Before this patch, the regular_column_transformation constructor, which
we used in Alternator GSIs to generates a view key from a regular-column
cell, accepted a cell of any size. As a reviewer (Avi) noticed, very
long cells are possible, well beyond what Scylla allows for keys (64KB),
and because regular_column_transformation stores such values in a
contiguous "bytes" object it can cause stalls.
But allowing oversized attributes creates an even more accute problem:
While view building (backfilling in DynamoDB jargon), if we encounter
an oversized (>64KB) key, the view building step will fail and the
entire view building will hang forever.
This patch fixes both problems by adding to regular_column_transformation's
constructor the check that if the cell is 64KB or larger, an empty value
is returned for the key. This causes the backfilling to silently skip
this item, which is what we expect to happen (backfilling cannot do
anything to fix or reject the pre-existing items in the best table).
A test test_gsi_updatetable.py::test_gsi_backfill_oversized_key is
introduced to reproduce this problem and its fix. The test adds a 65KB
attribute to a base table, and then adds GSIs to this table with this
attribute as its partition key or its sort key. Before this patch, the
backfilling process for the new GSIs hangs, and never completes.
After this patch, the backfilling completes and as expected contains
other base-table items but not the item with the oversized attribute.
The new test also passes on DynamoDB.
However, while implementing this fix I realized that issue #10347 also
exists for GSIs. Issue #10347 is about the fact that DynamoDB limits
partition key and sort key attributes to 2048 and 1024 bytes,
respectively. In the fix described above we only handled the accute case
of lengths above 64 KB, but we should actually skip items whose GSI
keys are over 2048 or 1024 bytes - not 64KB. This extra checking is
not handled in this patch, and is part of a wider existing issue:
Refs #10347
Signed-off-by: Nadav Har'El <nyh@scylladb.com>
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