sql: fix nil pointer error in RunPostDeserializationChanges

[RichardJCai]

In some restore paths, the privilegeDescriptor would be nil in
RunPostDeserializationChanges, avoid doing work on a nil pointer.

Release note: None

Fixes #76063
Fixes #76062
Fixes #76042

[cockroach-teamcity]

This change is 

[adityamaru]

Thanks!

[ajwerner]

I suppose

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @ajwerner and @RichardJCai)

---

pkg/sql/catalog/dbdesc/database_desc_builder.go, line 100 at r1 (raw file):

) (hasChanged bool) {
	// If privileges are nil, there is nothing to convert.
	// This case can happen during restore where privileges are not yet created.

I wish I understood this. Why does this happen? Can we fix it in the restore side?

[RichardJCai]

I suppose

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @ajwerner and @RichardJCai)

pkg/sql/catalog/dbdesc/database_desc_builder.go, line 100 at r1 (raw file):

) (hasChanged bool) {
	// If privileges are nil, there is nothing to convert.
	// This case can happen during restore where privileges are not yet created.

I wish I understood this. Why does this happen? Can we fix it in the restore side?

I'll dig into this more, wanted to unblock some restore tests locally on master.

#76042 (comment)

Here's an example of the stacktrace thats erroring.

		var b catalog.DescriptorBuilder
		if tableDesc, isTable := desc.(catalog.TableDescriptor); isTable {
			b = tabledesc.NewBuilderForFKUpgrade(tableDesc.TableDesc(), skipFKsWithNoMatchingTable)
		} else {
			b = desc.NewBuilder()
		}
		b.RunPostDeserializationChanges()

seems like for whatever reason, RunPostDeserializationChanges is being called here with a descriptor that has nil privileges.

[RichardJCai]

Thanks!

bors r+

[craig]

Build succeeded:

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