Re: [HACKERS] Backend problem with large objects

> >Many thanks indeed for this. Unfortunately it doesn't completely work:  it
> >fixes the problem as reported, but when, instead of writing five
> >characters, one at a time, I write five at once, the backend dies in
> >the same place it did before. Here's the C code slightly modified to
> >reproduce the problem:
> 
> Give me some time. I'm not sure if I could solve new problem, though.
> --
> Tatsuo Ishii

I think I have fixed the problem you mentioned. Ian, could you apply
included patches and test again? Note that those are for 6.4.2 and
additions to the previous patches.

BTW, lobj strangely added a 0 filled disk block at the head of the
heap. As a result, even 1-byte-user-data lobj consumes at least 16384
bytes (2 disk blocks)! Included patches also fix this problem.

To Bruce:
Thanks for taking care of my previous patches for current.  If
included patch is ok, I will make one for current.

---------------------------- cut here ---------------------------------
*** postgresql-6.4.2/src/backend/storage/large_object/inv_api.c.orig    Sun Dec 13 14:08:19 1998
--- postgresql-6.4.2/src/backend/storage/large_object/inv_api.c    Fri Feb 12 20:21:05 1999
***************
*** 624,648 ****         || obj_desc->offset < obj_desc->lowbyte         || !ItemPointerIsValid(&(obj_desc->htid)))
{         /* initialize scan key if not done */         if (obj_desc->iscan == (IndexScanDesc) NULL)         {
 
-             ScanKeyData skey;
-              /*              * As scan index may be prematurely closed (on commit), we              * must use object
currentoffset (was 0) to reinitialize the              * entry [ PA ].              */
 
-             ScanKeyEntryInitialize(&skey, 0x0, 1, F_INT4GE,
-                                    Int32GetDatum(obj_desc->offset));             obj_desc->iscan =
index_beginscan(obj_desc->index_r,                                (bool) 0, (uint16) 1,
&skey);        }
 
-          do         {             res = index_getnext(obj_desc->iscan, ForwardScanDirection);
--- 630,655 ----         || obj_desc->offset < obj_desc->lowbyte         || !ItemPointerIsValid(&(obj_desc->htid)))
{
+         ScanKeyData skey;
+ 
+         ScanKeyEntryInitialize(&skey, 0x0, 1, F_INT4GE,
+                        Int32GetDatum(obj_desc->offset));          /* initialize scan key if not done */         if
(obj_desc->iscan== (IndexScanDesc) NULL)         {             /*              * As scan index may be prematurely
closed(on commit), we              * must use object current offset (was 0) to reinitialize the              * entry [
PA].              */             obj_desc->iscan =                 index_beginscan(obj_desc->index_r,
             (bool) 0, (uint16) 1,                                 &skey);
 
+         } else {
+             index_rescan(obj_desc->iscan, false, &skey);         }         do         {             res =
index_getnext(obj_desc->iscan,ForwardScanDirection);
 
***************
*** 666,672 ****             tuple = heap_fetch(obj_desc->heap_r, SnapshotNow,
&res->heap_iptr,buffer);             pfree(res);
 
!         } while (tuple == (HeapTuple) NULL);          /* remember this tid -- we may need it for later reads/writes
*/         ItemPointerCopy(&tuple->t_ctid, &obj_desc->htid);
 
--- 673,679 ----             tuple = heap_fetch(obj_desc->heap_r, SnapshotNow,
&res->heap_iptr,buffer);             pfree(res);
 
!         } while (!HeapTupleIsValid(tuple));          /* remember this tid -- we may need it for later reads/writes */
        ItemPointerCopy(&tuple->t_ctid, &obj_desc->htid);
 
***************
*** 675,680 ****
--- 682,691 ----     {         tuple = heap_fetch(obj_desc->heap_r, SnapshotNow,
&(obj_desc->htid),buffer);
 
+         if (!HeapTupleIsValid(tuple)) {
+           elog(ERROR,
+                "inv_fetchtup: heap_fetch failed");
+         }     }      /*
***************
*** 746,757 ****      nblocks = RelationGetNumberOfBlocks(hr); 
!     if (nblocks > 0)         buffer = ReadBuffer(hr, nblocks - 1);
!     else         buffer = ReadBuffer(hr, P_NEW);
! 
!     page = BufferGetPage(buffer);      /*      * If the last page is too small to hold all the data, and it's too
--- 757,771 ----      nblocks = RelationGetNumberOfBlocks(hr); 
!     if (nblocks > 0) {         buffer = ReadBuffer(hr, nblocks - 1);
!         page = BufferGetPage(buffer);
!     }
!     else {         buffer = ReadBuffer(hr, P_NEW);
!         page = BufferGetPage(buffer);
!         PageInit(page, BufferGetPageSize(buffer), 0);
!     }      /*      * If the last page is too small to hold all the data, and it's too
***************
*** 865,876 ****          nblocks = RelationGetNumberOfBlocks(hr); 
!         if (nblocks > 0)             newbuf = ReadBuffer(hr, nblocks - 1);
!         else             newbuf = ReadBuffer(hr, P_NEW); 
-         newpage = BufferGetPage(newbuf);         freespc = IFREESPC(newpage);          /*
--- 879,894 ----          nblocks = RelationGetNumberOfBlocks(hr); 
!         if (nblocks > 0) {             newbuf = ReadBuffer(hr, nblocks - 1);
!             newpage = BufferGetPage(newbuf);
!         }
!         else {             newbuf = ReadBuffer(hr, P_NEW);
+             newpage = BufferGetPage(newbuf);
+             PageInit(newpage, BufferGetPageSize(newbuf), 0);
+         }          freespc = IFREESPC(newpage);          /*
***************
*** 973,978 ****
--- 991,999 ----     WriteBuffer(buffer);     if (newbuf != buffer)         WriteBuffer(newbuf);
+ 
+     /* Tuple id is no longer valid */
+     ItemPointerSetInvalid(&(obj_desc->htid));      /* done */     return nwritten;