MDEV-36507 fix dbug_print_row concurrent access

7544fd4 had to make use of a static array to avoid memory
use-after-free or leak.

Instead, let us make a function returning String, this is the only way
to automatically manage the memory after the function returned.

To make it all correct, move constructor is added. Normally, it is
expected, that the constructor will be elided upon return of an object
by value, but if something goes different, or -fno-elide-constructors is
used, we can have a problem. So this was a move constructor avoids
copy elision-related UB.

dbug_print_row returning char* is still there for convenient use in a
debugger.

Author: FooBarrior

sql/filesort.cc

@@ -637,19 +637,8 @@ static uchar *read_buffpek_from_file(IO_CACHE *buffpek_pointers, uint count,
 
 static char dbug_row_print_buf[4096];
 
-/*
-  Print table's current row into a buffer and return a pointer to it.
-
-  This is intended to be used from gdb:
-  
-    (gdb) p dbug_print_table_row(table)
-      $33 = "SUBQUERY2_t1(col_int_key,col_varchar_nokey)=(7,c)"
-    (gdb)
 
-  Only columns in table->read_set are printed
-*/
-
-const char* dbug_print_row(TABLE *table, const uchar *rec, bool print_names)
+String dbug_format_row(TABLE *table, const uchar *rec, bool print_names)
 {
   Field **pfield;
   char row_buff_tmp[512];
@@ -717,14 +706,37 @@ const char* dbug_print_row(TABLE *table, const uchar *rec, bool print_names)
       output.append(tmp.ptr(), tmp.length());
     }
   }
-  output.append(")");
-  if (output.c_ptr() == dbug_row_print_buf)
-    return dbug_row_print_buf;
-  else
+  output.append(')');
+
+  return output;
+}
+
+/**
+  A function to display a row in debugger.
+
+  Example usage:
+  (gdb) p dbug_print_row(table, table->record[1])
+*/
+const char *dbug_print_row(TABLE *table, const uchar *rec)
+{
+  String row= dbug_format_row(table, table->record[0]);
+  if (row.length() > sizeof dbug_row_print_buf - 1)
     return "Couldn't fit into buffer";
+  memcpy(dbug_row_print_buf, row.c_ptr(), row.length());
+  return dbug_row_print_buf;
 }
 
+/**
+  Print table's current row into a buffer and return a pointer to it.
+
+  This is intended to be used from gdb:
 
+    (gdb) p dbug_print_table_row(table)
+      $33 = "SUBQUERY2_t1(col_int_key,col_varchar_nokey)=(7,c)"
+    (gdb)
+
+  Only columns in table->read_set are printed
+*/
 const char* dbug_print_table_row(TABLE *table)
 {
   return dbug_print_row(table, table->record[0]);

sql/handler.cc

@@ -7366,7 +7366,8 @@ int handler::ha_write_row(const uchar *buf)
 
   TABLE_IO_WAIT(tracker, PSI_TABLE_WRITE_ROW, MAX_KEY, error,
                       { error= write_row(buf); })
-  DBUG_PRINT("dml", ("INSERT: %s = %d", dbug_print_row(table, buf, false), error));
+  DBUG_PRINT("dml", ("INSERT: %s = %d",
+                     dbug_format_row(table, buf, false).c_ptr_safe(), error));
 
   MYSQL_INSERT_ROW_DONE(error);
   if (likely(!error))
@@ -7425,8 +7426,10 @@ int handler::ha_update_row(const uchar *old_data, const uchar *new_data)
 
   TABLE_IO_WAIT(tracker, PSI_TABLE_UPDATE_ROW, active_index, 0,
                       { error= update_row(old_data, new_data);})
-  DBUG_PRINT("dml", ("UPDATE: %s => %s = %d", dbug_print_row(table, old_data, false),
-                     dbug_print_row(table, new_data, false), error));
+  DBUG_PRINT("dml", ("UPDATE: %s => %s = %d",
+                     dbug_format_row(table, old_data, false).c_ptr_safe(),
+                     dbug_format_row(table, new_data, false).c_ptr_safe(),
+                     error));
 
   MYSQL_UPDATE_ROW_DONE(error);
   if (likely(!error))
@@ -7506,7 +7509,8 @@ int handler::ha_delete_row(const uchar *buf)
 
   TABLE_IO_WAIT(tracker, PSI_TABLE_DELETE_ROW, active_index, error,
     { error= delete_row(buf);})
-  DBUG_PRINT("dml", ("DELETE: %s = %d", dbug_print_row(table, buf, false), error));
+  DBUG_PRINT("dml", ("DELETE: %s = %d",
+                     dbug_format_row(table, buf, false).c_ptr_safe(), error));
   MYSQL_DELETE_ROW_DONE(error);
   if (likely(!error))
   {

sql/handler.h

@@ -5324,6 +5324,6 @@ int get_select_field_pos(Alter_info *alter_info, int select_field_count,
                          bool versioned);
 
 #ifndef DBUG_OFF
-const char* dbug_print_row(TABLE *table, const uchar *rec, bool print_names= true);
+String dbug_format_row(TABLE *table, const uchar *rec, bool print_names= true);
 #endif /* DBUG_OFF */
 #endif /* HANDLER_INCLUDED */

sql/sql_string.h

@@ -512,6 +512,10 @@ class Binary_string: public Static_binary_string
     extra_alloc= 0;
     alloced= thread_specific= 0;
   }
+  Binary_string(Binary_string &&str) noexcept
+  {
+    move(str);
+  }
 
   ~Binary_string() { free(); }
 
@@ -856,6 +860,8 @@ class String: public Charset, public Binary_string
     Binary_string(str, len)
   { }
   String(const String &str) = default;
+  String(String &&str) noexcept
+   :Charset(std::move(str)), Binary_string(std::move(str)){}
 
   void set(String &str,size_t offset,size_t arg_length)
   {