When describing a table, we need to do it carefully: if some
columns were dropped, we must specify that explicitly by
```
ALTER TABLE {table} DROP {column} USING TIMESTAMP ...
```
in the result of the DESCRIBE statement. Failing to do so
could lead to data resurrection.
However, if a table has been altered many, many times,
we might end up with a huge create statement. Constructing
it could, in turn, trigger an oversized allocation.
Some tests ran into that very problem in fact.
In this commit, we want to mitigate the problem: instead of
allocating a contiguous chunk of memory for the create
statement, we use `fragmented_ostringstream` and `managed_string`
to possibly keep data scattered in memory. It makes handling
`cql3::description` less convenient in the code, but since
the struct is pretty much immediately serialized after
creating it, it's a very good trade-off.
We provide a reproducer. It consistently passes with this commit,
while having about 50% chance of failure before it (based on my
own experiments). Playing with the parameters of the test
doesn't seem to improve that chance, so let's keep it as-is.
Fixesscylladb/scylladb#24018
This is a prerequiste for a following commit. We want
to move towards using non-contiguous memory chunks
to avoid making large allocations.
This commit does NOT change the behavior of Scylla
at all. The rows corresponding to the result of a DESCRIBE
statement are represented by an instance of `result_set`.
Before these changes, we encoded descriptions using `bytes`
and then passed them into a `result_set` using its method
`add_row`. What it does is turn the instances of `bytes`
into instances of `managed_bytes` and append them at the
end of its internal vector. In these changes, we do it
on our own and use another overload of the method.
the log.hh under the root of the tree was created keep the backward
compatibility when seastar was extracted into a separate library.
so log.hh should belong to `utils` directory, as it is based solely
on seastar, and can be used all subsystems.
in this change, we move log.hh into utils/log.hh to that it is more
modularized. and this also improves the readability, when one see
`#include "utils/log.hh"`, it is obvious that this source file
needs the logging system, instead of its own log facility -- please
note, we do have two other `log.hh` in the tree.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
In these changes, we describe the purpose of the type
and make it reusable for other parts of the code.
That includes ditching the existing constructors,
leaving the formatting of its fields to the user
of the interface.
The removed constructors have been replaced by
free functions so that existing code can still
use them the way it did before.
We move the declaration of `description` to dedicated files
to be able to create instances of it from other parts of
the code.
`describe_statement.cc` has been functioning as an
intermediary between objects that can be described
and the end user. It will still perform that duty,
but we want to let other modules be able to generate
descriptions on their own, without having to share
an additional layer of abstraction in form of types
inheriting from `data_dictionary::keyspace_element`.
Those types may not perform any other function than
that and thus may be redundant.
Adjusting `description` to its new purpose will happen
in an upcoming commit.
