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 cea6edae4a37cef2519ff32045c46c0a3c73e4d0..6ac4cb28402ce3466467f683e2da5c638783869a 100644
--- a/base/logging.cc
+++ b/base/logging.cc
@@ -49,6 +49,7 @@ typedef pthread_mutex_t* MutexHandle;
 #include <algorithm>
 #include <cstring>
 #include <ctime>
+#include <deque>
 #include <iomanip>
 #include <ostream>
 #include <string>
@@ -58,6 +59,7 @@ 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"
@@ -65,6 +67,7 @@ typedef pthread_mutex_t* MutexHandle;
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/synchronization/lock_impl.h"
+#include "base/synchronization/read_write_lock.h"
 #include "base/threading/platform_thread.h"
 #include "base/vlog.h"
 #if defined(OS_POSIX)
@@ -123,8 +126,11 @@ 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 listeners that get notified of every log message we process.
+base::LazyInstance<std::deque<LogMessageListener*>>::Leaky
+    log_message_listeners = LAZY_INSTANCE_INITIALIZER;
+base::LazyInstance<base::subtle::ReadWriteLock>::Leaky
+    log_message_listener_lock = LAZY_INSTANCE_INITIALIZER;
 
 // Helper functions to wrap platform differences.
 
@@ -342,6 +348,25 @@ void CloseLogFileUnlocked() {
 
 }  // namespace
 
+LogMessageListener::LogMessageListener() {
+  base::subtle::AutoWriteLock lock(log_message_listener_lock.Get());
+  log_message_listeners.Get().push_front(this);
+}
+
+LogMessageListener::~LogMessageListener() {
+  base::subtle::AutoWriteLock lock(log_message_listener_lock.Get());
+  auto& listeners = log_message_listeners.Get();
+  size_t count = listeners.size();
+  listeners.erase(std::remove(listeners.begin(), listeners.end(), this),
+                  listeners.end());
+  DCHECK_EQ(count - 1, listeners.size());
+}
+
+size_t LogMessageListenerCountForTesting() {
+  base::subtle::AutoReadLock lock(log_message_listener_lock.Get());
+  return log_message_listeners.Get().size();
+}
+
 // This is never instantiated, it's just used for EAT_STREAM_PARAMETERS to have
 // an object of the correct type on the LHS of the unused part of the ternary
 // operator.
@@ -414,7 +439,12 @@ 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 ||
+  bool listeners_empty;
+  {
+    base::subtle::AutoReadLock lock(log_message_listener_lock.Get());
+    listeners_empty = log_message_listeners.Get().empty();
+  }
+  return g_logging_destination != LOG_NONE || !listeners_empty ||
          severity >= kAlwaysPrintErrorLevel;
 }
 
@@ -448,14 +478,6 @@ void SetLogAssertHandler(LogAssertHandlerFunction handler) {
   log_assert_handler = handler;
 }
 
-void SetLogMessageHandler(LogMessageHandlerFunction handler) {
-  log_message_handler = handler;
-}
-
-LogMessageHandlerFunction GetLogMessageHandler() {
-  return log_message_handler;
-}
-
 // Explicit instantiations for commonly used comparisons.
 template std::string* MakeCheckOpString<int, int>(
     const int&, const int&, const char* names);
@@ -542,12 +564,12 @@ 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;
+  {
+    base::subtle::AutoReadLock lock(log_message_listener_lock.Get());
+    // Broadcast to log message listeners first.
+    for (auto* listener : log_message_listeners.Get()) {
+      listener->OnMessage(severity_, file_, line_, message_start_, str_newline);
+    }
   }
 
   if ((g_logging_destination & LOG_TO_SYSTEM_DEBUG_LOG) != 0) {