Fix incorrect index behavior in COPY FROM with partitioned tables

86b85044e rewrote how COPY FROM works to allow multiple tuple buffers to
exist to once thus allowing multi-inserts to be used in more cases with
partitioned tables.  That commit neglected to update the estate's
es_result_relation_info when flushing the insert buffer to the partition
making it possible for the index tuples to be added into an index on the
wrong partition.

Fix this and also add an Assert in ExecInsertIndexTuples to help ensure
that we never make this mistake again.

Reported-by: Haruka Takatsuka
Author: Ashutosh Sharma
Discussion: https://postgr.es/m/15832-b1bf336a4ee246b5@postgresql.org

src/backend/commands/copy.c

@@ -2446,6 +2446,9 @@ CopyMultiInsertBufferFlush(CopyMultiInsertInfo *miinfo,
 	ResultRelInfo *resultRelInfo = buffer->resultRelInfo;
 	TupleTableSlot **slots = buffer->slots;
 
+	/* Set es_result_relation_info to the ResultRelInfo we're flushing. */
+	estate->es_result_relation_info = resultRelInfo;
+
 	/*
 	 * Print error context information correctly, if one of the operations
 	 * below fail.

src/backend/executor/execIndexing.c

@@ -299,6 +299,9 @@ ExecInsertIndexTuples(TupleTableSlot *slot,
 	indexInfoArray = resultRelInfo->ri_IndexRelationInfo;
 	heapRelation = resultRelInfo->ri_RelationDesc;
 
+	/* Sanity check: slot must belong to the same rel as the resultRelInfo. */
+	Assert(slot->tts_tableOid == RelationGetRelid(heapRelation));
+
 	/*
 	 * We will use the EState's per-tuple context for evaluating predicates
 	 * and index expressions (creating it if it's not already there).

src/test/regress/input/copy.source

@@ -187,4 +187,17 @@ copy parted_copytest from '@abs_builddir@/results/parted_copytest.csv';
 select tableoid::regclass,count(*),sum(a) from parted_copytest
 group by tableoid order by tableoid::regclass::name;
 
+truncate table parted_copytest;
+create index on parted_copytest (b);
+drop trigger part_ins_trig on parted_copytest_a2;
+
+copy parted_copytest from stdin;
+1	1	str1
+2	2	str2
+\.
+
+-- Ensure index entries were properly added during the copy.
+select * from parted_copytest where b = 1;
+select * from parted_copytest where b = 2;
+
 drop table parted_copytest;

src/test/regress/output/copy.source

@@ -147,4 +147,21 @@ group by tableoid order by tableoid::regclass::name;
  parted_copytest_a2 |    10 |  10055
 (2 rows)
 
+truncate table parted_copytest;
+create index on parted_copytest (b);
+drop trigger part_ins_trig on parted_copytest_a2;
+copy parted_copytest from stdin;
+-- Ensure index entries were properly added during the copy.
+select * from parted_copytest where b = 1;
+ a | b |  c   
+---+---+------
+ 1 | 1 | str1
+(1 row)
+
+select * from parted_copytest where b = 2;
+ a | b |  c   
+---+---+------
+ 2 | 2 | str2
+(1 row)
+
 drop table parted_copytest;