Add CMemoryLogger

Logger that collects messages in memory. This is useful to collect the log messages for a particular operation and show them in a user interface when the operation failed. Messages can also be forwarded to a parent logger if it's set, regardless of this logger's filter, so the messages also appear in the console normally.

src/base/log.cpp

@@ -549,3 +549,38 @@ void CFutureLogger::OnFilterChange()
 		pLogger->SetFilter(m_Filter);
 	}
 }
+
+void CMemoryLogger::Log(const CLogMessage *pMessage)
+{
+	if(m_pParentLogger)
+	{
+		m_pParentLogger->Log(pMessage);
+	}
+	if(m_Filter.Filters(pMessage))
+	{
+		return;
+	}
+	const CLockScope LockScope(m_MessagesMutex);
+	m_vMessages.push_back(*pMessage);
+}
+
+std::vector<CLogMessage> CMemoryLogger::Lines()
+{
+	const CLockScope LockScope(m_MessagesMutex);
+	return m_vMessages;
+}
+
+std::string CMemoryLogger::ConcatenatedLines()
+{
+	const CLockScope LockScope(m_MessagesMutex);
+	std::string Result;
+	for(const CLogMessage &Message : m_vMessages)
+	{
+		if(!Result.empty())
+		{
+			Result += '\n';
+		}
+		Result += Message.m_aLine;
+	}
+	return Result;
+}

src/base/logger.h

@@ -6,6 +6,7 @@
 
 #include <atomic>
 #include <memory>
+#include <string>
 #include <vector>
 
 typedef void *IOHANDLE;
@@ -247,6 +248,30 @@ class CFutureLogger : public ILogger
 	void OnFilterChange() override;
 };
 
+/**
+ * @ingroup Log
+ *
+ * Logger that collects messages in memory. This is useful to collect the log
+ * messages for a particular operation and show them in a user interface when
+ * the operation failed. Use only temporarily with @link CLogScope @endlink
+ * or it will result in excessive memory usage.
+ *
+ * Messages are also forwarded to the parent logger if it's set, regardless
+ * of this logger's filter.
+ */
+class CMemoryLogger : public ILogger
+{
+	ILogger *m_pParentLogger = nullptr;
+	std::vector<CLogMessage> m_vMessages GUARDED_BY(m_MessagesMutex);
+	CLock m_MessagesMutex;
+
+public:
+	void SetParent(ILogger *pParentLogger) { m_pParentLogger = pParentLogger; }
+	void Log(const CLogMessage *pMessage) override REQUIRES(!m_MessagesMutex);
+	std::vector<CLogMessage> Lines() REQUIRES(!m_MessagesMutex);
+	std::string ConcatenatedLines() REQUIRES(!m_MessagesMutex);
+};
+
 /**
  * @ingroup Log
  *