Heap lock levels for REINDEX INDEX CONCURRENTLY not quite right?

Hi,

While looking at https://www.postgresql.org/message-id/20190430070552.jzqgcy4ihalx7nur%40alap3.anarazel.de
I noticed that

/*
* ReindexIndex
* Recreate a specific index.
*/
void
ReindexIndex(RangeVar *indexRelation, int options, bool concurrent)
{
Oid indOid;
Oid heapOid = InvalidOid;
Relation irel;
char persistence;

/*
* Find and lock index, and check permissions on table; use callback to
* obtain lock on table first, to avoid deadlock hazard. The lock level
* used here must match the index lock obtained in reindex_index().
*/
indOid = RangeVarGetRelidExtended(indexRelation,
concurrent ? ShareUpdateExclusiveLock : AccessExclusiveLock,
0,
RangeVarCallbackForReindexIndex,
(void *) &heapOid);

doesn't pass concurrent-ness to RangeVarCallbackForReindexIndex(). Which
then goes on to lock the table

static void
RangeVarCallbackForReindexIndex(const RangeVar *relation,
Oid relId, Oid oldRelId, void *arg)

if (OidIsValid(*heapOid))
LockRelationOid(*heapOid, ShareLock);

without knowing that it should use ShareUpdateExclusive. Which
e.g. ReindexTable knows:

/* The lock level used here should match reindex_relation(). */
heapOid = RangeVarGetRelidExtended(relation,
concurrent ? ShareUpdateExclusiveLock : ShareLock,
0,
RangeVarCallbackOwnsTable, NULL);

so there's a lock upgrade hazard.

Creating a table
CREATE TABLE blarg(id serial primary key);
and then using pgbench to reindex it:
REINDEX INDEX CONCURRENTLY blarg_pkey;

indeed proves that there's a problem:

2019-04-30 08:12:58.679 PDT [30844][7/925] ERROR: 40P01: deadlock detected
2019-04-30 08:12:58.679 PDT [30844][7/925] DETAIL: Process 30844 waits for ShareUpdateExclusiveLock on relation 50661 of database 13408; blocked by process 30848.
Process 30848 waits for ShareUpdateExclusiveLock on relation 50667 of database 13408; blocked by process 30844.
Process 30844: REINDEX INDEX CONCURRENTLY blarg_pkey;
Process 30848: REINDEX INDEX CONCURRENTLY blarg_pkey;
2019-04-30 08:12:58.679 PDT [30844][7/925] HINT: See server log for query details.
2019-04-30 08:12:58.679 PDT [30844][7/925] LOCATION: DeadLockReport, deadlock.c:1140
2019-04-30 08:12:58.679 PDT [30844][7/925] STATEMENT: REINDEX INDEX CONCURRENTLY blarg_pkey;

I assume the fix woudl be to pass a struct {LOCKMODE lockmode; Oid
heapOid;} to RangeVarCallbackForReindexIndex().

Greetings,

Andres Freund