Reopen debug.log on SIGHUP

The best log rotation method formerly available was to configure
logrotate with the copytruncate option.  As described in the logrotate
documentation, "there is a very small time slice between copying the
file and truncating it, so some logging data might be lost".

By sending SIGHUP to the server process, one can now reopen the debug
log file without losing any data.

src/util.cpp

@@ -70,6 +70,7 @@ bool fTestNet = false;
 bool fNoListen = false;
 bool fLogTimestamps = false;
 CMedianFilter<int64> vTimeOffsets(200,0);
+bool fReopenDebugLog = false;
 
 // Init openssl library multithreading support
 static CCriticalSection** ppmutexOpenSSL;
@@ -213,6 +214,14 @@ inline int OutputDebugStringF(const char* pszFormat, ...)
             static boost::mutex mutexDebugLog;
             boost::mutex::scoped_lock scoped_lock(mutexDebugLog);
 
+            // reopen the log file, if requested
+            if (fReopenDebugLog) {
+                fReopenDebugLog = false;
+                boost::filesystem::path pathDebug = GetDataDir() / "debug.log";
+                if (freopen(pathDebug.string().c_str(),"a",fileout) != NULL)
+                    setbuf(fileout, NULL); // unbuffered
+            }
+
             // Debug print useful for profiling
             if (fLogTimestamps && fStartedNewLine)
                 fprintf(fileout, "%s ", DateTimeStrFormat("%x %H:%M:%S", GetTime()).c_str());