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
ofdatabase 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