sstables: parse(summary): reserve positions vector

[bhalevy]

We know the number of positions in advance
so reserve the chunked_vector capacity for that.

• Small optimization, no backport needed

[scylladb-promoter]

🔴 CI State: FAILURE

✅ - Build
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✅ - dtest with tablets
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Build Details:

• Duration: 4 hr 1 min
• Builder: spider7.cloudius-systems.com

[denesb]

This looks like we wanted to reset the positions to empty vector, in case it had content.
I see we don't do this consistently with all members, but maybe we should still keep the reset just in case.

[bhalevy]

okay, I'll make sure it's still needed.

[bhalevy]

I don't see where the summary component is reused, so I don't think this resetting is required.
We only call parse from

scylladb/sstables/sstables.cc

Line 1318 in cd2a2bd

co_return co_await read_simple<component_type::Summary>(_components->summary);

After checking

scylladb/sstables/sstables.cc

Lines 1308 to 1310 in cd2a2bd

	if (_components->summary) {
	co_return;
	}

So we don't reuse the object.

[raphaelsc]

I'd suggest to add an explanation in the commit why it can go away. I'd expect a separate commit for removing the reset, but maybe that's bikeshedding since it's a simple change.

[nyh]

I would add a comment in front of this function saying it can only be called once.
But, wouldn't it be easier just to allow it to be called twice? I.e., leave the reset as in the old code - and then add the reserve() you wanted to add?

[bhalevy]

But we didn't reset s.entries before so it was never supposed to work this way.
We don't need to invent new use cases to excuse some old artifact.

[nyh]

That's a good point.

[bhalevy]

• rebased
• added note to git commit message about why it is safe to drop thew existing reset of positions.

[scylladb-promoter]

🟢 CI State: SUCCESS

✅ - Build
✅ - dtest with topology changes
✅ - dtest with tablets
✅ - dtest
✅ - Docker Test
✅ - Offline-installer Artifact Tests
✅ - Unit Tests

Build Details:

• Duration: 3 hr 45 min
• Builder: spider5.cloudius-systems.com

[bhalevy]

@scylladb/scylla-maint please consider merging