libdeng2: Protect log entries against multithreaded access

The libdeng2 Log is a multithreaded system. Entries may be created in
any thread, and they get collected into a central LogBuffer. The buffer
is flushed whenever a new entry triggers the flush condition, which
means flushing may occur from any thread.

Log entry arguments are appended after the creation of the entry and
even after it has been inserted to the buffer. Therefore it is possible
that an entry is being flushed while another thread is still adding
arguments to it.

This scenario is now handled by locking the entry while it is being
converted to text or while an argument is being added.

doomsday/libdeng2/include/de/core/log.h

@@ -270,7 +270,7 @@ class DENG2_PUBLIC Log
  *
  * @ingroup core
  */
-class DENG2_PUBLIC LogEntry
+class DENG2_PUBLIC LogEntry : public Lockable
 {
 public:
     /**
@@ -434,7 +434,9 @@ class DENG2_PUBLIC LogEntry
     template <typename ValueType>
     inline LogEntry &operator << (ValueType const &v) {
         if(!_disabled) {
+            lock();
             _args.push_back(new Arg(v));
+            unlock();
         }
         return *this;
     }

doomsday/libdeng2/src/core/log.cpp

@@ -21,6 +21,7 @@
 #include "de/Time"
 #include "de/Date"
 #include "de/LogBuffer"
+#include "de/Guard"
 #include "logtextstyle.h"
 
 #include <QMap>
@@ -76,6 +77,8 @@ LogEntry::LogEntry(Log::LogLevel level, String const &section, int sectionDepth,
 
 LogEntry::~LogEntry()
 {
+    DENG2_GUARD(this); // cannot delete if someone has a lock
+
     for(Args::iterator i = _args.begin(); i != _args.end(); ++i) 
     {
         delete *i;
@@ -84,6 +87,8 @@ LogEntry::~LogEntry()
 
 String LogEntry::asText(Flags const &formattingFlags, int shortenSection) const
 {
+    DENG2_GUARD(this); // others shouldn't touch the entry while we're converting
+
     /// @todo This functionality belongs in an entry formatter class.
 
     Flags flags = formattingFlags;