Fix alpha start in ludicrous mode

[ashish-goswami]

Fixes: #5601
Fixes: DGRAPH-1669

Currently we are unable to start alpha server in ludicrous mode, because of reasons explained
below.

Things we do at alpha start:

1. At start we insert dgraph.type, dgraph.graphql.schema and dgraph.graphql.xid predicates as
   our initial schema.
2. We also insert dgraph.graphql type(having predicates dgraph.graphql.schema and
   dgraph.graphql.xid) as initial types.
3. We also upsert data related to graphql schema(which results in upserts for predicates
   dgraph.graphql.schema and dgraph.graphql.xid).

While inserting types we verify that predicates used in the types should either already be present
in schema or should be part of same mutation where we are trying to insert this type.

In ludicrous mode we mark any proposal done without error as soon as it is retrieved as part of commited entries, as opposed to after applying proposal (this is done in normal mode).

Events which led to alpha getting stuck:

1. We propose schema mutation for dgraph.type, which is marked as done immediately. This
   proposal is still getting applied.
2. Since we marked above proposal done without error, we propose next schema mutation for
   dgraph.graphql.schema, which again is marked done without error. But applying of this proposal
   fails as indexing was already in progress for dgraph.type. But we never retry this schema
   mutation.
3. Next we propose dgraph.graphql.xid, which is applied successfully(indexing is done until now
   for dgraph.type).
4. When we insert dgraph.graphql type, verifyTypes() fails, because it never finds
   dgraphq.graphql.schema predicate.

Proposed solution:
Assumption that proposal is never failed while applying, hence it should be marked as done without
error immediately, should hold only for data mutations and not schema mutations.

This PR checks if proposal is data mutation(having edges > 0), while marking it done
immediately. This leads to retrying of schema proposal if they fail once because indexing is already
in progress.

Note: We didn't observe this before commit(aef7072), because predicate dgraph.graphql.schema was getting inserted via graphql upserts(#3 of start steps). Now after
commit(aef7072) we cannot insert data for predicates starting with dgraph. prefix unless those are already present.

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This change is 

[martinmr]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @ashish-goswami, @manishrjain, and @vvbalaji-dgraph)

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worker/draft.go, line 1161 at r1 (raw file):

						if x.WorkerConfig.LudicrousMode &&
							proposal.Mutations != nil && len(proposal.Mutations.Edges) > 0 {

I think this can be written as

if x.WorkerConfig.LudicrousMode && len(proposal.Mutations.GetEdges()) > 0

GetEdges will take care of the nil case on its own.

[ashish-goswami]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @manishrjain, @martinmr, and @vvbalaji-dgraph)

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worker/draft.go, line 1161 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

						if x.WorkerConfig.LudicrousMode &&
							proposal.Mutations != nil && len(proposal.Mutations.Edges) > 0 {

I think this can be written as

if x.WorkerConfig.LudicrousMode && len(proposal.Mutations.GetEdges()) > 0

GetEdges will take care of the nil case on its own.

Done.

[martinmr]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)

[vvbalaji-dgraph]

Skipping ludicrous mode for schema mutations seems to be the right approach. If 'Getedges' is the right mechanism for identifying schema changes then it would be good to have a comment to describe our intent here.

[parasssh]

Good catch! LGTM