Re: display offset along with block number in vacuum errors

[Justin Pryzby]

On Fri, Jul 24, 2020 at 11:18:43PM +0530, Mahendra Singh Thalor wrote:
> Hi hackers,
> We discussed in another email thread[1], that it will be helpful if we can
> display offset along with block number in vacuum error. Here, proposing a
> patch to add offset along with block number in vacuum errors.

Thanks.  I happened to see both threads, only by chance.

I'd already started writing the same as your 0001, which is essentially the
same as yours.

Here:

@@ -1924,14 +1932,22 @@ lazy_vacuum_page(Relation onerel, BlockNumber blkno, 
Buffer buffer,
                BlockNumber tblk;
                OffsetNumber toff;
                ItemId          itemid;
+               LVSavedErrInfo loc_saved_err_info;
 
                tblk = 
ItemPointerGetBlockNumber(&dead_tuples->itemptrs[tupindex]);
                if (tblk != blkno)
                        break;                          /* past end of tuples 
for this block */
                toff = 
ItemPointerGetOffsetNumber(&dead_tuples->itemptrs[tupindex]);
+
+               /* Update error traceback information */
+               update_vacuum_error_info(vacrelstats, &loc_saved_err_info, 
VACUUM_ERRCB_PHASE_VACUUM_HEAP,
+                                                                blkno, toff);
                itemid = PageGetItemId(page, toff);
                ItemIdSetUnused(itemid);
                unused[uncnt++] = toff;
+
+               /* Revert to the previous phase information for error traceback 
*/
+               restore_vacuum_error_info(vacrelstats, &loc_saved_err_info);
        }

I'm not sure why you use restore_vacuum_error_info() at all.  It's already
called at the end of lazy_vacuum_page() (and others) to allow functions to
clean up after their own state changes, rather than requiring callers to do it.
I don't think you should use it in a loop, nor introduce another
LVSavedErrInfo.

Since phase and blkno are already set, I think you should just set
vacrelstats->offnum = toff, rather than calling update_vacuum_error_info().
Similar to whats done in lazy_vacuum_heap():

                tblk = 
ItemPointerGetBlockNumber(&vacrelstats->dead_tuples->itemptrs[tupindex]);
                vacrelstats->blkno = tblk;

I think you should also do:

@@ -2976,6 +2984,7 @@ heap_page_is_all_visible(Relation rel, Buffer buf,
                ItemId          itemid;
                HeapTupleData tuple;
 
+               vacrelstats->offset = offnum;

I'm not sure, but maybe you'd also want to do the same in more places:

@@ -2024,6 +2030,7 @@ lazy_check_needs_freeze(Buffer buf, bool *hastup)
@@ -2790,6 +2797,7 @@ count_nondeletable_pages(Relation onerel, LVRelStats 
*vacrelstats)

-- 
Justin