valgrind errors around dsa.c

Hi,

newly added tests exercise parallel bitmap scans. And they trigger
valgrind errors:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2017-04-07%2007%3A10%3A01

==4567== VALGRINDERROR-BEGIN
==4567== Conditional jump or move depends on uninitialised value(s)
==4567== at 0x5FD62A: check_for_freed_segments (dsa.c:2219)
==4567== by 0x5FD97E: dsa_get_address (dsa.c:934)
==4567== by 0x5FDA2A: init_span (dsa.c:1339)
==4567== by 0x5FE6D1: ensure_active_superblock (dsa.c:1696)
==4567== by 0x5FEBBD: alloc_object (dsa.c:1452)
==4567== by 0x5FEBBD: dsa_allocate_extended (dsa.c:693)
==4567== by 0x3C7A83: pagetable_allocate (tidbitmap.c:1536)
==4567== by 0x3C7A83: pagetable_create (simplehash.h:342)
==4567== by 0x3C7A83: tbm_create_pagetable (tidbitmap.c:323)
==4567== by 0x3C8DAD: tbm_get_pageentry (tidbitmap.c:1246)
==4567== by 0x3C98A1: tbm_add_tuples (tidbitmap.c:432)
==4567== by 0x22510C: btgetbitmap (nbtree.c:460)
==4567== by 0x21A8D1: index_getbitmap (indexam.c:726)
==4567== by 0x38AD48: MultiExecBitmapIndexScan (nodeBitmapIndexscan.c:91)
==4567== by 0x37D353: MultiExecProcNode (execProcnode.c:621)
==4567== Uninitialised value was created by a heap allocation
==4567== at 0x602FD5: palloc (mcxt.c:872)
==4567== by 0x5FF73B: create_internal (dsa.c:1242)
==4567== by 0x5FF8F5: dsa_create_in_place (dsa.c:473)
==4567== by 0x37CA32: ExecInitParallelPlan (execParallel.c:532)
==4567== by 0x38C324: ExecGather (nodeGather.c:152)
==4567== by 0x37D247: ExecProcNode (execProcnode.c:551)
==4567== by 0x39870F: ExecNestLoop (nodeNestloop.c:156)
==4567== by 0x37D1B7: ExecProcNode (execProcnode.c:512)
==4567== by 0x3849D4: fetch_input_tuple (nodeAgg.c:686)
==4567== by 0x387764: agg_retrieve_direct (nodeAgg.c:2306)
==4567== by 0x387A11: ExecAgg (nodeAgg.c:2117)
==4567== by 0x37D217: ExecProcNode (execProcnode.c:539)
==4567==

It could be that these are spurious due to shared memory - valgrind
doesn't track definedness across processes - but the fact that memory
allocated by palloc is the source of the undefined memory makes me doubt
that.

Greetings,

Andres Freund