Improve database detection logic in datachecksumsworker

The worker need to know whether a database which failed checksum
processing still exists, or has been dropped.  This improves the
detection logic by checking for being partially dropped.

Author: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Reviewed-by: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Reviewed-by: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Discussion: https://postgr.es/m/9197F930-DDEB-4CAC-82A2-16FEC715CCE8@yesql.se

src/backend/postmaster/datachecksum_state.c

@@ -848,8 +848,7 @@ ProcessDatabase(DataChecksumsWorkerDatabase *db)
 
 		/*
 		 * Heuristic to see if the database was dropped, and if it was we can
-		 * treat it as not an error, else treat as fatal and error out. TODO:
-		 * this could probably be improved with a tighter check.
+		 * treat it as not an error, else treat as fatal and error out.
 		 */
 		if (DatabaseExists(db->dboid))
 			return DATACHECKSUMSWORKER_FAILED;
@@ -1314,7 +1313,9 @@ DataChecksumsShmemRequest(void *arg)
  * DatabaseExists
  *
  * Scans the system catalog to check if a database with the given Oid exists
- * and returns true if it is found, else false.
+ * and returns true if it is found and valid, else false. Note, we cannot use
+ * database_is_invalid_oid here as it will ERROR out, and we want to gracefully
+ * handle errors.
  */
 static bool
 DatabaseExists(Oid dboid)
@@ -1324,6 +1325,7 @@ DatabaseExists(Oid dboid)
 	SysScanDesc scan;
 	bool		found;
 	HeapTuple	tuple;
+	Form_pg_database pg_database_tuple;
 
 	StartTransactionCommand();
 
@@ -1337,6 +1339,14 @@ DatabaseExists(Oid dboid)
 	tuple = systable_getnext(scan);
 	found = HeapTupleIsValid(tuple);
 
+	/* If the Oid exists, ensure that it's not partially dropped */
+	if (found)
+	{
+		pg_database_tuple = (Form_pg_database) GETSTRUCT(tuple);
+		if (database_is_invalid_form(pg_database_tuple))
+			found = false;
+	}
+
 	systable_endscan(scan);
 	table_close(rel, AccessShareLock);