Frequent HLL bitstream_unpack crashes

We’ve been seeing nearly daily crashes from a PostgreSQL 9.6 application that is heavily
dependent on the HLL extension (v 2.10.2). All these crashes are from inside the HLL
bitstream_unpack function. Usually they’re from an INSERT VALUES statement, but
occasionally they are from an hll_cardinality call in a query. The format of the HLL cells
is always set using expressions like: hll_empty(17,5,-1,1) || hll_hash_bigint(879826435) || ...

I think I’ve identified the root cause, but I’d like someone who is familiar with the code
in the HLL library to confirm my hypothesis:

In bitstream_unpack it pulls a full quadword of data out of the bitstream using the
brc_curp pointer. Usually this is not a problem. However, if the brc_curp pointer is
less than 8 bytes from the end of the bitstream data, then that quadword read is
reading past the end of the actual bitstream data. Because of the subsequent bit
reordering, shifting, and masking this has no effect of the answers. However, when
the end of the bitstream is very close to the end of an OS page then the quadword
read will attempt to read the next OS page, and if that next OS page does not exist
in this process, then it will SEGV.

Assuming I’ve correctly identified the problem, the attached patch should fix the issue.

Regards,
Steve Kirk

[hll_2_10_2_patch.txt](https://github.com/citusdata/postgresql-hll/files/3816240/hll_2_10_2_patch.txt)

BTW, The crash stacks all look roughly like:

#0 bitstream_unpack (brcp=brcp@entry=0x7ffc69814290) at hll.c:263
#1 0x0000147cbfcbe8ac in sparse_unpack (i_size=, i_bitp=0x7ffc69814327 "", i_nfilled=10403, i_log2nregs=, i_width=5, i_regp=0x7ffc69814320 "\002") at hll.c:359
#2 multiset_unpack (o_msp=o_msp@entry=0x7ffc698142f0, i_bitp=i_bitp@entry=0x147a16bf903c "\023\221\177", i_size=, o_encoded_type=o_encoded_type@entry=0x0) at hll.c:1188
#3 0x0000147cbfcc0823 in hll_union (fcinfo=) at hll.c:2042
#4 0x0000000000617132 in ExecMakeFunctionResultNoSets (fcache=0x147cbb3eb948, econtext=0x147a1d6cd530, isNull=0x147a1d6ce09d "", isDone=) at execQual.c:2041
#5 0x000000000061cace in ExecTargetList (tupdesc=, isDone=0x0, itemIsDone=0x147a1d6cffa0, isnull=0x147a1d6ce090 "", values=0x147a1d6ce000, econtext=0x147a1d6cd530, targetlist=0x147cbb3ecc28) at execQual.c:5423
#6 ExecProject (projInfo=, isDone=isDone@entry=0x0) at execQual.c:5647
#7 0x000000000062f10b in ExecOnConflictUpdate (returning=, canSetTag=1 '\001', estate=0x147a1d6c8038, excludedSlot=0x147a1d6c9bc0, planSlot=0x147a1d6c9bc0, conflictTid=0x7ffc69854520, resultRelInfo=0x147a1d6c81c8, mtstate=0x147a1d6c82d8) at nodeModifyTable.c:1234
#8 ExecInsert (canSetTag=1 '\001', estate=0x147a1d6c8038, onconflict=ONCONFLICT_UPDATE, arbiterIndexes=0x147a1654fd28, planSlot=0x147a1d6c9bc0, slot=0x147a1d6c9bc0, mtstate=0x147a1d6c82d8) at nodeModifyTable.c:410
#9 ExecModifyTable (node=node@entry=0x147a1d6c82d8) at nodeModifyTable.c:1512
#10 0x00000000006162a8 in ExecProcNode (node=node@entry=0x147a1d6c82d8) at execProcnode.c:396
#11 0x0000000000612727 in ExecutePlan (dest=0x147a1654d888, direction=, numberTuples=0, sendTuples=, operation=CMD_INSERT, use_parallel_mode=, planstate=0x147a1d6c82d8, estate=0x147a1d6c8038) at execMain.c:1567
#12 standard_ExecutorRun (queryDesc=0x147cbb3c6438, direction=, count=0) at execMain.c:339