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 0771b47c182e5c18c868f1124a0784bf411d74a9..f1140b29aaae1976b1025ccafcf3206ffce127aa 100644
--- a/base/logging.cc
+++ b/base/logging.cc
@@ -49,15 +49,18 @@ typedef pthread_mutex_t* MutexHandle;
 #include <algorithm>
 #include <cstring>
 #include <ctime>
+#include <deque>
 #include <iomanip>
 #include <ostream>
 #include <string>
+#include <unordered_set>
 
 #include "base/base_switches.h"
 #include "base/command_line.h"
 #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"
@@ -123,8 +126,13 @@ 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;
+// Log message listeners that get notified of every log message we process
+// before log message handlers.
+base::LazyInstance<std::unordered_set<LogMessageListenerFunction>>::Leaky
+    log_message_listeners = LAZY_INSTANCE_INITIALIZER;
 
 // Helper functions to wrap platform differences.
 
@@ -342,6 +350,16 @@ void CloseLogFileUnlocked() {
 
 }  // namespace
 
+ScopedLogMessageListener::ScopedLogMessageListener(
+    LogMessageListenerFunction listener)
+    : listener_(listener) {
+  AddLogMessageListener(listener);
+}
+
+ScopedLogMessageListener::~ScopedLogMessageListener() {
+  RemoveLogMessageListener(listener_);
+}
+
 LoggingSettings::LoggingSettings()
     : logging_dest(LOG_DEFAULT),
       log_file(nullptr),
@@ -409,7 +427,9 @@ 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() ||
+         !log_message_listeners.Get().empty() ||
          severity >= kAlwaysPrintErrorLevel;
 }
 
@@ -443,12 +463,27 @@ 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);
+}
+
+void PopLogMessageHandler(LogMessageHandlerFunction expected) {
+  CHECK_EQ(GetTopLogMessageHandler(), expected);
+  log_message_handlers.Get().pop_front();
+}
+
+LogMessageHandlerFunction GetTopLogMessageHandler() {
+  if (log_message_handlers.Get().empty())
+    return nullptr;
+  return log_message_handlers.Get().front();
 }
 
-LogMessageHandlerFunction GetLogMessageHandler() {
-  return log_message_handler;
+void AddLogMessageListener(LogMessageListenerFunction listener) {
+  log_message_listeners.Get().insert(listener);
+}
+
+void RemoveLogMessageListener(LogMessageListenerFunction listener) {
+  CHECK_EQ(1u, log_message_listeners.Get().erase(listener));
 }
 
 // Explicit instantiations for commonly used comparisons.
@@ -537,12 +572,17 @@ 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;
+  // Broadcast to log message listeners first.
+  for (auto* listeners : log_message_listeners.Get()) {
+    listeners(severity_, file_, line_, message_start_, str_newline);
+  }
+
+  // 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) {