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

base/logging.cc

Index: base/logging.cc
diff --git a/base/logging.cc b/base/logging.cc
index 98cecd094fd84e7312c9a3e9998dfb44cfd05925..8322d905d16b800caf2126d4e75c77d2c91e5c65 100644
--- a/base/logging.cc
+++ b/base/logging.cc
@@ -51,6 +51,7 @@ typedef pthread_mutex_t* MutexHandle;
 #include <algorithm>
 #include <cstring>
 #include <ctime>
+#include <deque>
 #include <iomanip>
 #include <ostream>
 #include <string>
@@ -60,6 +61,7 @@ typedef pthread_mutex_t* MutexHandle;
 #include "base/debug/alias.h"
 #include "base/debug/debugger.h"
 #include "base/debug/stack_trace.h"
+#include "base/lazy_instance.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/string_util.h"
@@ -125,8 +127,9 @@ bool show_error_dialogs = false;
 // An assert handler override specified by the client to be called instead of
 // the debug message dialog and process termination.
 LogAssertHandlerFunction log_assert_handler = nullptr;
-// A log message handler that gets notified of every log message we process.
-LogMessageHandlerFunction log_message_handler = nullptr;
+// Log message handlers that get notified of every log message we process.
+base::LazyInstance<std::deque<LogMessageHandlerFunction>>::Leaky
+    log_message_handlers = LAZY_INSTANCE_INITIALIZER;
 
 // Helper functions to wrap platform differences.
 
@@ -400,7 +403,8 @@ bool ShouldCreateLogMessage(int severity) {
   // Return true here unless we know ~LogMessage won't do anything. Note that
   // ~LogMessage writes to stderr if severity_ >= kAlwaysPrintErrorLevel, even
   // when g_logging_destination is LOG_NONE.
-  return g_logging_destination != LOG_NONE || log_message_handler ||
+  return g_logging_destination != LOG_NONE ||
+         !log_message_handlers.Get().empty() ||
          severity >= kAlwaysPrintErrorLevel;
 }
 
@@ -434,12 +438,18 @@ void SetLogAssertHandler(LogAssertHandlerFunction handler) {
   log_assert_handler = handler;
 }
 
-void SetLogMessageHandler(LogMessageHandlerFunction handler) {
-  log_message_handler = handler;
+void PushLogMessageHandler(LogMessageHandlerFunction handler) {
+  log_message_handlers.Get().push_front(handler);
 }
 
-LogMessageHandlerFunction GetLogMessageHandler() {
-  return log_message_handler;
+void PopLogMessageHandler() {
+  log_message_handlers.Get().pop_front();
+}
+
+LogMessageHandlerFunction GetTopLogMessageHandler() {
+  if (log_message_handlers.Get().empty())
+    return nullptr;
+  return log_message_handlers.Get().front();
 }
 
 // Explicit instantiations for commonly used comparisons.
@@ -528,12 +538,12 @@ LogMessage::~LogMessage() {
   stream_ << std::endl;
   std::string str_newline(stream_.str());
 
-  // Give any log message handler first dibs on the message.
-  if (log_message_handler &&
-      log_message_handler(severity_, file_, line_,
-                          message_start_, str_newline)) {
-    // The handler took care of it, no further processing.
-    return;
+  // Give log message handlers first dibs on the message.
+  for (auto* handler : log_message_handlers.Get()) {
+    if (handler(severity_, file_, line_, message_start_, str_newline)) {
+      // The handler took care of it, no further processing.
+      return;
+    }
   }
 
   if ((g_logging_destination & LOG_TO_SYSTEM_DEBUG_LOG) != 0) {