RE: Adding REPACK [concurrently]

[Hayato Kuroda (Fujitsu)]

Dear Álvaro,

Thanks for updating the patch. I have two comments/questions.

01.
```
--- a/src/backend/access/index/genam.c
+++ b/src/backend/access/index/genam.c
@@ -394,6 +394,14 @@ systable_beginscan(Relation heapRelation,
        SysScanDesc sysscan;
        Relation        irel;
 
+       /*
+        * If this backend promised that it won't access shared catalogs during
+        * logical decoding, this it the right place to verify.
+        */
+       Assert(!HistoricSnapshotActive() ||
+                  accessSharedCatalogsInDecoding ||
+                  !heapRelation->rd_rel->relisshared);
```

Not sure it's OK to use Assert(). elog(ERROR) might be better if we want to 
really
avoid the case.

02. SnapBuildProcessRunningXacts

Per my understanding, the db_specic snapshot can be also serialized. Is it
possibility tha normal logical decoding system restores the snapshot and obtain
the wrong result?

Best regards,
Hayato Kuroda
FUJITSU LIMITED