Fix debug flush race from open-simh PR 454

Fix the debug-output flush race described in open-simh/simh#454, but
with a much simpler local implementation.

The old debug flush path drained SIMH's internal debug buffer, then
closed and reopened ordinary debug files to force the stdio buffers out.
That temporarily cleared and replaced the global debug stream, which
could race with other threads emitting debug output.

Keep the internal debug-buffer flush, then call fflush() on the current
debug stream. This preserves the useful flush behavior without closing
or replacing sim_deb.

Upstream-PR: open-simh/simh#454

Author: pmetzger

src/core/scp.c

@@ -10388,36 +10388,18 @@ static void _sim_debug_write (const char *buf, size_t len)
 _sim_debug_write_flush (buf, len, FALSE);
 }
 
+/* Flush pending debug output. */
 static t_stat _sim_debug_flush (void)
 {
-int32 saved_quiet = sim_quiet;
-int32 saved_sim_switches = sim_switches;
-int32 saved_deb_switches = sim_deb_switches;
-struct timespec saved_deb_basetime = sim_deb_basetime;
-char saved_debug_filename[CBUFSIZE];
+    if (sim_deb == NULL)                                /* no debug? */
+        return SCPE_OK;
 
-if (sim_deb == NULL)                                    /* no debug? */
-    return SCPE_OK;
+    _sim_debug_write_flush ("", 0, TRUE);
 
-_sim_debug_write_flush ("", 0, TRUE);
+    if (fflush (sim_deb) != 0)
+        return SCPE_IOERR;
 
-if (sim_deb == sim_log) {                               /* debug is log */
-    fflush (sim_deb);                                   /* fflush is the best we can do */
     return SCPE_OK;
-    }
-
-if (!(saved_deb_switches & SWMASK ('B'))) {
-    strcpy (saved_debug_filename, sim_logfile_name (sim_deb, sim_deb_ref));
-
-    sim_quiet = 1;
-    sim_set_deboff (0, NULL);
-    sim_switches = saved_deb_switches;
-    sim_set_debon (0, saved_debug_filename);
-    sim_deb_basetime = saved_deb_basetime;
-    sim_switches = saved_sim_switches;
-    sim_quiet = saved_quiet;
-    }
-return SCPE_OK;
 }