Bug#12368495 CRASH AND/OR VALGRIND ERRORS WITH REVERSE FUNCTION AND CHARSET CONVERTS

Item_func_trim::val_str: we were using the non-mb algorithm for skipping leading spaces
in a multibyte-charset string.

sql/item_strfunc.cc

@@ -1005,6 +1005,7 @@ String *Item_func_reverse::val_str(String *str)
       if ((l= my_ismbchar(res->charset(),ptr,end)))
       {
         tmp-= l;
+        DBUG_ASSERT(tmp >= tmp_value.ptr());
         memcpy(tmp,ptr,l);
         ptr+= l;
       }
@@ -1751,18 +1752,35 @@ String *Item_func_trim::val_str(String *str)
   ptr= (char*) res->ptr();
   end= ptr+res->length();
   r_ptr= remove_str->ptr();
-  while (ptr+remove_length <= end && !memcmp(ptr,r_ptr,remove_length))
-    ptr+=remove_length;
 #ifdef USE_MB
   if (use_mb(res->charset()))
   {
+    while (ptr + remove_length <= end)
+    {
+      uint num_bytes= 0;
+      while (num_bytes < remove_length)
+      {
+        uint len;
+        if ((len= my_ismbchar(res->charset(), ptr + num_bytes, end)))
+          num_bytes+= len;
+        else
+          ++num_bytes;
+      }
+      if (num_bytes != remove_length)
+        break;
+      if (memcmp(ptr, r_ptr, remove_length))
+        break;
+      ptr+= remove_length;
+    }
     char *p=ptr;
     register uint32 l;
  loop:
     while (ptr + remove_length < end)
     {
-      if ((l=my_ismbchar(res->charset(), ptr,end))) ptr+=l;
-      else ++ptr;
+      if ((l= my_ismbchar(res->charset(), ptr,end)))
+        ptr+= l;
+      else
+        ++ptr;
     }
     if (ptr + remove_length == end && !memcmp(ptr,r_ptr,remove_length))
     {
@@ -1775,6 +1793,8 @@ String *Item_func_trim::val_str(String *str)
   else
 #endif /* USE_MB */
   {
+    while (ptr+remove_length <= end && !memcmp(ptr,r_ptr,remove_length))
+      ptr+=remove_length;
     while (ptr + remove_length <= end &&
 	   !memcmp(end-remove_length,r_ptr,remove_length))
       end-=remove_length;