x/build/maintner: negative byte downloads

[dmitshur]

I've seen negative byte downloads in the logs of programs that use the maintner client. E.g.:

2018/08/08 14:01:07 Downloading -6670 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 14:09:37 Downloading -17228 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 14:19:31 Downloading -38666 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 14:27:18 Downloading -10229 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 15:12:23 Downloading -13379 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 15:17:38 Downloading -16171 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 15:24:00 Downloading -28018 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 15:30:51 Downloading -28436 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 16:17:27 Downloading -811 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 16:17:43 Downloading -495 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 16:55:01 Downloading -20370 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:10:43 Downloading -4056 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:10:58 Downloading -323 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:11:13 Downloading -323 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:11:28 Downloading -323 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:17:46 Downloading -33025 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:18:01 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:18:16 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:18:31 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:18:47 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:19:02 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:19:17 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:19:32 Downloading -209 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 17:55:33 Downloading -288 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 18:33:27 Downloading -521 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 18:33:42 Downloading -275 bytes of https://maintner.golang.org/logs/38 ...
2018/08/08 18:33:57 Downloading -275 bytes of https://maintner.golang.org/logs/38 ...

That doesn't make sense and shouldn't be possible.

I think it happens near/during net split (maintner.ErrSplit) situations only, so it's not very critical, but would be nice to understand why it happens and fix.

[gopherbot]

Change https://go.dev/cl/414434 mentions this issue: maintner: don't try to download negative bytes

[dmitshur]

The downloading code was just missing the case for where the client had more bytes than server (due to server restarting with some uncommitted transactions).