MDEV-31991 Split class Database_qualified_name

- Moving some of Database_qualidied_name methods into a new class
  Identifier_chain2.

- Changing the data type of the following variables from
  Database_qualified_name to Identifier_chain2:

  * q_pkg_proc in LEX::call_statement_start()
  * q_pkg_func in LEX::make_item_func_call_generic()

Rationale:

The data type of Database_qualified_name::m_db will be changed
to Lex_ident_db soon. So Database_qualified_name won't be able
to store the `pkg.routine` part of `db.pkg.routine` any more,
because `pkg` must not depend on lower-case-table-names.

sql/item.cc

@@ -2786,11 +2786,11 @@ Item_sp::func_name_cstring(THD *thd, bool is_package_function) const
       quoted `pkg` and `func` separately, so the entire result looks like:
          `db`.`pkg`.`func`
     */
-    Database_qualified_name tmp= Database_qualified_name::split(m_name->m_name);
-    DBUG_ASSERT(tmp.m_db.length);
-    append_identifier(thd, &qname, &tmp.m_db);
+    Identifier_chain2 tmp= Identifier_chain2::split(m_name->m_name);
+    DBUG_ASSERT(tmp[0].length);
+    append_identifier(thd, &qname, &tmp[0]);
     qname.append('.');
-    append_identifier(thd, &qname, &tmp.m_name);
+    append_identifier(thd, &qname, &tmp[1]);
   }
   else
     append_identifier(thd, &qname, &m_name->m_name);

sql/sql_class.h

@@ -7773,6 +7773,61 @@ class Use_relaxed_field_copy: public Sql_mode_save,
 };
 
 
+class Identifier_chain2
+{
+  LEX_CSTRING m_name[2];
+public:
+  Identifier_chain2()
+   :m_name{Lex_cstring(), Lex_cstring()}
+  { }
+  Identifier_chain2(const LEX_CSTRING &a, const LEX_CSTRING &b)
+   :m_name{a, b}
+  { }
+
+  const LEX_CSTRING& operator [] (size_t i) const
+  {
+    return m_name[i];
+  }
+
+  static Identifier_chain2 split(const LEX_CSTRING &txt)
+  {
+    DBUG_ASSERT(txt.str[txt.length] == '\0'); // Expect 0-terminated input
+    const char *dot= strchr(txt.str, '.');
+    if (!dot)
+      return Identifier_chain2(Lex_cstring(), txt);
+    size_t length0= dot - txt.str;
+    Lex_cstring name0(txt.str, length0);
+    Lex_cstring name1(txt.str + length0 + 1, txt.length - length0 - 1);
+    return Identifier_chain2(name0, name1);
+  }
+
+  // Export as a qualified name string: 'db.name'
+  size_t make_qname(char *dst, size_t dstlen) const
+  {
+    return my_snprintf(dst, dstlen, "%.*s.%.*s",
+                       (int) m_name[0].length, m_name[0].str,
+                       (int) m_name[1].length, m_name[1].str);
+  }
+
+  // Export as a qualified name string, allocate on mem_root.
+  bool make_qname(MEM_ROOT *mem_root, LEX_CSTRING *dst) const
+  {
+    const uint dot= !!m_name[0].length;
+    char *tmp;
+    /* format: [pkg + dot] + name + '\0' */
+    dst->length= m_name[0].length + dot + m_name[1].length;
+    if (unlikely(!(dst->str= tmp= (char*) alloc_root(mem_root,
+                                                     dst->length + 1))))
+      return true;
+    snprintf(tmp, dst->length + 1, "%.*s%.*s%.*s",
+            (int) m_name[0].length, (m_name[0].length ? m_name[0].str : ""),
+            dot, ".",
+            (int) m_name[1].length, m_name[1].str);
+    return false;
+  }
+};
+
+
 /**
   This class resembles the SQL Standard schema qualified object name:
   <schema qualified name> ::= [ <schema name> <period> ] <qualified identifier>
@@ -7813,41 +7868,15 @@ class Database_qualified_name
   void copy(MEM_ROOT *mem_root, const LEX_CSTRING &db,
                                 const LEX_CSTRING &name);
 
-  static Database_qualified_name split(const LEX_CSTRING &txt)
-  {
-    DBUG_ASSERT(txt.str[txt.length] == '\0'); // Expect 0-terminated input
-    const char *dot= strchr(txt.str, '.');
-    if (!dot)
-      return Database_qualified_name(NULL, 0, txt.str, txt.length);
-    size_t dblen= dot - txt.str;
-    Lex_cstring db(txt.str, dblen);
-    Lex_cstring name(txt.str + dblen + 1, txt.length - dblen - 1);
-    return Database_qualified_name(db, name);
-  }
-
   // Export db and name as a qualified name string: 'db.name'
   size_t make_qname(char *dst, size_t dstlen) const
   {
-    return my_snprintf(dst, dstlen, "%.*s.%.*s",
-                       (int) m_db.length, m_db.str,
-                       (int) m_name.length, m_name.str);
+    return Identifier_chain2(m_db, m_name).make_qname(dst, dstlen);
   }
   // Export db and name as a qualified name string, allocate on mem_root.
   bool make_qname(MEM_ROOT *mem_root, LEX_CSTRING *dst) const
   {
-    const uint dot= !!m_db.length;
-    char *tmp;
-    /* format: [database + dot] + name + '\0' */
-    dst->length= m_db.length + dot + m_name.length;
-    if (unlikely(!(dst->str= tmp= (char*) alloc_root(mem_root,
-                                                     dst->length + 1))))
-      return true;
-    snprintf(tmp, dst->length + 1, "%.*s%.*s%.*s",
-            (int) m_db.length, (m_db.length ? m_db.str : ""),
-            dot, ".",
-            (int) m_name.length, m_name.str);
-    DBUG_SLOW_ASSERT(Lex_ident_fs(m_db).ok_for_lower_case_names());
-    return false;
+    return Identifier_chain2(m_db, m_name).make_qname(mem_root, dst);
   }
 
   bool make_package_routine_name(MEM_ROOT *mem_root,
@@ -7858,9 +7887,7 @@ class Database_qualified_name
     size_t length= package.length + 1 + routine.length + 1;
     if (unlikely(!(tmp= (char *) alloc_root(mem_root, length))))
       return true;
-    m_name.length= my_snprintf(tmp, length, "%.*s.%.*s",
-                               (int) package.length, package.str,
-                               (int) routine.length, routine.str);
+    m_name.length= Identifier_chain2(package, routine).make_qname(tmp, length);
     m_name.str= tmp;
     return false;
   }

sql/sql_lex.cc

@@ -9310,7 +9310,7 @@ bool LEX::call_statement_start(THD *thd,
                                const Lex_ident_sys_st *proc)
 {
   Database_qualified_name q_db_pkg(db, pkg);
-  Database_qualified_name q_pkg_proc(pkg, proc);
+  Identifier_chain2 q_pkg_proc(*pkg, *proc);
   sp_name *spname;
 
   sql_command= SQLCOM_CALL;
@@ -9603,7 +9603,7 @@ Item *LEX::make_item_func_call_generic(THD *thd,
   static Lex_cstring dot(".", 1);
   Lex_ident_sys db(thd, cdb), pkg(thd, cpkg), func(thd, cfunc);
   Database_qualified_name q_db_pkg(db, pkg);
-  Database_qualified_name q_pkg_func(pkg, func);
+  Identifier_chain2 q_pkg_func(pkg, func);
   sp_name *qname;
 
   if (db.is_null() || pkg.is_null() || func.is_null())