Bug#57799 READ UNCOMMITTED access failure of off-page DYNAMIC or COMPRESSED

columns again

This is follow-up to Bug #54358. Not all occurrences of the bug were fixed.
We need to check all calls to btr_copy_externally_stored_field_prefix_low()
and do the right thing when the pointer to the off-page column is null
(full of zero bytes).

It turns out that only the call to btr_copy_externally_stored_field_prefix()
in row_sel_sec_rec_is_for_blob() needs to be changed.

For fetching complete off-page columns rather than prefixes, the function
btr_rec_copy_externally_stored_field() already checks if the pointer
is null (all-zero).  Two of its callers (row_merge_copy_blobs() and
row_sel_fetch_columns()) are never executed as READ COMMITTED and can
rightfully assert that the fetch succeeded.  The third caller,
row_sel_store_mysql_rec(), already does the right thing.

The calls in row_upd_ext_fetch() and trx_undo_page_fetch_ext() must
expect that the off-page column exists. Update and rollback are
locking operations, never READ UNCOMMITTED.

storage/innodb_plugin/ChangeLog

+2010-11-02	The InnoDB Team
+
+	* row/row0sel.c:
+	Fix Bug#57799 READ UNCOMMITTED access failure of off-page
+	DYNAMIC or COMPRESSED columns again
+
 2010-10-24	The InnoDB Team
 
 	* row/row0mysql.c

storage/innodb_plugin/row/row0sel.c

@@ -106,6 +106,18 @@ row_sel_sec_rec_is_for_blob(
 	ulint	len;
 	byte	buf[DICT_MAX_INDEX_COL_LEN];
 
+	ut_a(clust_len >= BTR_EXTERN_FIELD_REF_SIZE);
+
+	if (UNIV_UNLIKELY
+	    (!memcmp(clust_field + clust_len - BTR_EXTERN_FIELD_REF_SIZE,
+		     field_ref_zero, BTR_EXTERN_FIELD_REF_SIZE))) {
+		/* The externally stored field was not written yet.
+		This record should only be seen by
+		recv_recovery_rollback_active() or any
+		TRX_ISO_READ_UNCOMMITTED transactions. */
+		return(FALSE);
+	}
+
 	len = btr_copy_externally_stored_field_prefix(buf, sizeof buf,
 						      zip_size,
 						      clust_field, clust_len);