ovsdb-idl: Make it possible to omit or pay less attention to columns.

ovs-vswitchd has no need to replicate some parts of the database.  In
particular, it doesn't need to replicate the bits that it never reads,
such as the external_ids column in the Open_vSwitch table.  This saves
some memory, CPU time, and bandwidth to the database.

Another type of column that benefits from special treatment is "write-only
columns", that is, those that ovs-vswitchd writes and keeps up-to-date but
never expects another client to write, such as the cur_cfg column in the
Open_vSwitch table.  If the IDL reports that the database has changed when
ovs-vswitchd updates such a column, then ovs-vswitchd reconfigures itself
for no reason, wasting CPU time.  This commit also adds support for such
columns.

1 files changed, 23 insertions, 0 deletions

diff --git a/lib/ovsdb-idl-provider.h b/lib/ovsdb-idl-provider.h
index c86396c9..040a6999 100644
--- a/lib/ovsdb-idl-provider.h
+++ b/lib/ovsdb-idl-provider.h
@@ -52,8 +52,31 @@ struct ovsdb_idl_table_class {
     size_t allocation_size;
 };
 
+enum ovsdb_idl_mode {
+    /* Client reads and may write this column and wants to be alerted upon
+     * updates to it.
+     *
+     * This is the default. */
+    OVSDB_IDL_MODE_RW,
+
+    /* Client may read and write this column, but doesn't care to be alerted
+     * when it is updated.
+     *
+     * This is useful for columns that a client treats as "write-only", that
+     * is, it updates them but doesn't want to get alerted about its own
+     * updates.  It also won't be alerted about other clients' updates, so this
+     * is suitable only for use by a client that "owns" a particular column. */
+    OVSDB_IDL_MODE_WO,
+
+    /* Client won't read or write this column at all.  The IDL code can't
+     * prevent reading the column, but writing will cause assertion
+     * failures. */
+    OVSDB_IDL_MODE_NONE
+};
+
 struct ovsdb_idl_table {
     const struct ovsdb_idl_table_class *class;
+    unsigned char *modes;    /* One of OVSDB_MODE_*, indexed by column. */
     struct shash columns;    /* Contains "const struct ovsdb_idl_column *"s. */
     struct hmap rows;        /* Contains "struct ovsdb_idl_row"s. */
     struct ovsdb_idl *idl;   /* Containing idl. */