Repair Large Object bugs demonstrated by Ian Grant's example. inv_write

was inappropriately relying on rel->rd_nblocks to tell if the LO is
empty (apparently a hack to get around a long-dead index bug), causing
misbehavior on a written-but-never-vacuumed LO.  Also, inv_read failed
to cope gracefully with 'holes' (unwritten regions) in the object.

src/backend/storage/large_object/inv_api.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/storage/large_object/inv_api.c,v 1.69 2000/06/05 07:28:45 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/storage/large_object/inv_api.c,v 1.70 2000/06/15 06:07:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -187,6 +187,7 @@ inv_create(int flags)
 	retval->idesc = RelationGetDescr(indr);
 	retval->offset = retval->lowbyte = retval->highbyte = 0;
 	ItemPointerSetInvalid(&(retval->htid));
+	retval->flags = 0;
 
 	if (flags & INV_WRITE)
 	{
@@ -198,7 +199,7 @@ inv_create(int flags)
 		LockRelation(r, ShareLock);
 		retval->flags = IFS_RDLOCK;
 	}
-	retval->flags |= IFS_ATEOF;
+	retval->flags |= IFS_ATEOF;	/* since we know the object is empty */
 
 	return retval;
 }
@@ -235,6 +236,7 @@ inv_open(Oid lobjId, int flags)
 	retval->idesc = RelationGetDescr(indrel);
 	retval->offset = retval->lowbyte = retval->highbyte = 0;
 	ItemPointerSetInvalid(&(retval->htid));
+	retval->flags = 0;
 
 	if (flags & INV_WRITE)
 	{
@@ -373,14 +375,8 @@ inv_seek(LargeObjectDesc *obj_desc, int offset, int whence)
 	if (whence == SEEK_CUR)
 	{
 		offset += obj_desc->offset;		/* calculate absolute position */
-		return inv_seek(obj_desc, offset, SEEK_SET);
 	}
-
-	/*
-	 * if you seek past the end (offset > 0) I have no clue what happens
-	 * :-(				  B.L.	 9/1/93
-	 */
-	if (whence == SEEK_END)
+	else if (whence == SEEK_END)
 	{
 		/* need read lock for getsize */
 		if (!(obj_desc->flags & IFS_RDLOCK))
@@ -391,8 +387,8 @@ inv_seek(LargeObjectDesc *obj_desc, int offset, int whence)
 		offset += _inv_getsize(obj_desc->heap_r,
 							   obj_desc->hdesc,
 							   obj_desc->index_r);
-		return inv_seek(obj_desc, offset, SEEK_SET);
 	}
+	/* now we can assume that the operation is SEEK_SET */
 
 	/*
 	 * Whenever we do a seek, we turn off the EOF flag bit to force
@@ -416,9 +412,6 @@ inv_seek(LargeObjectDesc *obj_desc, int offset, int whence)
 	 * stores the offset of the last byte that appears on it, and we have
 	 * an index on this.
 	 */
-
-
-	/* right now, just assume that the operation is SEEK_SET */
 	if (obj_desc->iscan != (IndexScanDesc) NULL)
 	{
 		d = Int32GetDatum(offset);
@@ -426,7 +419,6 @@ inv_seek(LargeObjectDesc *obj_desc, int offset, int whence)
 	}
 	else
 	{
-
 		ScanKeyEntryInitialize(&skey, 0x0, 1, F_INT4GE,
 							   Int32GetDatum(offset));
 
@@ -489,9 +481,27 @@ inv_read(LargeObjectDesc *obj_desc, char *buf, int nbytes)
 
 		/* copy the data from this block into the buffer */
 		d = heap_getattr(&tuple, 2, obj_desc->hdesc, &isNull);
+		fsblock = (struct varlena *) DatumGetPointer(d);
 		ReleaseBuffer(buffer);
 
-		fsblock = (struct varlena *) DatumGetPointer(d);
+		/*
+		 * If block starts beyond current seek point, then we are looking
+		 * at a "hole" (unwritten area) in the object.  Return zeroes for
+		 * the "hole".
+		 */
+		if (obj_desc->offset < obj_desc->lowbyte)
+		{
+			int		nzeroes = obj_desc->lowbyte - obj_desc->offset;
+
+			if (nzeroes > (nbytes - nread))
+				nzeroes = (nbytes - nread);
+			MemSet(buf, 0, nzeroes);
+			buf += nzeroes;
+			nread += nzeroes;
+			obj_desc->offset += nzeroes;
+			if (nread >= nbytes)
+				break;
+		}
 
 		off = obj_desc->offset - obj_desc->lowbyte;
 		ncopy = obj_desc->highbyte - obj_desc->offset + 1;
@@ -537,14 +547,11 @@ inv_write(LargeObjectDesc *obj_desc, char *buf, int nbytes)
 		Buffer		buffer;
 
 		/*
-		 * Fetch the current inversion file system block.  If the class
-		 * storing the inversion file is empty, we don't want to do an
-		 * index lookup, since index lookups choke on empty files (should
-		 * be fixed someday).
+		 * Fetch the current inversion file system block.  We can skip
+		 * the work if we already know we are at EOF.
 		 */
 
-		if ((obj_desc->flags & IFS_ATEOF)
-			|| obj_desc->heap_r->rd_nblocks == 0)
+		if (obj_desc->flags & IFS_ATEOF)
 			tuple.t_data = NULL;
 		else
 			inv_fetchtup(obj_desc, &tuple, &buffer);
@@ -659,6 +666,7 @@ inv_fetchtup(LargeObjectDesc *obj_desc, HeapTuple tuple, Buffer *buffer)
 		}
 		else
 			index_rescan(obj_desc->iscan, false, &skey);
+
 		do
 		{
 			res = index_getnext(obj_desc->iscan, ForwardScanDirection);
@@ -1149,7 +1157,8 @@ inv_indextup(LargeObjectDesc *obj_desc, HeapTuple tuple)
 		pfree(res);
 }
 
-/*
+#ifdef NOT_USED
+
 static void
 DumpPage(Page page, int blkno)
 {
@@ -1239,7 +1248,8 @@ ItemPointerFormExternal(ItemPointer pointer)
 
 		return itemPointerString;
 }
-*/
+
+#endif
 
 static int
 _inv_getsize(Relation hreln, TupleDesc hdesc, Relation ireln)