backupccl: fix error during span-merging optimization

[pbardea]

This commit fixes a bug where a backup may crash if a full backup with
revision history is taken on a table that has index-merging
opportunities on descriptor revisions that lie before the GC TTL of the
table.

Since the merging of index-spans during backup planning is entirely an
optimization (in an effort to reduce the number of spans we place a
protectedts over), it is safe to ignore such errors and treat it as
un-mergable.

Fixes #66797.

Release note (bug fix): Fix an error that backup would produce in some
full backups with revision history. Previously some full backups would
erroneously produce an error in the form of batch timestamp <some_timestamp> must be after replica GC threshold <some_other_timestamp>.

[cockroach-teamcity]

This change is 

[adityamaru]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @pbardea)

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pkg/ccl/backupccl/backup_planning.go, line 366 at r1 (raw file):

		if rawTbl != nil && rawTbl.Public() {
			tbl := tabledesc.NewBuilder(rawTbl).BuildImmutableTable()
			// endTime is safe to pass in here because if the scan at endTime is different than

i'm a little confused by this comment since we are still passing in rev.Time.

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pkg/ccl/backupccl/backup_test.go, line 8424 at r1 (raw file):

// doesn't produce an error when there are span-merging opportunities on
// descriptor revisions from before the GC threshold of the table.
func TestSpanMergeingBeforeGCThreshold(t *testing.T) {

really nice test

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pkg/ccl/backupccl/backup_test.go, line 8439 at r1 (raw file):

	/*
	Produce a table with the following indexes over time:
				|

the formatting looks a little off in Reviewable.

[pbardea]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @adityamaru and @pbardea)

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pkg/ccl/backupccl/backup_planning.go, line 366 at r1 (raw file):

Previously, adityamaru (Aditya Maru) wrote…

i'm a little confused by this comment since we are still passing in rev.Time.

Thanks for catching... this was leftover from some prototyping.

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pkg/ccl/backupccl/backup_test.go, line 8439 at r1 (raw file):

Previously, adityamaru (Aditya Maru) wrote…

the formatting looks a little off in Reviewable.

Played around with it and I think this did the trick...

[pbardea]

Played around with the formatting of the comment a bit more to make the linter happy. I think all editors/github/reviewable should all see the same thing now though.
TFTR!
bors r+

[craig]

Build succeeded:

• GitHub CI (Cockroach)