Allow truncating raft logs via debug tool

[manishrjain]

This PR would add a flag in debug tool, which would allow dropping data from Raft log entries of type EntryNormal.

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This change is 

[golangcibot]

line is 111 characters (from lll)

[danielmai]

Reviewed 1 of 2 files at r1, 1 of 1 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @manishrjain)

[codexnull]

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

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dgraph/cmd/debug/run.go, line 92 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

line is 111 characters (from lll)

I feel like "truncate" is the wrong term here. Truncate means to cut off the end (e.g. truncate a file), and I think here you're removing things from the start. Possible alternatives: drop, prune, expire,

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dgraph/cmd/debug/run.go, line 93 at r2 (raw file):

	flag.StringVarP(&opt.wdir, "wal", "w", "", "Directory where Raft write-ahead logs are stored.")
	flag.Uint64VarP(&opt.wtruncateUntil, "truncate", "t", 0,
		"Remove data from Raft entries up until this index.")

"until" is a ambiguous about whether the index itself is removed or not. I'd suggest changing it to "until and including" or "until but not including" to be clear.

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dgraph/cmd/debug/run.go, line 855 at r2 (raw file):

				case ent.Type == raftpb.EntryNormal && ent.Index < opt.wtruncateUntil:
					if len(ent.Data) == 0 {
						continue

Is there a reason for not removing empty entries?

[manishrjain]

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

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dgraph/cmd/debug/run.go, line 93 at r2 (raw file):

Previously, codexnull (Javier Alvarado) wrote…

"until" is a ambiguous about whether the index itself is removed or not. I'd suggest changing it to "until and including" or "until but not including" to be clear.

Done.

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dgraph/cmd/debug/run.go, line 855 at r2 (raw file):

Previously, codexnull (Javier Alvarado) wrote…

Is there a reason for not removing empty entries?

Entries aren't being removed. Only their data is being dropped.