Fix for BUG#7716: in in_string::set() take into account that the value returned

by item->val_str() may be a substring of the passed string. 
Disallow string=its_substring assignment in String::operator=().


mysql-test/r/func_misc.result:
  Testcase for BUG#7716
mysql-test/t/func_misc.test:
  Testcase for BUG#7716
sql/item_cmpfunc.cc:
  Fix for BUG#7716: in in_string::set() take into account that the string returned 
  by item->val_str(S) may be not S but use the buffer owned by S.
sql/sql_string.h:
  * Added assert: String& String::operator=(const String&) may not be used to do 
    assignments like str = string_that_uses_buffer_owned_by_str
  * Added String::uses_buffer_owned_by().

1 files changed, 10 insertions, 0 deletions

diff --git a/sql/sql_string.h b/sql/sql_string.h
index a8fb9574c0b..9136dddbbf2 100644
--- a/sql/sql_string.h
+++ b/sql/sql_string.h
@@ -182,6 +182,11 @@ public:
   {
     if (&s != this)
     {
+      /*
+        It is forbidden to do assignments like 
+        some_string = substring_of_that_string
+       */
+      DBUG_ASSERT(!s.uses_buffer_owned_by(this));
       free();
       Ptr=s.Ptr ; str_length=s.str_length ; Alloced_length=s.Alloced_length;
       alloced=0;
@@ -313,4 +318,9 @@ public:
 
   /* Swap two string objects. Efficient way to exchange data without memcpy. */
   void swap(String &s);
+
+  inline bool uses_buffer_owned_by(const String *s) const
+  {
+    return (s->alloced && Ptr >= s->Ptr && Ptr < s->Ptr + s->str_length);
+  }
 };