Make sure linux log files don't get deleted while still logging to them

* We need to use file locks to ensure the last process knows it can
  delete the log, as well as delete any inherited file descriptors when
  forking to launch a new executable.
* To accomplish this, tracking the single logfile handle is now part of
  the OS-specific code instead of common logging code. It makes little
  difference either way as we don't support multiple log files.

renderdoc/common/common.cpp

@@ -253,7 +253,7 @@ uint64_t Log2Floor(uint64_t value)
 #endif
 
 static string logfile;
-static void *logfileHandle = NULL;
+static bool logfileOpened = false;
 
 const char *rdclog_getfilename()
 {
@@ -268,20 +268,21 @@ void rdclog_filename(const char *filename)
   if(filename && filename[0])
     logfile = filename;
 
-  FileIO::logfile_close(logfileHandle);
+  FileIO::logfile_close(NULL);
+
+  logfileOpened = false;
 
   if(!logfile.empty())
   {
-    logfileHandle = FileIO::logfile_open(filename);
+    logfileOpened = FileIO::logfile_open(filename);
 
-    if(logfileHandle && previous.c_str())
+    if(logfileOpened && previous.c_str())
     {
       vector<unsigned char> previousContents;
       FileIO::slurp(previous.c_str(), previousContents);
 
       if(!previousContents.empty())
-        FileIO::logfile_append(logfileHandle, (const char *)&previousContents[0],
-                               previousContents.size());
+        FileIO::logfile_append((const char *)&previousContents[0], previousContents.size());
 
       FileIO::Delete(previous.c_str());
     }
@@ -295,11 +296,10 @@ void rdclog_enableoutput()
   log_output_enabled = true;
 }
 
-void rdclog_closelog()
+void rdclog_closelog(const char *filename)
 {
   log_output_enabled = false;
-  if(logfileHandle)
-    FileIO::logfile_close(logfileHandle);
+  FileIO::logfile_close(filename);
 }
 
 void rdclog_flush()
@@ -326,10 +326,10 @@ void rdclogprint_int(LogType type, const char *fullMsg, const char *msg)
     OSUtility::WriteOutput(OSUtility::Output_StdErr, msg);
 #endif
 #if ENABLED(OUTPUT_LOG_TO_DISK)
-  if(logfileHandle)
+  if(logfileOpened)
   {
     // strlen used as byte length - str is UTF-8 so this is NOT number of characters
-    FileIO::logfile_append(logfileHandle, fullMsg, strlen(fullMsg));
+    FileIO::logfile_append(fullMsg, strlen(fullMsg));
   }
 #endif
 }