Debug tool to modify Raft snapshot and hardstate

[manishrjain]

This PR adds a way for the debug tool to operate upon the Raft logs to set a future snapshot and hardstate. This can be used to fix up a broken follower, who has fallen way behind the leader.

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This change is 

[martinmr]

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

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dgraph/cmd/debug/run.go, line 96 at r1 (raw file):

		"Remove data from Raft entries until but not including this index.")
	flag.StringVarP(&opt.wsetSnapshot, "snap", "s", "",
		"Set snapshot term,index,readts to this.")

"Set the snapshot term, index, and readTs. Value must be a comma-separated list containing the value for these variables in that order."

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dgraph/cmd/debug/run.go, line 891 at r1 (raw file):

			store := raftwal.Init(db, rid, gid)
			switch {
			case len(opt.wsetSnapshot) > 0:

The logic to set the snapshot is quite long. Unless there's a good reason for it, I think this code should go into its own function, just like printRaft.

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dgraph/cmd/debug/run.go, line 954 at r1 (raw file):

				x.Check(err)
				err = store.CreateSnapshot(dsnap.Index, &cs, data)
				fmt.Printf("Created snapshot with error: %v\n", err)

only print this if there's an error.

[martinmr]

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

[martinmr]

Just a couple comments but overall it looks good.

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

[manishrjain]

Reviewable status: 1 of 3 files reviewed, 3 unresolved discussions (waiting on @danielmai and @martinmr)

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dgraph/cmd/debug/run.go, line 96 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

"Set the snapshot term, index, and readTs. Value must be a comma-separated list containing the value for these variables in that order."

Done.

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dgraph/cmd/debug/run.go, line 891 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

The logic to set the snapshot is quite long. Unless there's a good reason for it, I think this code should go into its own function, just like printRaft.

Done. Moved all the logic out of this file into a new wal.go file, so all raft stuff is separate. Also, moved the nested funcs outside.

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dgraph/cmd/debug/run.go, line 954 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

only print this if there's an error.

Done.