Making tables read-only

Author: arssher

pg_shardman--0.0.1.sql

@@ -403,6 +403,38 @@ BEGIN
 END
 $$ LANGUAGE plpgsql;
 
+-- Make table read-only
+CREATE FUNCTION readonly_table_on(relation regclass)
+	RETURNS void AS $$
+BEGIN
+	-- Grab exclusive lock on table to finish existing transactions
+	PERFORM shardman.ae_lock_table(relation);
+	-- Create go away trigger to prevent any new ones
+	PERFORM shardman.readonly_table_off(relation);
+	EXECUTE format(
+		'CREATE TRIGGER shardman_readonly BEFORE INSERT OR UPDATE OR DELETE OR
+		TRUNCATE ON %I FOR EACH STATEMENT EXECUTE PROCEDURE shardman.go_away();',
+		relation);
+	EXECUTE format(
+		'ALTER TABLE %I ENABLE ALWAYS TRIGGER shardman_readonly;', relation);
+END
+$$ LANGUAGE plpgsql STRICT;
+CREATE FUNCTION go_away() RETURNS TRIGGER AS $$
+BEGIN
+	RAISE EXCEPTION 'The "%" table is read only.', TG_TABLE_NAME
+		USING HINT = 'Probably table copy is in progress';
+  RETURN NULL;
+END;
+$$ LANGUAGE plpgsql;
+CREATE FUNCTION ae_lock_table(relation regclass) RETURNS void
+	AS 'pg_shardman' LANGUAGE C;
+-- And make it writable again
+CREATE FUNCTION readonly_table_off(relation regclass)
+	RETURNS void AS $$
+BEGIN
+	EXECUTE format('DROP TRIGGER IF EXISTS shardman_readonly ON %s', relation);
+END $$ LANGUAGE plpgsql STRICT;
+
 CREATE FUNCTION gen_create_table_sql(relation text, connstring text) RETURNS text
     AS 'pg_shardman' LANGUAGE C;
 

src/shard.c

@@ -554,8 +554,8 @@ epoll_subscribe(int epfd, MoveMPartState *mmps)
 }
 
 /*
- * Actually run MoveMPart state machine. Return value says when (if ever)
- * we want to be executed again.
+ * Actually run MoveMPart state machine. On return, mmps values say when (if
+ * ever) we want to be executed again.
  */
 void
 exec_move_mpart(MoveMPartState *mmps)

src/udf.c

@@ -9,6 +9,7 @@
 #include "funcapi.h"
 #include "access/htup_details.h"
 #include "catalog/pg_type.h"
+#include "storage/lmgr.h"
 #include "libpq-fe.h"
 
 /*
@@ -268,3 +269,15 @@ pq_conninfo_parse(PG_FUNCTION_ARGS)
 	PQconninfoFree(opts);
 	PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
 }
+
+/*
+ * Obtain AccessExclusiveLock on table
+ */
+PG_FUNCTION_INFO_V1(ae_lock_table);
+Datum
+ae_lock_table(PG_FUNCTION_ARGS)
+{
+	Oid relid = PG_GETARG_OID(0);
+	LockRelationOid(relid, AccessExclusiveLock);
+	PG_RETURN_VOID();
+}