Move thread re-initialisation out of cache_thread

It mostly duplicates code of do_handle_one_connection(). Move it there,
so that it is easier to simplify it in future.

Moved PSI_CALL_delete_current_thread()/_db_pop_() out of mutex. Call them
consistently in all cases, unlike before:
- called for reused threads
- called for flushed threads
- called for threads that go down after 5 minutes timeout
- not called for threads attempting to cache themselves during flush
- not called for threads attempting to cache themselves when cache is full

Part of MDEV-19515 - Improve connect speed

sql/mysqld.cc

@@ -2663,31 +2663,30 @@ void unlink_thd(THD *thd)
 */
 
 
-bool cache_thread(THD *thd)
+CONNECT *cache_thread(THD *thd)
 {
   struct timespec abstime;
   DBUG_ENTER("cache_thread");
   DBUG_ASSERT(thd);
   set_timespec(abstime, THREAD_CACHE_TIMEOUT);
 
+  /*
+    Delete the instrumentation for the job that just completed,
+    before parking this pthread in the cache (blocked on COND_thread_cache).
+  */
+  PSI_CALL_delete_current_thread();
+
+#ifndef DBUG_OFF
+  while (_db_is_pushed_())
+    _db_pop_();
+#endif
+
   mysql_mutex_lock(&LOCK_thread_cache);
   if (cached_thread_count < thread_cache_size && !kill_cached_threads)
   {
     /* Don't kill the thread, just put it in cache for reuse */
     DBUG_PRINT("info", ("Adding thread to cache"));
     cached_thread_count++;
-
-    /*
-      Delete the instrumentation for the job that just completed,
-      before parking this pthread in the cache (blocked on COND_thread_cache).
-    */
-    PSI_CALL_delete_current_thread();
-
-#ifndef DBUG_OFF
-    while (_db_is_pushed_())
-      _db_pop_();
-#endif
-
     while (!wake_thread)
     {
       int error= mysql_cond_timedwait(&COND_thread_cache, &LOCK_thread_cache,
@@ -2712,35 +2711,7 @@ bool cache_thread(THD *thd)
     {
       wake_thread--;
       mysql_mutex_unlock(&LOCK_thread_cache);
-
-      if (!(connect->create_thd(thd)))
-      {
-        /* Out of resources. Free thread to get more resources */
-        connect->close_and_delete();
-        DBUG_RETURN(0);
-      }
-      delete connect;
-
-      /*
-        We have to call store_globals to update mysys_var->id and lock_info
-        with the new thread_id
-      */
-      thd->store_globals();
-
-      /*
-        Create new instrumentation for the new THD job,
-        and attach it to this running pthread.
-      */
-      PSI_CALL_set_thread(PSI_CALL_new_thread(key_thread_one_connection,
-                                              thd, thd->thread_id));
-
-      /* reset abort flag for the thread */
-      thd->mysys_var->abort= 0;
-      thd->thr_create_utime= microsecond_interval_timer();
-      thd->start_utime= thd->thr_create_utime;
-
-      server_threads.insert(thd);
-      DBUG_RETURN(1);
+      DBUG_RETURN(connect);
     }
   }
   mysql_mutex_unlock(&LOCK_thread_cache);

sql/mysqld.h

@@ -79,7 +79,7 @@ void close_connection(THD *thd, uint sql_errno= 0);
 void handle_connection_in_main_thread(CONNECT *thd);
 void create_thread_to_handle_connection(CONNECT *connect);
 void unlink_thd(THD *thd);
-bool cache_thread(THD *thd);
+CONNECT *cache_thread(THD *thd);
 void flush_thread_cache();
 void refresh_status(THD *thd);
 bool is_secure_file_path(char *path);

sql/sql_connect.cc

@@ -1426,8 +1426,36 @@ void do_handle_one_connection(CONNECT *connect, bool put_in_cache)
 
     unlink_thd(thd);
     if (IF_WSREP(thd->wsrep_applier, false) || !put_in_cache ||
-        !cache_thread(thd))
+        !(connect= cache_thread(thd)))
       break;
+
+    if (!(connect->create_thd(thd)))
+    {
+      /* Out of resources. Free thread to get more resources */
+      connect->close_and_delete();
+      break;
+    }
+    delete connect;
+
+    /*
+      We have to call store_globals to update mysys_var->id and lock_info
+      with the new thread_id
+    */
+    thd->store_globals();
+
+    /*
+      Create new instrumentation for the new THD job,
+      and attach it to this running pthread.
+    */
+    PSI_CALL_set_thread(PSI_CALL_new_thread(key_thread_one_connection,
+                                            thd, thd->thread_id));
+
+    /* reset abort flag for the thread */
+    thd->mysys_var->abort= 0;
+    thd->thr_create_utime= microsecond_interval_timer();
+    thd->start_utime= thd->thr_create_utime;
+
+    server_threads.insert(thd);
   }
   delete thd;
 }