MDEV-32013 Add Field::val_lex_string_strmake()

There are two functions to extract a Field::val_str() value
as a LEX_STRING or LEX_CSTRING pointing to the data allocated on a MEM_ROOT:

  char *get_field(MEM_ROOT *mem, Field *field);
  bool get_field(MEM_ROOT *mem, Field *field, class String *res);

The first function requires strlen() calls to make a LEX_CSTRING/LEX_STRING.
The second function requires a redundant String buffer,
which is used only as a temporary proxy value pointing to a MEM_ROOT fragment
(and does not use any String dynamic allocation methods).

This patch add a native way to extract a Field::val_str() value
as a LEX_STRING or LEX_CSTRING pointing to a MEM_ROOT fragment.
It helps to remove redundant strlen() calls and redundant String buffers.

- Adding a new method:

    LEX_STRING Field::val_lex_string_strmake(MEM_ROOT *mem);

- Reusing the new method Field::val_lex_string_strmake() in;

    bool get_field(MEM_ROOT *mem, Field *field, String *res);

  Also, moving it from table.cc to a static function in sql_help.cc.
  It is used in sql_help.cc only, and we don't want it to be reused
  in other parts of the code (to avoid redundant String buffers).

- Reusing the new method Field::val_lex_string_strmake() in this function:

    char *get_field(MEM_ROOT *mem, Field *field);

- Replacing get_field() to Field::val_lex_string_strmake() in these files:

    sql_plugin.cc  (redundant String buffers were removed)
    sql_udf.cc     (redundant strlen() calls were removed)

Note, this function:

   char *get_field(MEM_ROOT *mem, Field *field);

is still used in a number of files:

   event_data_objects.cc
   event_db_repository.cc
   sql_acl.cc
   sql_servers.cc

These remaining calls will be removed by separate patches,
and get_field() will be removed after that.

sql/field.cc

@@ -11380,6 +11380,15 @@ void Field::register_field_in_read_map()
 }
 
 
+LEX_STRING Field::val_lex_string_strmake(MEM_ROOT *mem)
+{
+  StringBuffer<MAX_FIELD_WIDTH> str;
+  val_str(&str);
+  char *to= strmake_root(mem, str.ptr(), str.length());
+  return to ? LEX_STRING{to, str.length()} : LEX_STRING{NULL, 0};
+}
+
+
 bool Field::val_str_nopad(MEM_ROOT *mem_root, LEX_CSTRING *to)
 {
   StringBuffer<MAX_FIELD_WIDTH> str;

sql/field.h

@@ -1055,6 +1055,20 @@ public:
     return to->copy((const char *) ptr, pack_length());
   }
   String *val_int_as_str(String *val_buffer, bool unsigned_flag);
+
+  /*
+    Copy the Field::val_str() value to MEM_ROOT as a 0x00-teminated string.
+
+    @param    mem_root   The memory root to put the value to.
+    @returns             {NULL,0} in case of EOM, or the field value otherwise.
+
+    Only one 0x00 terminating byte is put in the end, even in case
+    of complex character sets like UCS2/UTF16/UTF32.
+    This is OK, since this method is used to read system tables,
+    which are in utf8.
+  */
+  LEX_STRING val_lex_string_strmake(MEM_ROOT *mem_root);
+
   /*
     Return the field value as a LEX_CSTRING, without padding to full length
     (MODE_PAD_CHAR_TO_FULL_LENGTH is temporarily suppressed during the call).

sql/sql_help.cc

@@ -69,6 +69,29 @@ enum enum_used_fields
 };
 
 
+/*
+  Allocate string field in MEM_ROOT and return it as String
+
+  SYNOPSIS
+    get_field()
+    mem         MEM_ROOT for allocating
+    field       Field for retrieving of string
+    res         result String
+*/
+
+static void get_field(MEM_ROOT *mem, Field *field, String *res)
+{
+  THD *thd= field->get_thd();
+  Sql_mode_instant_remove sms(thd, MODE_PAD_CHAR_TO_FULL_LENGTH);
+  LEX_STRING ls= field->val_lex_string_strmake(mem);
+  DBUG_ASSERT((!ls.str && !ls.length) || ls.str[ls.length] == '\0');
+  if (!ls.str)
+    res->length(0); // EOM
+  else
+    res->set((const char *) ls.str, ls.length, field->charset());
+}
+
+
 /*
   Fill st_find_field structure with pointers to fields
 

sql/sql_plugin.cc

@@ -1883,12 +1883,11 @@ static void plugin_load(MEM_ROOT *tmp_root)
   while (!(error= read_record_info.read_record()))
   {
     DBUG_PRINT("info", ("init plugin record"));
-    String str_name, str_dl;
-    get_field(tmp_root, table->field[0], &str_name);
-    get_field(tmp_root, table->field[1], &str_dl);
-
-    LEX_CSTRING name= {str_name.ptr(), str_name.length()};
-    LEX_CSTRING dl=   {str_dl.ptr(), str_dl.length()};
+    DBUG_ASSERT(new_thd == table->field[0]->get_thd());
+    DBUG_ASSERT(new_thd == table->field[1]->get_thd());
+    DBUG_ASSERT(!(new_thd->variables.sql_mode & MODE_PAD_CHAR_TO_FULL_LENGTH));
+    LEX_CSTRING name= table->field[0]->val_lex_string_strmake(tmp_root);
+    LEX_CSTRING dl= table->field[1]->val_lex_string_strmake(tmp_root);
 
     if (!name.length || !dl.length)
       continue;

sql/sql_udf.cc

@@ -208,10 +208,11 @@ void udf_init()
   while (!(error= read_record_info.read_record()))
   {
     DBUG_PRINT("info",("init udf record"));
-    LEX_CSTRING name;
-    name.str=get_field(&mem, table->field[0]);
-    name.length = (uint) safe_strlen(name.str);
-    char *dl_name= get_field(&mem, table->field[2]);
+    DBUG_ASSERT(!(new_thd->variables.sql_mode & MODE_PAD_CHAR_TO_FULL_LENGTH));
+    DBUG_ASSERT(table->field[0]->get_thd() == new_thd);
+    DBUG_ASSERT(table->field[2]->get_thd() == new_thd);
+    LEX_CSTRING name= table->field[0]->val_lex_string_strmake(&mem);
+    LEX_CSTRING dl_name= table->field[2]->val_lex_string_strmake(&mem);
     bool new_dl=0;
     Item_udftype udftype=UDFTYPE_FUNCTION;
     if (table->s->fields >= 4)			// New func table
@@ -224,7 +225,8 @@ void udf_init()
 
       On windows we must check both FN_LIBCHAR and '/'.
     */
-    if (!name.str || !dl_name || check_valid_path(dl_name, strlen(dl_name)) ||
+    if (!name.length || !dl_name.length ||
+        check_valid_path(dl_name.str, dl_name.length) ||
         check_string_char_length(&name, 0, NAME_CHAR_LEN,
                                  system_charset_info, 1))
     {
@@ -234,7 +236,7 @@ void udf_init()
     }
 
     if (!(tmp= add_udf(&name,(Item_result) table->field[1]->val_int(),
-                       dl_name, udftype)))
+                       dl_name.str, udftype)))
     {
       sql_print_error("Can't alloc memory for udf function: '%.64s'", name.str);
       continue;

sql/table.cc

@@ -5090,40 +5090,6 @@ rename_file_ext(const char * from,const char * to,const char * ext)
 }
 
 
-/*
-  Allocate string field in MEM_ROOT and return it as String
-
-  SYNOPSIS
-    get_field()
-    mem   	MEM_ROOT for allocating
-    field 	Field for retrieving of string
-    res         result String
-
-  RETURN VALUES
-    1   string is empty
-    0	all ok
-*/
-
-bool get_field(MEM_ROOT *mem, Field *field, String *res)
-{
-  const char *to;
-  StringBuffer<MAX_FIELD_WIDTH> str;
-  bool rc;
-  THD *thd= field->get_thd();
-  Sql_mode_instant_remove sms(thd, MODE_PAD_CHAR_TO_FULL_LENGTH);
-
-  field->val_str(&str);
-  if ((rc= !str.length() ||
-           !(to= strmake_root(mem, str.ptr(), str.length()))))
-  {
-    res->length(0);
-    return rc;
-  }
-  res->set(to, str.length(), field->charset());
-  return false;
-}
-
-
 /*
   Allocate string field in MEM_ROOT and return it as NULL-terminated string
 
@@ -5139,10 +5105,12 @@ bool get_field(MEM_ROOT *mem, Field *field, String *res)
 
 char *get_field(MEM_ROOT *mem, Field *field)
 {
-  String str;
-  bool rc= get_field(mem, field, &str);
-  DBUG_ASSERT(rc || str.ptr()[str.length()] == '\0');
-  return  rc ? NullS : (char *) str.ptr();
+  THD *thd= field->get_thd();
+  Sql_mode_instant_remove sms(thd, MODE_PAD_CHAR_TO_FULL_LENGTH);
+  LEX_STRING ls= field->val_lex_string_strmake(mem);
+  DBUG_ASSERT((!ls.str && !ls.length) || ls.str[ls.length] == '\0');
+  // Empty string "" is intentionally returned as NullS
+  return ls.length == 0 ? NullS : ls.str;
 }
 
 /*

sql/table.h

@@ -3404,7 +3404,6 @@ bool check_period_name(const char *name);
 bool check_table_name(const char *name, size_t length, bool check_for_path_chars);
 int rename_file_ext(const char * from,const char * to,const char * ext);
 char *get_field(MEM_ROOT *mem, Field *field);
-bool get_field(MEM_ROOT *mem, Field *field, class String *res);
 
 bool validate_comment_length(THD *thd, LEX_CSTRING *comment, size_t max_len,
                              uint err_code, const char *name);