debug_sync: Print all current active signals within the trace file during wait

When running with DBUG trace enabled, print all current active signals.

The output looks like this with the signals present in curr:
T@6 ... debug_sync_exec: wait for 'nothing'  at: 'now', curr: 'something,from_function,from_myvar'

sql/debug_sync.cc

@@ -18,6 +18,7 @@
 
 #include "mariadb.h"
 #include "debug_sync.h"
+#include <cstring>
 
 #if defined(ENABLED_DEBUG_SYNC)
 
@@ -351,6 +352,40 @@ void debug_sync_init_thread(THD *thd)
 }
 
 
+/**
+  Returns an allocated buffer containing a comma-separated C string of all
+  active signals.
+
+  Buffer must be freed by the caller.
+*/
+static const char *get_signal_set_as_string()
+{
+  mysql_mutex_assert_owner(&debug_sync_global.ds_mutex);
+  size_t req_size= 1; // In case of empty set for the end '\0' char.
+
+  for (size_t i= 0; i < debug_sync_global.ds_signal_set.size(); i++)
+    req_size+= debug_sync_global.ds_signal_set.at(i)->length + 1;
+
+  char *buf= (char *) my_malloc(PSI_NOT_INSTRUMENTED, req_size, MYF(0));
+  if (!buf)
+    return nullptr;
+  memset(buf, '\0', req_size);
+
+  char *cur_pos= buf;
+  for (size_t i= 0; i < debug_sync_global.ds_signal_set.size(); i++)
+  {
+    const LEX_CSTRING *signal= debug_sync_global.ds_signal_set.at(i);
+    memcpy(cur_pos, signal->str, signal->length);
+    if (i != debug_sync_global.ds_signal_set.size() - 1)
+      cur_pos[signal->length]= ',';
+    else
+      cur_pos[signal->length] = '\0';
+    cur_pos+= signal->length + 1;
+  }
+  return buf;
+}
+
+
 /**
   End the debug sync facility at thread end.
 
@@ -1581,12 +1616,22 @@ static void debug_sync_execute(THD *thd, st_debug_sync_action *action)
         restore_current_mutex = false;
 
       set_timespec(abstime, action->timeout);
-      // TODO turn this into a for loop printing.
       DBUG_EXECUTE("debug_sync_exec", {
+          const char *signal_set= get_signal_set_as_string();
+          if (!signal_set)
+          {
+            DBUG_PRINT("debug_sync_exec",
+                       ("Out of memory when fetching signal set"));
+          }
+          else
+          {
             /* Functions as DBUG_PRINT args can change keyword and line nr. */
             DBUG_PRINT("debug_sync_exec",
-                     ("wait for '%s'  at: '%s'",
-                      sig_wait, dsp_name));});
+                       ("wait for '%s'  at: '%s', curr: '%s'",
+                        sig_wait, dsp_name, signal_set));
+            my_free((void *)signal_set);
+          }});
+
 
       /*
         Wait until global signal string matches the wait_for string.