Allow multiple logging::LogMessage{Handler,Listener}s

base/logging.h

Index: base/logging.h
diff --git a/base/logging.h b/base/logging.h
index 7854e3934e974b4670930d6855f3ab8699a8376f..9201913e3a5da175fb3b78585e85a492c9b703bb 100644
--- a/base/logging.h
+++ b/base/logging.h
@@ -280,14 +280,44 @@ BASE_EXPORT void SetShowErrorDialogs(bool enable_dialogs);
 typedef void (*LogAssertHandlerFunction)(const std::string& str);
 BASE_EXPORT void SetLogAssertHandler(LogAssertHandlerFunction handler);
 
-// Sets the Log Message Handler that gets passed every log message before
-// it's sent to other log destinations (if any).
-// Returns true to signal that it handled the message and the message
-// should not be sent to other log destinations.
-typedef bool (*LogMessageHandlerFunction)(int severity,
-    const char* file, int line, size_t message_start, const std::string& str);
-BASE_EXPORT void SetLogMessageHandler(LogMessageHandlerFunction handler);
-BASE_EXPORT LogMessageHandlerFunction GetLogMessageHandler();
+// LogMessageHandler is the callback interface for log message handling.
+// OnMessage() is called for every log message before it's sent to other log

[grt (UTC plus 2), 2017/01/04 09:12:19]

nit: i think it's more clear to say messages are sent here after all listeners
and before other destinations. i also think it's more clear to define listeners
first since they are recommended (as per comment in LogMessageListener) and
because they see messages before handlers.

+// destinations (if any), but after LogMessageListener.
+// Return true in OnMessage() to signal that it handled the message and the
+// message should not be sent to other log destinations, including other
+// handlers added before the current one.
+class BASE_EXPORT LogMessageHandler {
+ public:
+  virtual ~LogMessageHandler();
+  virtual bool OnMessage(int severity,
+                         const char* file,
+                         int line,
+                         size_t message_start,
+                         const std::string& str) = 0;
+
+ protected:
+  LogMessageHandler();
+};
+
+BASE_EXPORT size_t LogMessageHandlerCountForTesting();
+
+// LogMessageListener is similar to LogMessageHandler except that this
+// interface doesn't support message hijacking, and is preferred over
+// LogMessageHandler if hijacking is not needed.
+class BASE_EXPORT LogMessageListener {
+ public:
+  virtual ~LogMessageListener();
+  virtual void OnMessage(int severity,
+                         const char* file,
+                         int line,
+                         size_t message_start,
+                         const std::string& str) = 0;
+
+ protected:
+  LogMessageListener();
+};
+
+BASE_EXPORT size_t LogMessageListenerCountForTesting();
 
 typedef int LogSeverity;
 const LogSeverity LOG_VERBOSE = -1;  // This is level 1 verbosity