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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 a77eaa6a95009a3441e0d475d1980259d45da072, 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.
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