Disallow uid as a predicate name.

[martinmr]

uid is used internally to report the UIDs of the nodes in Dgraph. Users
should get an error when trying to add a predicate of that name to the
schema or when sending mutations with uid as a predicate.

This PR enables checks for those situations.

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This change is 

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[pullrequest]

These changes look pretty solid. Nothing to comment on.

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[pawanrawal]

Reviewed 5 of 5 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain)

[poonai]

Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @manishrjain and @martinmr)

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worker/draft.go, line 255 at r1 (raw file):

	for attr, storageType := range schemaMap {
		if _, err := schema.State().TypeOf(attr); err != nil {

do we need any explicit error checking? createSchema if this error

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worker/mutation.go, line 210 at r1 (raw file):

		return err
	}
	if err := updateSchema(&s); err != nil {

Just return updateSchema(&s)

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x/keys.go, line 565 at r1 (raw file):

// predicate is a predicate that has a special meaning in Dgraph and its query
// language and should not be allowed as a user-defined  predicate.
var internalPredicateMap = map[string]struct{}{

where predicates are populated?

[martinmr]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @balajijinnah and @manishrjain)

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worker/draft.go, line 255 at r1 (raw file):

Previously, balajijinnah (balaji) wrote…

do we need any explicit error checking? createSchema if this error

I changed the return value of createSchema so I changed the code here to check for that error.

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worker/mutation.go, line 210 at r1 (raw file):

Previously, balajijinnah (balaji) wrote…

Just return updateSchema(&s)

Done.

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x/keys.go, line 565 at r1 (raw file):

Previously, balajijinnah (balaji) wrote…

where predicates are populated?

Right here. This is meant to act as a constant. The map should not modified. If another predicate (let's say _predicate_ is deemed as an internal predicate, it should be added here.

[poonai]

Reviewed 4 of 5 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @balajijinnah and @manishrjain)