MDEV-7999 - PROFILING routines take 0.2% when profiling disabled

PROFILING::start_new_query() optimizations:
- no need to check "current": added assertion instead
- "enabled" now means "is enabled currently" instead of "was enabled at query
  start". Old meaning was useless, new meaning echoes OPTION_PROFILING so
  that start_new_query() can be defined in sql_profile.h.
- remnants of start_new_query() moved to sql_profile.h so it can be inlined

PROFILING::start_new_query() overhead dropped 0.08% -> out of radar.

PROFILING::set_query_source() optimizations:
- no need to check "enabled": !enabled && current is impossible
- remnants of set_query_source() moved to sql_profile.h so it can be inlined

PROFILING::set_query_source() overhead dropped 0.02% -> out of radar.

PROFILING::finish_current_query() optimizations:
- moved "current" check out to sql_profile.h so it can be inlined

PROFILING::finish_current_query() overhead dropped 0.10% -> out of radar.

sql/sql_profile.cc

@@ -337,36 +337,6 @@ PROFILING::~PROFILING()
     delete current;
 }
 
-
-/**
-  Prepare to start processing a new query.  It is an error to do this
-  if there's a query already in process; nesting is not supported.
-
-  @param  initial_state  (optional) name of period before first state change
-*/
-void PROFILING::start_new_query(const char *initial_state)
-{
-  DBUG_ENTER("PROFILING::start_new_query");
-
-  /* This should never happen unless the server is radically altered. */
-  if (unlikely(current != NULL))
-  {
-    DBUG_PRINT("warning", ("profiling code was asked to start a new query "
-                           "before the old query was finished.  This is "
-                           "probably a bug."));
-    finish_current_query();
-  }
-
-  enabled= ((thd->variables.option_bits & OPTION_PROFILING) != 0);
-
-  if (! enabled) DBUG_VOID_RETURN;
-
-  DBUG_ASSERT(current == NULL);
-  current= new QUERY_PROFILE(this, initial_state);
-
-  DBUG_VOID_RETURN;
-}
-
 /**
   Throw away the current profile, because it's useless or unwanted
   or corrupted.
@@ -386,36 +356,31 @@ void PROFILING::discard_current_query()
   saved, and maintain the profile history size.  Naturally, this may not
   succeed if the profile was previously discarded, and that's expected.
 */
-void PROFILING::finish_current_query()
+void PROFILING::finish_current_query_impl()
 {
   DBUG_ENTER("PROFILING::finish_current_profile");
-  if (current != NULL)
+  DBUG_ASSERT(current);
+
+  /* The last fence-post, so we can support the span before this. */
+  status_change("ending", NULL, NULL, 0);
+
+  if (enabled &&   /* ON at end? */
+      (current->query_source != NULL) &&
+      (! current->entries.is_empty()))
   {
-    /* The last fence-post, so we can support the span before this. */
-    status_change("ending", NULL, NULL, 0);
+    current->profiling_query_id= next_profile_id();   /* assign an id */
 
-    if ((enabled) &&                                    /* ON at start? */
-        ((thd->variables.option_bits & OPTION_PROFILING) != 0) &&   /* and ON at end? */
-        (current->query_source != NULL) &&
-        (! current->entries.is_empty()))
-    {
-      current->profiling_query_id= next_profile_id();   /* assign an id */
+    history.push_back(current);
+    last= current; /* never contains something that is not in the history. */
 
-      history.push_back(current);
-      last= current; /* never contains something that is not in the history. */
-      current= NULL;
-    }
-    else
-    {
-      delete current;
-      current= NULL;
-    }
+    /* Maintain the history size. */
+    while (history.elements > thd->variables.profiling_history_size)
+      delete history.pop();
   }
+  else
+    delete current;
 
-  /* Maintain the history size. */
-  while (history.elements > thd->variables.profiling_history_size)
-    delete history.pop();
-
+  current= NULL;
   DBUG_VOID_RETURN;
 }
 
@@ -475,26 +440,6 @@ bool PROFILING::show_profiles()
   DBUG_RETURN(FALSE);
 }
 
-/**
-  At a point in execution where we know the query source, save the text
-  of it in the query profile.
-
-  This must be called exactly once per descrete statement.
-*/
-void PROFILING::set_query_source(char *query_source_arg, uint query_length_arg)
-{
-  DBUG_ENTER("PROFILING::set_query_source");
-
-  if (! enabled)
-    DBUG_VOID_RETURN;
-
-  if (current != NULL)
-    current->set_query_source(query_source_arg, query_length_arg);
-  else
-    DBUG_PRINT("info", ("no current profile to send query source to"));
-  DBUG_VOID_RETURN;
-}
-
 /**
   Fill the information schema table, "query_profile", as defined in show.cc .
   There are two ways to get to this function:  Selecting from the information
@@ -720,4 +665,10 @@ int PROFILING::fill_statistics_info(THD *thd_arg, TABLE_LIST *tables, Item *cond
 
   DBUG_RETURN(0);
 }
+
+
+void PROFILING::reset()
+{
+  enabled= thd->variables.option_bits & OPTION_PROFILING;
+}
 #endif /* ENABLED_PROFILING */

sql/sql_profile.h

@@ -268,32 +268,62 @@ private:
 public:
   PROFILING();
   ~PROFILING();
-  void set_query_source(char *query_source_arg, uint query_length_arg);
 
-  void start_new_query(const char *initial_state= "starting");
+  /**
+    At a point in execution where we know the query source, save the text
+    of it in the query profile.
+
+    This must be called exactly once per descrete statement.
+  */
+  void set_query_source(char *query_source_arg, uint query_length_arg)
+  {
+    if (unlikely(current))
+      current->set_query_source(query_source_arg, query_length_arg);
+  }
+
+  /**
+    Prepare to start processing a new query.  It is an error to do this
+    if there's a query already in process; nesting is not supported.
+
+    @param  initial_state  (optional) name of period before first state change
+  */
+  void start_new_query(const char *initial_state= "starting")
+  {
+    DBUG_ASSERT(!current);
+    if (unlikely(enabled))
+      current= new QUERY_PROFILE(this, initial_state);
+  }
 
   void discard_current_query();
 
-  void finish_current_query();
+  void finish_current_query()
+  {
+    if (unlikely(current))
+      finish_current_query_impl();
+  }
+
+  void finish_current_query_impl();
 
   void status_change(const char *status_arg,
                      const char *function_arg,
                      const char *file_arg, unsigned int line_arg)
   {
     if (unlikely(current))
-    {
-      DBUG_ASSERT(enabled);
       current->new_status(status_arg, function_arg, file_arg, line_arg);
-    }
   }
 
-  inline void set_thd(THD *thd_arg) { thd= thd_arg; };
+  inline void set_thd(THD *thd_arg)
+  {
+    thd= thd_arg;
+    reset();
+  }
 
   /* SHOW PROFILES */
   bool show_profiles();
 
   /* ... from INFORMATION_SCHEMA.PROFILING ... */
   int fill_statistics_info(THD *thd, TABLE_LIST *tables, Item *cond);
+  void reset();
 };
 
 #  endif /* ENABLED_PROFILING */

sql/sys_vars.cc

@@ -3652,10 +3652,18 @@ static Sys_var_bit Sys_unique_checks(
        DEFAULT(TRUE), NO_MUTEX_GUARD, IN_BINLOG);
 
 #ifdef ENABLED_PROFILING
+static bool update_profiling(sys_var *self, THD *thd, enum_var_type type)
+{
+  if (type == OPT_SESSION)
+    thd->profiling.reset();
+  return false;
+}
+
 static Sys_var_bit Sys_profiling(
        "profiling", "profiling",
        NO_SET_STMT SESSION_VAR(option_bits), NO_CMD_LINE, OPTION_PROFILING,
-       DEFAULT(FALSE));
+       DEFAULT(FALSE), NO_MUTEX_GUARD, NOT_IN_BINLOG, ON_CHECK(0),
+       ON_UPDATE(update_profiling));
 
 static Sys_var_ulong Sys_profiling_history_size(
        "profiling_history_size", "Limit of query profiling memory",