Test to validate vacuum doesn't remove tuples that are still needed.

src/test/isolation/expected/heap-serializable-vacuum.out

+Parsed test spec with 2 sessions
+
+starting permutation: s1begin s1delete s2begin s1commit s1vacuum s2select
+step s1begin: begin;
+step s1delete: delete from heaptest;
+step s2begin: BEGIN ISOLATION LEVEL SERIALIZABLE;
+		  select 123 as "establish snapshot";
+establish snapshot
+
+123            
+step s1commit: commit;
+step s1vacuum: vacuum heaptest;
+step s2select: select count(*) from heaptest;
+count          
+
+100            

src/test/isolation/isolation_schedule

 
 # GPDB-specific tests
-
+test: heap-serializable-vacuum
 test: ao-serializable-read
 test: ao-serializable-vacuum
 test: ao-insert-eof

src/test/isolation/specs/heap-serializable-vacuum.spec

+# Test behaviour with distributed (serializable) snapshots and vacuum.
+#
+# The crux of this test is that session 2 begins a serializable
+# transaction, but it doesn't immediately do any actions in that transaction
+# that would open the connections to the QEs. Hence, GetSnapshotData() isn't
+# called in the QEs for session 2 the "s2select" stage. Meanwhile, session 1
+# deletes rows from the table that session 2 later needs to see, and runs
+# vacuum. If there is no mechanism to prevent the global "oldest xmin" from
+# advancing, it will merrily vacuum away tuples that session 2 needs to see
+# later.
+#
+# In a single-node system, the dummy "establish snapshot" query will set
+# MyProc->xmin and prevent the tuples it needs to see later from being
+# vacuumed away. In MPP, the dummy query establishes a distributed snapshot,
+# but that doesn't immediately set the xmin values in QEs.
+setup
+{
+    create table heaptest (i int, j int);
+    insert into heaptest select i, i from generate_series(0, 99) i;
+}
+
+teardown
+{
+    drop table if exists heaptest;
+}
+
+session "s1"
+step "s1begin"  { begin; }
+step "s1delete"	{ delete from heaptest; }
+step "s1commit"  { commit; }
+
+step "s1vacuum"	{ vacuum heaptest; }
+
+session "s2"
+step "s2begin"	{ BEGIN ISOLATION LEVEL SERIALIZABLE;
+		  select 123 as "establish snapshot"; }
+step "s2select"	{ select count(*) from heaptest; }
+teardown	{ abort; }
+
+permutation "s1begin" "s1delete" "s2begin" "s1commit" "s1vacuum" "s2select"