Pad encData to 17 bytes before decoding

[animesh2049]

Fixes #4020
We tried to reproduce the issue.

Setup

• 1 Alpha
• 1 Zero
• Live loader processing 1 million dataset

Linux

• We couldn't reproduce the issue. It works fine irrespective of length of the encoded data given that we append at least 3 bytes.

Windows

• Doesn't work with padding of only 3 bytes. Stack trace is below.
• Works fine when concurrency parameter is set to 1 in live loader
• Works fine when encoded data is appended with enough 0 bytes to ensure a length of 17 bytes

(dlv) bt                                                                                                                                                                                                                  0  0x0000000000437ce0 in runtime.fatalthrow
    at c:/go/src/runtime/panic.go:820
 1  0x0000000000437b69 in runtime.throw
    at c:/go/src/runtime/panic.go:774
 2  0x000000000044c443 in runtime.sigpanic
    at c:/go/src/runtime/signal_windows.go:236
 3  0x0000000000fe79b1 in github.com/dgraph-io/dgraph/vendor/github.com/dgryski/go-groupvarint.Decode4
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/vendor/github.com/dgryski/go-groupvarint/decode_amd64.s:10
 4  0x0000000000fe9baa in github.com/dgraph-io/dgraph/codec.(*Decoder).unpackBlock.func1
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/codec/codec.go:159
 5  0x0000000000fe86aa in github.com/dgraph-io/dgraph/codec.(*Decoder).unpackBlock
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/codec/codec.go:160
 6  0x0000000000fe8b02 in github.com/dgraph-io/dgraph/codec.(*Decoder).Seek
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/codec/codec.go:192
 7  0x0000000001004b46 in github.com/dgraph-io/dgraph/posting.(*pIterator).init
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/posting/list.go:127
 8  0x0000000001007fa2 in github.com/dgraph-io/dgraph/posting.(*List).iterate
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/posting/list.go:615
 9  0x0000000001008800 in github.com/dgraph-io/dgraph/posting.(*List).length
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/posting/list.go:691
10  0x0000000000ffe520 in github.com/dgraph-io/dgraph/posting.(*Txn).addMutationHelper
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/posting/index.go:356
11  0x0000000000ffefdd in github.com/dgraph-io/dgraph/posting.(*List).AddMutationWithIndex
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/posting/index.go:394
12  0x00000000011d4b8b in github.com/dgraph-io/dgraph/worker.runMutation
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/worker/mutation.go:115
13  0x000000000120b9f4 in github.com/dgraph-io/dgraph/worker.(*node).applyMutations.func3
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/worker/draft.go:289
14  0x000000000120bc49 in github.com/dgraph-io/dgraph/worker.(*node).applyMutations.func4
    at C:/Users/manga/go/src/github.com/dgraph-io/dgraph/worker/draft.go:318
15  0x0000000000468131 in runtime.goexit
    at c:/go/src/runtime/asm_amd64.s:1357

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This change is 

[pullrequest]

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[pullrequest]

Looks good to me. I'd recommend moving 17 into a variable with a descriptive name or comment, especially since it's now used in two places. But no biggie either way

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Reviewed with ❤️ by PullRequest

[dgryski]

Might want to note this is to workaround dgryski/go-groupvarint#1

[martinmr]

except for a small question.

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @animesh2049 and @manishrjain)

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codec/codec.go, line 142 at r1 (raw file):

			// decoding and expects it to be of length >= 4 at all the stages. Padding
			// with zero to make sure length is always >= 4.
			encData = append(encData, bytes.Repeat([]byte{0}, 17-len(encData))...)

What happens if len(encData) is greater than 17? I am assuming nothing is appended. If that's the case, feel free to resolve this issue.

[manishrjain]

Can you please add dgryski/go-groupvarint#1 in a comment above the padding. Also, in our chat, I had asked to add a comment about the number 17s significance. So, please do that as well.

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @animesh2049 and @manishrjain)

[animesh2049]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @martinmr)

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codec/codec.go, line 142 at r1 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

What happens if len(encData) is greater than 17? I am assuming nothing is appended. If that's the case, feel free to resolve this issue.

Yes.