+    try {
+      if (ob === undefined) {
+        return "undefined";
+      }
+      if (ob === null) {
+        return "null";
+      }
+      // Pass through primitive types and objects that unbox to primitive types.
+      if ((typeof(ob) != "object" || typeof(ob.valueOf()) != "object") &&
+          typeof(ob) != "function") {
+        return ob;
+      }
+      if (ob instanceof Ci.nsIException) {
+        return ob.toString() + " " + Log.stackTrace(ob);
+      }
+      else if (isError(ob)) {
+        return Log._formatError(ob);
+      }
+      // Just JSONify it. Filter out our internal fields and those the caller has
+      // already handled.
+      return JSON.stringify(ob, (key, val) => {
+        if (INTERNAL_FIELDS.has(key)) {
+          return undefined;
+        }
+        return val;
+      });
+    }
+    catch (e) {
+      dumpError("Exception trying to format object for log message: " + Log.exceptionStr(e));
+    }
+    // Fancy formatting failed. Just toSource() it - but even this may fail!
+    try {
+      return ob.toSource();
+    } catch (_) { }
+    try {
+      return "" + ob;
+    } catch (_) {
+      return "[object]"
+    }
+  }
+}
+
+/*
+ * Appenders
+ * These can be attached to Loggers to log to different places
+ * Simply subclass and override doAppend to implement a new one
+ */
+
+function Appender(formatter) {
+  this._name = "Appender";
+  this._formatter = formatter? formatter : new BasicFormatter();
+}
+Appender.prototype = {
+  level: Log.Level.All,
+
+  append: function App_append(message) {
+    if (message) {
+      this.doAppend(this._formatter.format(message));
+    }
+  },
+  toString: function App_toString() {
+    return this._name + " [level=" + this.level +
+      ", formatter=" + this._formatter + "]";
+  },
+  doAppend: function App_doAppend(formatted) {}
+};
+
+/*
+ * DumpAppender
+ * Logs to standard out
+ */
+
+function DumpAppender(formatter) {
+  Appender.call(this, formatter);
+  this._name = "DumpAppender";
+}
+DumpAppender.prototype = {
+  __proto__: Appender.prototype,
+
+  doAppend: function DApp_doAppend(formatted) {
+    dump(formatted + "\n");
+  }
+};
+
+/*
+ * ConsoleAppender
+ * Logs to the javascript console
+ */
+
+function ConsoleAppender(formatter) {
+  Appender.call(this, formatter);
+  this._name = "ConsoleAppender";
+}
+ConsoleAppender.prototype = {
+  __proto__: Appender.prototype,
+
+  // XXX this should be replaced with calls to the Browser Console
+  append: function App_append(message) {
+    if (message) {
+      let m = this._formatter.format(message);
+      if (message.level > Log.Level.Warn) {
+        Cu.reportError(m);
+        return;
+      }
+      this.doAppend(m);
+    }
+  },
+
+  doAppend: function CApp_doAppend(formatted) {
+    Cc["@mozilla.org/consoleservice;1"].
+      getService(Ci.nsIConsoleService).logStringMessage(formatted);
+  }
+};
+
+/**
+ * Append to an nsIStorageStream
+ *
+ * This writes logging output to an in-memory stream which can later be read
+ * back as an nsIInputStream. It can be used to avoid expensive I/O operations
+ * during logging. Instead, one can periodically consume the input stream and
+ * e.g. write it to disk asynchronously.
+ */
+function StorageStreamAppender(formatter) {
+  Appender.call(this, formatter);
+  this._name = "StorageStreamAppender";
+}
+
+StorageStreamAppender.prototype = {
+  __proto__: Appender.prototype,
+
+  _converterStream: null, // holds the nsIConverterOutputStream
+  _outputStream: null,    // holds the underlying nsIOutputStream
+
+  _ss: null,
+
+  get outputStream() {
+    if (!this._outputStream) {
+      // First create a raw stream. We can bail out early if that fails.
+      this._outputStream = this.newOutputStream();
+      if (!this._outputStream) {
+        return null;
+      }
+
+      // Wrap the raw stream in an nsIConverterOutputStream. We can reuse
+      // the instance if we already have one.
+      if (!this._converterStream) {
+        this._converterStream = Cc["@mozilla.org/intl/converter-output-stream;1"]
+                                  .createInstance(Ci.nsIConverterOutputStream);
+      }
+      this._converterStream.init(
+        this._outputStream, "UTF-8", STREAM_SEGMENT_SIZE,
+        Ci.nsIConverterInputStream.DEFAULT_REPLACEMENT_CHARACTER);
+    }
+    return this._converterStream;
+  },
+
+  newOutputStream: function newOutputStream() {
+    let ss = this._ss = Cc["@mozilla.org/storagestream;1"]
+                          .createInstance(Ci.nsIStorageStream);
+    ss.init(STREAM_SEGMENT_SIZE, PR_UINT32_MAX, null);
+    return ss.getOutputStream(0);
+  },
+
+  getInputStream: function getInputStream() {
+    if (!this._ss) {
+      return null;
+    }
+    return this._ss.newInputStream(0);
+  },
+
+  reset: function reset() {
+    if (!this._outputStream) {
+      return;
+    }
+    this.outputStream.close();
+    this._outputStream = null;
+    this._ss = null;
+  },
+
+  doAppend: function (formatted) {
+    if (!formatted) {
+      return;
+    }
+    try {
+      this.outputStream.writeString(formatted + "\n");
+    } catch (ex) {
+      if (ex.result == Cr.NS_BASE_STREAM_CLOSED) {
+        // The underlying output stream is closed, so let's open a new one
+        // and try again.
+        this._outputStream = null;
+      } try {
+          this.outputStream.writeString(formatted + "\n");
+      } catch (ex) {
+        // Ah well, we tried, but something seems to be hosed permanently.
+      }
+    }
+  }
+};
+
+/**
+ * File appender
+ *
+ * Writes output to file using OS.File.
+ */
+function FileAppender(path, formatter) {
+  Appender.call(this, formatter);
+  this._name = "FileAppender";
+  this._encoder = new TextEncoder();
+  this._path = path;
+  this._file = null;
+  this._fileReadyPromise = null;
+
+  // This is a promise exposed for testing/debugging the logger itself.
+  this._lastWritePromise = null;
+}
+
+FileAppender.prototype = {
+  __proto__: Appender.prototype,
+
+  _openFile: function () {
+    return Task.spawn(function* _openFile() {
+      try {
+        this._file = yield OS.File.open(this._path,
+                                        {truncate: true});
+      } catch (err) {
+        if (err instanceof OS.File.Error) {
+          this._file = null;
+        } else {
+          throw err;
+        }
+      }
+    }.bind(this));
+  },
+
+  _getFile: function() {
+    if (!this._fileReadyPromise) {
+      this._fileReadyPromise = this._openFile();
+    }
+
+    return this._fileReadyPromise;
+  },
+
+  doAppend: function (formatted) {
+    let array = this._encoder.encode(formatted + "\n");
+    if (this._file) {
+      this._lastWritePromise = this._file.write(array);
+    } else {
+      this._lastWritePromise = this._getFile().then(_ => {
+        this._fileReadyPromise = null;
+        if (this._file) {
+          return this._file.write(array);
+        }
+        return undefined;
+      });
+    }
+  },
+
+  reset: function () {
+    let fileClosePromise = this._file.close();
+    return fileClosePromise.then(_ => {
+      this._file = null;
+      return OS.File.remove(this._path);
+    });
+  }
+};
+
+/**
+ * Bounded File appender
+ *
+ * Writes output to file using OS.File. After the total message size
+ * (as defined by formatted.length) exceeds maxSize, existing messages
+ * will be discarded, and subsequent writes will be appended to a new log file.
+ */
+function BoundedFileAppender(path, formatter, maxSize=2*ONE_MEGABYTE) {
+  FileAppender.call(this, path, formatter);
+  this._name = "BoundedFileAppender";
+  this._size = 0;
+  this._maxSize = maxSize;
+  this._closeFilePromise = null;
+}
+
+BoundedFileAppender.prototype = {
+  __proto__: FileAppender.prototype,
+
+  doAppend: function (formatted) {
+    if (!this._removeFilePromise) {
+      if (this._size < this._maxSize) {
+        this._size += formatted.length;
+        return FileAppender.prototype.doAppend.call(this, formatted);
+      }
+      this._removeFilePromise = this.reset();
+    }
+    this._removeFilePromise.then(_ => {
+      this._removeFilePromise = null;
+      this.doAppend(formatted);
+    });
+    return undefined;
+  },
+
+  reset: function () {
+    let fileClosePromise;
+    if (this._fileReadyPromise) {
+      // An attempt to open the file may still be in progress.
+      fileClosePromise = this._fileReadyPromise.then(_ => {
+        return this._file.close();
+      });
+    } else {
+      fileClosePromise = this._file.close();
+    }
+
+    return fileClosePromise.then(_ => {
+      this._size = 0;
+      this._file = null;
+      return OS.File.remove(this._path);
+    });
+  }
+};
+