Implement semi-consistent read to reduce lock conflicts at the cost

of breaking serializability.  (Bug #3300)

ha_innobase::unlock_row(): reset the "did semi consistent read" flag

ha_innobase::was_semi_consistent_read(),
ha_innobase::try_semi_consistent_read(): new methods

row_prebuilt_t, row_create_prebuilt(): add field row_read_type for
keeping track of semi-consistent reads

row_vers_build_for_semi_consistent_read(),
row_sel_build_committed_vers_for_mysql(): new functions

row_search_for_mysql(): implement semi-consistent reads

include/row0mysql.h

@@ -612,6 +612,31 @@ struct row_prebuilt_struct {
 					that was decided in ha_innodb.cc,
 					::store_lock(), ::external_lock(),
 					etc. */
+	ulint		row_read_type;	/* ROW_READ_WITH_LOCKS if row locks
+					should be the obtained for records
+					under an UPDATE or DELETE cursor.
+					If innodb_locks_unsafe_for_binlog
+					is TRUE, this can be set to
+					ROW_READ_TRY_SEMI_CONSISTENT, so that
+					if the row under an UPDATE or DELETE
+					cursor was locked by another
+					transaction, InnoDB will resort
+					to reading the last committed value
+					('semi-consistent read').  Then,
+					this field will be set to
+					ROW_READ_DID_SEMI_CONSISTENT to
+					indicate that.  If the row does not
+					match the WHERE condition, MySQL will
+					invoke handler::unlock_row() to
+					clear the flag back to
+					ROW_READ_TRY_SEMI_CONSISTENT and
+					to simply skip the row.  If
+					the row matches, the next call to
+					row_search_for_mysql() will lock
+					the row.
+					This eliminates lock waits in some
+					cases; note that this breaks
+					serializability. */
 	ulint		mysql_prefix_len;/* byte offset of the end of
 					the last requested column */
 	ulint		mysql_row_len;	/* length in bytes of a row in the
@@ -657,6 +682,10 @@ struct row_prebuilt_struct {
 #define ROW_RETRIEVE_PRIMARY_KEY	1
 #define ROW_RETRIEVE_ALL_COLS		2
 
+/* Values for row_read_type */
+#define ROW_READ_WITH_LOCKS		0
+#define ROW_READ_TRY_SEMI_CONSISTENT	1
+#define ROW_READ_DID_SEMI_CONSISTENT	2
 
 #ifndef UNIV_NONINL
 #include "row0mysql.ic"