Fix crash when using printf format specifiers in traceEvent (#3874)

rts/Trace.c

@@ -299,21 +299,29 @@ void trace_(char *msg, ...)
     va_end(ap);
 }
 
-void traceUserMsg(Capability *cap, char *msg)
+static void traceFormatUserMsg(Capability *cap, char *msg, ...)
 {
+    va_list ap;
+    va_start(ap,msg);
+
 #ifdef DEBUG
     if (RtsFlags.TraceFlags.tracing == TRACE_STDERR) {
-        traceCap_stderr(cap, msg, NULL);
+        traceCap_stderr(cap, msg, ap);
     } else
 #endif
     {
         if (eventlog_enabled) {
-            postUserMsg(cap, msg);
+            postUserMsg(cap, msg, ap);
         }
     }
     dtraceUserMsg(cap->no, msg);
 }
 
+void traceUserMsg(Capability *cap, char *msg)
+{
+    traceFormatUserMsg(cap, "%s", msg);
+}
+
 void traceThreadStatus_ (StgTSO *tso USED_IF_DEBUG)
 {
 #ifdef DEBUG

rts/eventlog/EventLog.c

@@ -448,9 +448,9 @@ void postCapMsg(Capability *cap, char *msg, va_list ap)
     postLogMsg(&capEventBuf[cap->no], EVENT_LOG_MSG, msg, ap);
 }
 
-void postUserMsg(Capability *cap, char *msg)
+void postUserMsg(Capability *cap, char *msg, va_list ap)
 {
-    postLogMsg(&capEventBuf[cap->no], EVENT_USER_MSG, msg, NULL);
+    postLogMsg(&capEventBuf[cap->no], EVENT_USER_MSG, msg, ap);
 }    
 
 void closeBlockMarker (EventsBuf *ebuf)

rts/eventlog/EventLog.h

@@ -39,7 +39,7 @@ void postEvent(Capability *cap, EventTypeNum tag);
 
 void postMsg(char *msg, va_list ap);
 
-void postUserMsg(Capability *cap, char *msg);
+void postUserMsg(Capability *cap, char *msg, va_list ap);
 
 void postCapMsg(Capability *cap, char *msg, va_list ap);