[2/2,Tracing] Add trace-file command to open/close/flush trace file

diff --git a/monitor.c b/monitor.c
index 090e13c..1e35a6b 100644
--- a/monitor.c
+++ b/monitor.c
@@ -540,6 +540,23 @@  static void do_change_trace_event_state(Monitor *mon, const QDict *qdict)
     bool new_state = qdict_get_bool(qdict, "option");
     change_trace_event_state(tp_name, new_state);
 }
+
+static void do_trace_file(Monitor *mon, const QDict *qdict)
+{
+    const char *op = qdict_get_try_str(qdict, "op");
+
+    if (!op) {
+        st_print_trace_file_status((FILE *)mon, &monitor_fprintf);
+    } else if (!strcmp(op, "on")) {
+        st_set_trace_file_enabled(true);
+    } else if (!strcmp(op, "off")) {
+        st_set_trace_file_enabled(false);
+    } else if (!strcmp(op, "flush")) {
+        st_flush_trace_buffer();
+    } else {
+        monitor_printf(mon, "unexpected argument \"%s\"\n", op);
+    }
+}
 #endif
 
 static void user_monitor_complete(void *opaque, QObject *ret_data)
diff --git a/qemu-monitor.hx b/qemu-monitor.hx
index c5ba3d2..25887bd 100644
--- a/qemu-monitor.hx
+++ b/qemu-monitor.hx
@@ -273,6 +273,20 @@  STEXI
 @findex trace-event
 changes status of a trace event
 ETEXI
+
+    {
+        .name       = "trace-file",
+        .args_type  = "op:s?",
+        .params     = "op [on|off|flush]",
+        .help       = "open, close, or flush trace file",
+        .mhandler.cmd = do_trace_file,
+    },
+
+STEXI
+@item trace-file on|off|flush
+@findex trace-file
+Open, close, or flush the trace file.  If no argument is given, the status of the trace file is displayed.
+ETEXI
 #endif
 
     {
diff --git a/simpletrace.c b/simpletrace.c
index 29fd6a9..71110b3 100644
--- a/simpletrace.c
+++ b/simpletrace.c
@@ -20,6 +20,13 @@  enum {
 static TraceRecord trace_buf[TRACE_BUF_LEN];
 static unsigned int trace_idx;
 static FILE *trace_fp;
+static bool trace_file_enabled = true;
+
+void st_print_trace_file_status(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...))
+{
+    stream_printf(stream, "Trace file \"" CONFIG_TRACE_FILE "\" %s.\n",
+                  getpid(), trace_file_enabled ? "on" : "off");
+}
 
 static bool open_trace_file(void)
 {
@@ -34,22 +41,52 @@  static bool open_trace_file(void)
     return trace_fp != NULL;
 }
 
-static void flush_trace_buffer(void)
+static void flush_trace_file(void)
 {
+    /* If the trace file is not open yet, open it now */
     if (!trace_fp) {
-        if (open_trace_file()) {
-            atexit(flush_trace_buffer);
+        if (!open_trace_file()) {
+            /* Avoid repeatedly trying to open file on failure */
+            trace_file_enabled = false;
+            return;
         }
+        atexit(st_flush_trace_buffer);
     }
+
     if (trace_fp) {
         size_t unused; /* for when fwrite(3) is declared warn_unused_result */
         unused = fwrite(trace_buf, trace_idx * sizeof(trace_buf[0]), 1, trace_fp);
     }
+}
+
+void st_flush_trace_buffer(void)
+{
+    if (trace_file_enabled) {
+        flush_trace_file();
+    }
 
     /* Discard written trace records */
     trace_idx = 0;
 }
 
+void st_set_trace_file_enabled(bool enable)
+{
+    if (enable == trace_file_enabled) {
+        return; /* no change */
+    }
+
+    /* Flush/discard trace buffer */
+    st_flush_trace_buffer();
+
+    /* To disable, close trace file */
+    if (!enable) {
+        fclose(trace_fp);
+        trace_fp = NULL;
+    }
+
+    trace_file_enabled = enable;
+}
+
 static void trace(TraceEventID event, unsigned long x1,
                   unsigned long x2, unsigned long x3,
                   unsigned long x4, unsigned long x5)
@@ -76,7 +113,7 @@  static void trace(TraceEventID event, unsigned long x1,
     rec->x5 = x5;
 
     if (++trace_idx == TRACE_BUF_LEN) {
-        flush_trace_buffer();
+        st_flush_trace_buffer();
     }
 }
 
diff --git a/tracetool b/tracetool
index 8d8f27c..999ef7e 100755
--- a/tracetool
+++ b/tracetool
@@ -141,6 +141,9 @@  void trace4(TraceEventID event, unsigned long x1, unsigned long x2, unsigned lon
 void trace5(TraceEventID event, unsigned long x1, unsigned long x2, unsigned long x3, unsigned long x4, unsigned long x5);
 void st_print_trace(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...));
 void st_print_trace_events(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...));
+void st_print_trace_file_status(FILE *stream, int (*stream_printf)(FILE *stream, const char *fmt, ...));
+void st_flush_trace_buffer(void);
+void st_set_trace_file_enabled(bool enable);
 void change_trace_event_state(const char *tname, bool tstate);
 EOF