Re: Fix uninitialized PruneFreezeResult in pruneheap and vacuumlazy

Hi Chao Li,Hackers,

> PruneFreezeResult presult; <== here presult is not initialized

>   const PruneFreezeResult *presult, <== presult is a const pointer, so prune_freeze_setup won’t update its content

>   PruneState *prstate)

> {

>     prstate->deadoffsets = (OffsetNumber *) presult->deadoffsets; <== presult->deadoffsets could be a random value

> }

> ```

> Attached is a simple fix by just initializing presult in the first place with {0}. 

    Initializing ’presult‘ under my operating system is very effective.

    I have tested the change, and "make check" passed.

Hi Hackers,

While reviewing Melanie's patch [1], I found this bug where presult is not initialized. Let me explain the logic.

In the first place:

```

static int

lazy_scan_prune(LVRelState *vacrel,

Buffer buf,

BlockNumber blkno,

Page page,

Buffer vmbuffer,

bool all_visible_according_to_vm,

bool *has_lpdead_items,

bool *vm_page_frozen)

{

Relation rel = vacrel->rel;

PruneFreezeResult presult; <== here presult is not initialized

heap_page_prune_and_freeze(&params,

  &presult, <== uninitialized presult is passed into heap_page_prune_and_freeze

  &vacrel->offnum,

  &vacrel->NewRelfrozenXid, &vacrel->NewRelminMxid);

```

Then in heap_page_prune_and_freeze():

```

void

heap_page_prune_and_freeze(PruneFreezeParams *params,

  PruneFreezeResult *presult,

  OffsetNumber *off_loc,

  TransactionId *new_relfrozen_xid,

  MultiXactId *new_relmin_mxid)

{

Buffer buffer = params->buffer;

Page page = BufferGetPage(buffer);

PruneState prstate;

bool do_freeze;

bool do_prune;

bool do_hint_prune;

bool did_tuple_hint_fpi;

int64 fpi_before = pgWalUsage.wal_fpi;

/* Initialize prstate */

prune_freeze_setup(params,

  new_relfrozen_xid, new_relmin_mxid,

  presult, &prstate); <== immediately pass uninitialized presult to prune_freeze_setup

```

Then in prune_freeze_setup():

```

static void

prune_freeze_setup(PruneFreezeParams *params,

  TransactionId *new_relfrozen_xid,

  MultiXactId *new_relmin_mxid,

  const PruneFreezeResult *presult, <== presult is a const pointer, so prune_freeze_setup won’t update its content

  PruneState *prstate)

{

    prstate->deadoffsets = (OffsetNumber *) presult->deadoffsets; <== presult->deadoffsets could be a random value

}

```

Attached is a simple fix by just initializing presult in the first place with {0}. 

[1] https://postgr.es/m/CAAKRu_bvQiesumRhgvbcym1T9Z9=ddGfUbi-dSNxLRc6JvL1-w@mail.gmail.com

Best regards,

--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/