Don't assume that a tuple's header size is unchanged during toasting.

This assumption can be wrong when the toaster is passed a raw on-disk
tuple, because the tuple might pre-date an ALTER TABLE ADD COLUMN operation
that added columns without rewriting the table.  In such a case the tuple's
natts value is smaller than what we expect from the tuple descriptor, and
so its t_hoff value could be smaller too.  In fact, the tuple might not
have a null bitmap at all, and yet our current opinion of it is that it
contains some trailing nulls.

In such a situation, toast_insert_or_update did the wrong thing, because
to save a few lines of code it would use the old t_hoff value as the offset
where heap_fill_tuple should start filling data.  This did not leave enough
room for the new nulls bitmap, with the result that the first few bytes of
data could be overwritten with null flag bits, as in a recent report from
Hubert Depesz Lubaczewski.

The particular case reported requires ALTER TABLE ADD COLUMN followed by
CREATE TABLE AS SELECT * FROM ... or INSERT ... SELECT * FROM ..., and
further requires that there be some out-of-line toasted fields in one of
the tuples to be copied; else we'll not reach the troublesome code.
The problem can only manifest in this form in 8.4 and later, because
before commit a77eaa6a, CREATE TABLE AS or
INSERT/SELECT wouldn't result in raw disk tuples getting passed directly
to heap_insert --- there would always have been at least a junkfilter in
between, and that would reconstitute the tuple header with an up-to-date
t_natts and hence t_hoff.  But I'm backpatching the tuptoaster change all
the way anyway, because I'm not convinced there are no older code paths
that present a similar risk.

src/backend/access/heap/tuptoaster.c

@@ -597,7 +597,6 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
 	if (newtup->t_data->t_infomask & HEAP_HASOID)
 		hoff += sizeof(Oid);
 	hoff = MAXALIGN(hoff);
-	Assert(hoff == newtup->t_data->t_hoff);
 	/* now convert to a limit on the tuple data size */
 	maxDataLen = TOAST_TUPLE_TARGET - hoff;
 
@@ -864,43 +863,54 @@ toast_insert_or_update(Relation rel, HeapTuple newtup, HeapTuple oldtup,
 	{
 		HeapTupleHeader olddata = newtup->t_data;
 		HeapTupleHeader new_data;
-		int32		new_len;
+		int32		new_header_len;
 		int32		new_data_len;
+		int32		new_tuple_len;
 
 		/*
-		 * Calculate the new size of the tuple.  Header size should not
-		 * change, but data size might.
+		 * Calculate the new size of the tuple.
+		 *
+		 * Note: we used to assume here that the old tuple's t_hoff must equal
+		 * the new_header_len value, but that was incorrect.  The old tuple
+		 * might have a smaller-than-current natts, if there's been an ALTER
+		 * TABLE ADD COLUMN since it was stored; and that would lead to a
+		 * different conclusion about the size of the null bitmap, or even
+		 * whether there needs to be one at all.
 		 */
-		new_len = offsetof(HeapTupleHeaderData, t_bits);
+		new_header_len = offsetof(HeapTupleHeaderData, t_bits);
 		if (has_nulls)
-			new_len += BITMAPLEN(numAttrs);
+			new_header_len += BITMAPLEN(numAttrs);
 		if (olddata->t_infomask & HEAP_HASOID)
-			new_len += sizeof(Oid);
-		new_len = MAXALIGN(new_len);
-		Assert(new_len == olddata->t_hoff);
+			new_header_len += sizeof(Oid);
+		new_header_len = MAXALIGN(new_header_len);
 		new_data_len = heap_compute_data_size(tupleDesc,
 											  toast_values, toast_isnull);
-		new_len += new_data_len;
+		new_tuple_len = new_header_len + new_data_len;
 
 		/*
 		 * Allocate and zero the space needed, and fill HeapTupleData fields.
 		 */
-		result_tuple = (HeapTuple) palloc0(HEAPTUPLESIZE + new_len);
-		result_tuple->t_len = new_len;
+		result_tuple = (HeapTuple) palloc0(HEAPTUPLESIZE + new_tuple_len);
+		result_tuple->t_len = new_tuple_len;
 		result_tuple->t_self = newtup->t_self;
 		result_tuple->t_tableOid = newtup->t_tableOid;
 		new_data = (HeapTupleHeader) ((char *) result_tuple + HEAPTUPLESIZE);
 		result_tuple->t_data = new_data;
 
 		/*
-		 * Put the existing tuple header and the changed values into place
+		 * Copy the existing tuple header, but adjust natts and t_hoff.
 		 */
-		memcpy(new_data, olddata, olddata->t_hoff);
+		memcpy(new_data, olddata, offsetof(HeapTupleHeaderData, t_bits));
+		HeapTupleHeaderSetNatts(new_data, numAttrs);
+		new_data->t_hoff = new_header_len;
+		if (olddata->t_infomask & HEAP_HASOID)
+			HeapTupleHeaderSetOid(new_data, HeapTupleHeaderGetOid(olddata));
 
+		/* Copy over the data, and fill the null bitmap if needed */
 		heap_fill_tuple(tupleDesc,
 						toast_values,
 						toast_isnull,
-						(char *) new_data + olddata->t_hoff,
+						(char *) new_data + new_header_len,
 						new_data_len,
 						&(new_data->t_infomask),
 						has_nulls ? new_data->t_bits : NULL);
@@ -1028,8 +1038,9 @@ toast_flatten_tuple_attribute(Datum value,
 	TupleDesc	tupleDesc;
 	HeapTupleHeader olddata;
 	HeapTupleHeader new_data;
-	int32		new_len;
+	int32		new_header_len;
 	int32		new_data_len;
+	int32		new_tuple_len;
 	HeapTupleData tmptup;
 	Form_pg_attribute *att;
 	int			numAttrs;
@@ -1100,33 +1111,39 @@ toast_flatten_tuple_attribute(Datum value,
 	}
 
 	/*
-	 * Calculate the new size of the tuple.  Header size should not change,
-	 * but data size might.
+	 * Calculate the new size of the tuple.
+	 *
+	 * This should match the reconstruction code in toast_insert_or_update.
 	 */
-	new_len = offsetof(HeapTupleHeaderData, t_bits);
+	new_header_len = offsetof(HeapTupleHeaderData, t_bits);
 	if (has_nulls)
-		new_len += BITMAPLEN(numAttrs);
+		new_header_len += BITMAPLEN(numAttrs);
 	if (olddata->t_infomask & HEAP_HASOID)
-		new_len += sizeof(Oid);
-	new_len = MAXALIGN(new_len);
-	Assert(new_len == olddata->t_hoff);
+		new_header_len += sizeof(Oid);
+	new_header_len = MAXALIGN(new_header_len);
 	new_data_len = heap_compute_data_size(tupleDesc,
 										  toast_values, toast_isnull);
-	new_len += new_data_len;
+	new_tuple_len = new_header_len + new_data_len;
 
-	new_data = (HeapTupleHeader) palloc0(new_len);
+	new_data = (HeapTupleHeader) palloc0(new_tuple_len);
 
 	/*
-	 * Put the tuple header and the changed values into place
+	 * Copy the existing tuple header, but adjust natts and t_hoff.
 	 */
-	memcpy(new_data, olddata, olddata->t_hoff);
+	memcpy(new_data, olddata, offsetof(HeapTupleHeaderData, t_bits));
+	HeapTupleHeaderSetNatts(new_data, numAttrs);
+	new_data->t_hoff = new_header_len;
+	if (olddata->t_infomask & HEAP_HASOID)
+		HeapTupleHeaderSetOid(new_data, HeapTupleHeaderGetOid(olddata));
 
-	HeapTupleHeaderSetDatumLength(new_data, new_len);
+	/* Reset the datum length field, too */
+	HeapTupleHeaderSetDatumLength(new_data, new_tuple_len);
 
+	/* Copy over the data, and fill the null bitmap if needed */
 	heap_fill_tuple(tupleDesc,
 					toast_values,
 					toast_isnull,
-					(char *) new_data + olddata->t_hoff,
+					(char *) new_data + new_header_len,
 					new_data_len,
 					&(new_data->t_infomask),
 					has_nulls ? new_data->t_bits : NULL);