storage: clone pointSynthesizingIter seek keys

[erikgrinaker]

The seek keys passed to pointSynthesizingIter seek methods may have originated from UnsafeKey(). Repositioning the parent iterator may therefore invalidate the seek key, but the code keeps using it afterwards.

This patch clones the seek key in a reusable buffer. The overhead of this should be negligible, considering the overall cost of a seek. MVCCGet benchmarks don't show a significant difference:

name                                                                  old time/op    new time/op    delta
MVCCGet_Pebble/batch=false/versions=1/valueSize=8/numRangeKeys=0-24     5.11µs ± 1%    5.13µs ± 1%    ~     (p=0.143 n=10+10)
MVCCGet_Pebble/batch=false/versions=1/valueSize=8/numRangeKeys=1-24     9.32µs ± 1%    9.41µs ± 1%  +0.96%  (p=0.001 n=10+10)
MVCCGet_Pebble/batch=false/versions=10/valueSize=8/numRangeKeys=0-24    19.1µs ± 2%    19.2µs ± 3%    ~     (p=0.075 n=10+10)
MVCCGet_Pebble/batch=false/versions=10/valueSize=8/numRangeKeys=1-24    25.7µs ± 2%    25.7µs ± 2%    ~     (p=0.853 n=10+10)
MVCCGet_Pebble/batch=true/versions=1/valueSize=8/numRangeKeys=0-24      3.10µs ± 2%    3.09µs ± 1%    ~     (p=0.985 n=10+10)
MVCCGet_Pebble/batch=true/versions=1/valueSize=8/numRangeKeys=1-24      5.05µs ± 0%    5.02µs ± 0%  -0.46%  (p=0.000 n=10+10)
MVCCGet_Pebble/batch=true/versions=10/valueSize=8/numRangeKeys=0-24     16.5µs ± 3%    16.4µs ± 2%    ~     (p=0.579 n=10+10)
MVCCGet_Pebble/batch=true/versions=10/valueSize=8/numRangeKeys=1-24     20.6µs ± 2%    20.7µs ± 1%  +0.60%  (p=0.022 n=9+10)

Touches #90642.
Epic: CRDB-2624.

Release note: None

[cockroach-teamcity]

This change is 

[erikgrinaker]

I'm not really sure this is strictly necessary, because I see that intentInterleavingIter is vulnerable to the same issue, and pebbleMVCCScanner appears to take care to clone the seek key itself for the same reason. That said, pebbleIterator does clone the seek key though, possibly unnecessarily.

[erikgrinaker]

Another site that seems very likely to be buggy:

cockroach/pkg/storage/point_synthesizing_iter.go

Lines 149 to 151 in 8cdaa31

	if ok, err := iter.updateIter(); ok && err == nil {
	iter.updateSeekGEPosition(parent.UnsafeKey())
	}

[jbowens]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @tbg)

[erikgrinaker]

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)