A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).

client/mysql.cc:
  A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).
  Introduced new get_current_db() function which is called from the com_use()
  and the com_go() if we get SERVER_STATUS_DB_DROPPED.
include/mysql_com.h:
  A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).
  SERVER_STATUS_DB_DROPPED flag added.
    Note: it is set to 256 to don't conflict with 5.0 ver.
sql/sql_db.cc:
  A fix (bug #4802 prompt in mysql client shows wrong database after dropping default db).
  SERVER_STATUS_DB_DROPPED flag is set/unset.

client/mysql.cc

@@ -1574,6 +1574,22 @@ static int reconnect(void)
   return 0;
 }
 
+static void get_current_db()
+{
+  MYSQL_RES *res;
+
+  my_free(current_db, MYF(MY_ALLOW_ZERO_PTR));
+  current_db= NULL;
+  /* In case of error below current_db will be NULL */
+  if (!mysql_query(&mysql, "SELECT DATABASE()") &&
+      (res= mysql_use_result(&mysql)))
+  {
+    MYSQL_ROW row= mysql_fetch_row(res);
+    if (row[0])
+      current_db= my_strdup(row[0], MYF(MY_WME));
+    mysql_free_result(res);
+  }
+}
 
 /***************************************************************************
  The different commands
@@ -1899,6 +1915,9 @@ com_go(String *buffer,char *line __attribute__((unused)))
   if (err >= 1)
     error= put_error(&mysql);
 
+  if (!status.batch && (mysql.server_status & SERVER_STATUS_DB_DROPPED))
+    get_current_db();
+
   return error;				/* New command follows */
 }
 
@@ -2614,24 +2633,7 @@ com_use(String *buffer __attribute__((unused)), char *line)
     under our feet, for example if DROP DATABASE or RENAME DATABASE
     (latter one not yet available by the time the comment was written)
   */
-  /*  Let's reset current_db, assume it's gone */
-  my_free(current_db, MYF(MY_ALLOW_ZERO_PTR));
-  current_db= 0;
-  /*
-    We don't care about in case of an error below because current_db
-    was just set to 0.
-  */
-  if (!mysql_query(&mysql, "SELECT DATABASE()") &&
-      (res= mysql_use_result(&mysql)))
-  {
-    row= mysql_fetch_row(res);
-    if (row[0])
-    {
-      current_db= my_strdup(row[0], MYF(MY_WME));
-    }
-    (void) mysql_fetch_row(res);               // Read eof
-    mysql_free_result(res);
-  }
+  get_current_db();
 
   if (!current_db || cmp_database(charset_info, current_db,tmp))
   {

include/mysql_com.h

@@ -127,6 +127,7 @@ enum enum_server_command
 #define SERVER_MORE_RESULTS_EXISTS 8    /* Multi query - next query exists */
 #define SERVER_QUERY_NO_GOOD_INDEX_USED 16
 #define SERVER_QUERY_NO_INDEX_USED      32
+#define SERVER_STATUS_DB_DROPPED        256 /* A database was dropped */
 
 #define MYSQL_ERRMSG_SIZE	512
 #define NET_READ_TIMEOUT	30		/* Timeout on read */

sql/sql_db.cc

@@ -634,7 +634,9 @@ int mysql_rm_db(THD *thd,char *db,bool if_exists, bool silent)
       thd->clear_error();
       mysql_bin_log.write(&qinfo);
     }
+    thd->server_status|= SERVER_STATUS_DB_DROPPED;
     send_ok(thd, (ulong) deleted);
+    thd->server_status&= !SERVER_STATUS_DB_DROPPED;
   }
 
 exit: