fix(Buffer): uint32 -> uint64 in slice methods

[mocurin]

Problem

I'm currently on Dgraph 21.12 unable to export my data.
Export fails on just two nodes out of 7 with the "Unexpected EOF" error:

Dec 14 11:31:51 dm-dgraph-04 dgraph[224355]: I1214 11:31:51.840130  224355 log.go:34] Export [01h26m22s] Scan (12): ~2.1 TiB/2.5 TiB at 177 MiB/sec. Sent: 801.5 GiB at 231 MiB/sec. jemalloc: 7.5 GiB
Dec 14 11:31:55 dm-dgraph-04 dgraph[224355]: W1214 11:31:55.408201  224355 log.go:36] Error while sending: unexpected EOF

Skipping rather long investigation of this issue I came to find length of slice, written to the Buffer during export exceed the size of uint32 (i've decoded varint before Value field in Badger KV struct with RDF's to get something around 4.5Gb, which is expected for a rather bloated reverse edge to the one of the most common nodes in my DB. Also count query returns 72 105 794 connected nodes which is, welp, quite a lot).

Not to mention that working with int which is almost always is int64 and then casually casting it to uint32 w/o any checks or warnings is as bad as it gets.

Solution

Find any 4 and Uint32 and carefully replace them with 8 and Uint64. As this happens only in slice-related methods the fix is quite easy. Locally tests run just fine, but i had to patch the sort one to accommodate for size changes. Also i did test 21.12-related badger version and tests run fine too.

Afterword

I'm somewhat in a hurry, so sorry for not being able to provide more info. I'll write something here some time later.
Also, as far as a can tell - any version of dgraph should be affected by this. Not sure how I turned out to be the first one.

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[mocurin]

Seems that coveralls report posting failed

[mangalaman93]

Thank you for the PR!