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diff --git a/xpcom/base/Logging.h b/xpcom/base/Logging.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_logging_h
+#define mozilla_logging_h
+
+#include <string.h>
+#include <stdarg.h>
+
+#include "prlog.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Atomics.h"
+#include "mozilla/Likely.h"
+#include "mozilla/MacroForEach.h"
+
+// This file is a placeholder for a replacement to the NSPR logging framework
+// that is defined in prlog.h. Currently it is just a pass through, but as
+// work progresses more functionality will be swapped out in favor of
+// mozilla logging implementations.
+
+// We normally have logging enabled everywhere, but measurements showed that
+// having logging enabled on Android is quite expensive (hundreds of kilobytes
+// for both the format strings for logging and the code to perform all the
+// logging calls).  Because retrieving logs from a mobile device is
+// comparatively more difficult for Android than it is for desktop and because
+// desktop machines tend to be less space/bandwidth-constrained than Android
+// devices, we've chosen to leave logging enabled on desktop, but disabled on
+// Android.  Given that logging can still be useful for development purposes,
+// however, we leave logging enabled on Android developer builds.
+#if !defined(ANDROID) || !defined(RELEASE_OR_BETA)
+#define MOZ_LOGGING_ENABLED 1
+#else
+#define MOZ_LOGGING_ENABLED 0
+#endif
+
+namespace mozilla {
+
+// While not a 100% mapping to PR_LOG's numeric values, mozilla::LogLevel does
+// maintain a direct mapping for the Disabled, Debug and Verbose levels.
+//
+// Mappings of LogLevel to PR_LOG's numeric values:
+//
+//   +---------+------------------+-----------------+
+//   | Numeric | NSPR Logging     | Mozilla Logging |
+//   +---------+------------------+-----------------+
+//   |       0 | PR_LOG_NONE      | Disabled        |
+//   |       1 | PR_LOG_ALWAYS    | Error           |
+//   |       2 | PR_LOG_ERROR     | Warning         |
+//   |       3 | PR_LOG_WARNING   | Info            |
+//   |       4 | PR_LOG_DEBUG     | Debug           |
+//   |       5 | PR_LOG_DEBUG + 1 | Verbose         |
+//   +---------+------------------+-----------------+
+//
+enum class LogLevel {
+  Disabled = 0,
+  Error,
+  Warning,
+  Info,
+  Debug,
+  Verbose,
+};
+
+/**
+ * Safely converts an integer into a valid LogLevel.
+ */
+LogLevel ToLogLevel(int32_t aLevel);
+
+class LogModule
+{
+public:
+  ~LogModule() { ::free(mName); }
+
+  /**
+   * Retrieves the module with the given name. If it does not already exist
+   * it will be created.
+   *
+   * @param aName The name of the module.
+   * @return A log module for the given name. This may be shared.
+   */
+  static LogModule* Get(const char* aName);
+
+  static void Init();
+
+  /**
+   * Sets the log file to the given filename.
+   */
+  static void SetLogFile(const char* aFilename);
+
+  /**
+   * @param aBuffer - pointer to a buffer
+   * @param aLength - the length of the buffer
+   *
+   * @return the actual length of the filepath.
+   */
+  static uint32_t GetLogFile(char *aBuffer, size_t aLength);
+
+  /**
+   * @param aAddTimestamp If we should log a time stamp with every message.
+   */
+  static void SetAddTimestamp(bool aAddTimestamp);
+
+  /**
+   * @param aIsSync If we should flush the file after every logged message.
+   */
+  static void SetIsSync(bool aIsSync);
+
+  /**
+   * Indicates whether or not the given log level is enabled.
+   */
+  bool ShouldLog(LogLevel aLevel) const { return mLevel >= aLevel; }
+
+  /**
+   * Retrieves the log module's current level.
+   */
+  LogLevel Level() const { return mLevel; }
+
+  /**
+   * Sets the log module's level.
+   */
+  void SetLevel(LogLevel level) { mLevel = level; }
+
+  /**
+   * Print a log message for this module.
+   */
+  void Printv(LogLevel aLevel, const char* aFmt, va_list aArgs) const;
+
+  /**
+   * Retrieves the module name.
+   */
+  const char* Name() const { return mName; }
+
+private:
+  friend class LogModuleManager;
+
+  explicit LogModule(const char* aName, LogLevel aLevel)
+    : mName(strdup(aName)), mLevel(aLevel)
+  {
+  }
+
+  LogModule(LogModule&) = delete;
+  LogModule& operator=(const LogModule&) = delete;
+
+  char* mName;
+  Atomic<LogLevel, Relaxed> mLevel;
+};
+
+/**
+ * Helper class that lazy loads the given log module. This is safe to use for
+ * declaring static references to log modules and can be used as a replacement
+ * for accessing a LogModule directly.
+ *
+ * Example usage:
+ *   static LazyLogModule sLayoutLog("layout");
+ *
+ *   void Foo() {
+ *     MOZ_LOG(sLayoutLog, LogLevel::Verbose, ("Entering foo"));
+ *   }
+ */
+class LazyLogModule final
+{
+public:
+  explicit constexpr LazyLogModule(const char* aLogName)
+    : mLogName(aLogName)
+    , mLog(nullptr)
+  {
+  }
+
+  operator LogModule*()
+  {
+    // NB: The use of an atomic makes the reading and assignment of mLog
+    //     thread-safe. There is a small chance that mLog will be set more
+    //     than once, but that's okay as it will be set to the same LogModule
+    //     instance each time. Also note LogModule::Get is thread-safe.
+    LogModule* tmp = mLog;
+    if (MOZ_UNLIKELY(!tmp)) {
+      tmp = LogModule::Get(mLogName);
+      mLog = tmp;
+    }
+
+    return tmp;
+  }
+
+private:
+  const char* const mLogName;
+  Atomic<LogModule*, ReleaseAcquire> mLog;
+};
+
+namespace detail {
+
+inline bool log_test(const PRLogModuleInfo* module, LogLevel level) {
+  MOZ_ASSERT(level != LogLevel::Disabled);
+  return module && module->level >= static_cast<int>(level);
+}
+
+/**
+ * A rather inefficient wrapper for PR_LogPrint that always allocates.
+ * PR_LogModuleInfo is deprecated so it's not worth the effort to do
+ * any better.
+ */
+void log_print(const PRLogModuleInfo* aModule,
+                      LogLevel aLevel,
+                      const char* aFmt, ...);
+
+inline bool log_test(const LogModule* module, LogLevel level) {
+  MOZ_ASSERT(level != LogLevel::Disabled);
+  return module && module->ShouldLog(level);
+}
+
+void log_print(const LogModule* aModule,
+               LogLevel aLevel,
+               const char* aFmt, ...);
+} // namespace detail
+
+} // namespace mozilla
+
+
+// Helper macro used convert MOZ_LOG's third parameter, |_args|, from a
+// parenthesized form to a varargs form. For example:
+//   ("%s", "a message") => "%s", "a message"
+#define MOZ_LOG_EXPAND_ARGS(...) __VA_ARGS__
+
+#if MOZ_LOGGING_ENABLED
+#define MOZ_LOG_TEST(_module,_level) mozilla::detail::log_test(_module, _level)
+#else
+// Define away MOZ_LOG_TEST here so the compiler will fold away entire
+// logging blocks via dead code elimination, e.g.:
+//
+//   if (MOZ_LOG_TEST(...)) {
+//     ...compute things to log and log them...
+//   }
+//
+// This also has the nice property that no special definition of MOZ_LOG is
+// required when logging is disabled.
+#define MOZ_LOG_TEST(_module,_level) false
+#endif
+
+#define MOZ_LOG(_module,_level,_args)     \
+  PR_BEGIN_MACRO             \
+    if (MOZ_LOG_TEST(_module,_level)) { \
+      mozilla::detail::log_print(_module, _level, MOZ_LOG_EXPAND_ARGS _args);         \
+    }                     \
+  PR_END_MACRO
+
+#undef PR_LOG
+#undef PR_LOG_TEST
+
+// This #define is a Logging.h-only knob!  Don't encourage people to get fancy
+// with their log definitions by exporting it outside of Logging.h.
+#undef MOZ_LOGGING_ENABLED
+
+#endif // mozilla_logging_h