Re: Extensions versus pg_upgrade

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Feb 8, 2011 at 10:25 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> My point is that the current restriction to just one containing
>> extension seems to me to be an implementation restriction, rather than
>> something inherent in the concept of extensions. I have no intention of
>> trying to relax that restriction in the near future --- I'm just
>> pointing out that it could become an interesting thing to do.

> OK. My point was that I think we should definitely *enforce* that
> restriction until we have a very clear vision of what it means to do
> anything else, so it sounds like we're basically in agreement.

Oh, for certain. I've been busy revising Dimitri's patch to use the
get_object_address infrastructure, and here's what I've got for the
actual implementation as distinct from syntax:

+ /*
+ * Execute ALTER THING SET EXTENSION
+ */
+ void
+ ExecAlterObjectExtensionStmt(AlterObjectExtensionStmt *stmt)
+ {
+ ObjectAddress object;
+ ObjectAddress extension;
+ Relation relation;
+
+ /*
+ * For now, insist on superuser privilege. Later we might want to
+ * relax this to ownership of the target object and the extension.
+ */
+ if (!superuser())
+ ereport(ERROR,
+ (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+ (errmsg("must be superuser to use ALTER SET EXTENSION"))));
+
+ /*
+ * Translate the parser representation that identifies this object into
+ * an ObjectAddress. get_object_address() will throw an error if the
+ * object does not exist, and will also acquire a lock on the target
+ * to guard against concurrent DROP and SET EXTENSION operations.
+ */
+ object = get_object_address(stmt->objtype, stmt->objname, stmt->objargs,
+ &relation, ShareUpdateExclusiveLock);
+
+ /*
+ * Complain if object is already attached to some extension.
+ */
+ if (getExtensionOfObject(object.classId, object.objectId) != InvalidOid)
+ ereport(ERROR,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("%s is already a member of an extension",
+ getObjectDescription(&object))));
+
+ /*
+ * OK, add the dependency.
+ */
+ extension.classId = ExtensionRelationId;
+ extension.objectId = get_extension_oid(stmt->extname, false);
+ extension.objectSubId = 0;
+
+ recordDependencyOn(&object, &extension, DEPENDENCY_EXTENSION);
+
+ /*
+ * If get_object_address() opened the relation for us, we close it to keep
+ * the reference count correct - but we retain any locks acquired by
+ * get_object_address() until commit time, to guard against concurrent
+ * activity.
+ */
+ if (relation != NULL)
+ relation_close(relation, NoLock);
+ }

regards, tom lane