Allow schema updates to reserved preds if the update is the same.

[martinmr]

Outright rejecting a schema update if it is for a reserved predicate has
caused some issues (in ratel and jepsen for example). Make the check
more robust by allowing the updates to go through if the update to a
reserved predicate is the same as a existing one.

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This change is 

[gitlw]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @martinmr)

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schema/schema.go, line 475 at r1 (raw file):

		return true
	}
	return true

Consider this return here when the provided pred is not a reserved predicate,
maybe we can change this function to be IsReservedPredicateAndChanged,
then this return should be change false, and we don't need to call the x.IsReservedPredicate in the caller.

[martinmr]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @gitlw)

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schema/schema.go, line 475 at r1 (raw file):

Previously, gitlw (Lucas Wang) wrote…

Consider this return here when the provided pred is not a reserved predicate,
maybe we can change this function to be IsReservedPredicateAndChanged,
then this return should be change false, and we don't need to call the x.IsReservedPredicate in the caller.

Done. I let the name unchanged but merged the checks into the same method.

[gitlw]

Reviewable status: 0 of 3 files reviewed, 1 unresolved discussion (waiting on @gitlw)

[manishrjain]

One comment.

Reviewed 1 of 3 files at r1, 2 of 2 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @gitlw and @martinmr)

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schema/schema.go, line 475 at r2 (raw file):

			continue
		}
		if proto.Equal(original, update) {

return !proto.Equal(...)