RCORE-2234 Crash in dart due to debug output when app is being torn down

CHANGELOG.md

@@ -7,6 +7,7 @@
 ### Fixed
 * <How do the end-user experience this issue? what was the impact?> ([#????](https://github.com/realm/realm-core/issues/????), since v?.?.?)
 * Using an empty KeyPath in C API would result in no filtering being done ([#7805](https://github.com/realm/realm-core/issues/7805))
+* Fix crash during client app shutdown when Logger log level is set higher than Info. ([#7969](https://github.com/realm/realm-core/issues/7969), since v13.23.3)
 
 ### Breaking changes
 * None.

src/realm/util/logger.cpp

@@ -25,8 +25,9 @@
 namespace realm::util {
 
 namespace {
-auto& s_logger_mutex = *new std::mutex;
+auto& s_default_logger_mutex = *new std::mutex;
 std::shared_ptr<util::Logger> s_default_logger;
+auto s_stderr_log_mutex = std::make_shared<std::mutex>();
 } // anonymous namespace
 
 size_t LogCategory::s_next_index = 0;
@@ -82,7 +83,7 @@ void LogCategory::set_default_level_threshold(Level level)
     for (auto c : m_children) {
         c->set_default_level_threshold(level);
     }
-    std::lock_guard lock(s_logger_mutex);
+    std::lock_guard lock(s_default_logger_mutex);
     if (s_default_logger)
         set_default_level_threshold(s_default_logger.get());
 }
@@ -110,13 +111,13 @@ void LogCategory::set_default_level_threshold(Logger* root) const
 
 void Logger::set_default_logger(std::shared_ptr<util::Logger> logger) noexcept
 {
-    std::lock_guard lock(s_logger_mutex);
+    std::lock_guard lock(s_default_logger_mutex);
     s_default_logger = logger;
 }
 
 std::shared_ptr<util::Logger>& Logger::get_default_logger() noexcept
 {
-    std::lock_guard lock(s_logger_mutex);
+    std::lock_guard lock(s_default_logger_mutex);
     if (!s_default_logger) {
         s_default_logger = std::make_shared<StderrLogger>();
     }
@@ -175,10 +176,15 @@ const std::string_view Logger::level_to_string(Level level) noexcept
     return "";
 }
 
+StderrLogger::StderrLogger() noexcept
+    : m_log_mutex(s_stderr_log_mutex)
+{
+}
+
 void StderrLogger::do_log(const LogCategory& cat, Level level, const std::string& message)
 {
-    static Mutex mutex;
-    LockGuard l(mutex);
+    REALM_ASSERT(m_log_mutex);
+    std::lock_guard l(*m_log_mutex.get());
     // std::cerr is unbuffered, so no need to flush
     std::cerr << cat.get_name() << " - " << get_level_prefix(level) << message << '\n'; // Throws
 }

src/realm/util/logger.hpp

@@ -24,6 +24,7 @@
 #include <realm/util/file.hpp>
 
 #include <cstring>
+#include <mutex>
 #include <ostream>
 #include <string>
 #include <utility>
@@ -344,7 +345,7 @@ std::basic_istream<C, T>& operator>>(std::basic_istream<C, T>&, Logger::Level&);
 /// level threshold.
 class StderrLogger : public Logger {
 public:
-    StderrLogger() noexcept = default;
+    StderrLogger() noexcept;
 
     StderrLogger(Level level) noexcept
         : Logger()
@@ -354,6 +355,9 @@ class StderrLogger : public Logger {
 
 protected:
     void do_log(const LogCategory& category, Level, const std::string&) final;
+
+private:
+    std::shared_ptr<std::mutex> m_log_mutex;
 };