What version of Go are you using (go version)?

$ go version
go version go1.13.5 darwin/amd64
github.com/golang/tools@61798d64

Does this issue reproduce with the latest release?

Yes

What did you do?

As a part of adding the LSP DocumentHighlight feature to govim I noticed that it sometimes hangs when I use it in medium sized projects.

In govim we vendor the internal lsp files from golang.org/x/tools and use them to communicate with gopls.

In the logs I see that govim successfully sends a $cancelRequest to gopls, and gets a response. But the request after (in this case a CodeAction) never returns.

When dumping the goroutines in govim I saw that requesting goroutine was waiting for a response on rchan at https://github.com/golang/tools/blob/1ace956b0e17ff85a6f9bdf6973af28a26234113/internal/jsonrpc2/jsonrpc2.go#L175.

It's a bit tricky to reproduce with my govim branch, so I created a quick hack in a forked version of tools, https://github.com/leitzler/tools/tree/cancel-hang-repro/repro.
There are two flags for the repro, -gopls that specifies gopls binary path, and -cancel-delay that specifies a delay in ns before the DocumentHighlight request is cancelled. Default it is 1000ns, which seems to work fine on my machine, but must probably be changed to reproduce the issue.
Too short and the client will cancel the request before it's even sent, too long and gopls will respond before the cancel occurs.

Note that according to gopls logs, it looks like the response is sent, but it never seems to reach the client.

The repro isn't beautiful but seems to work and, per discussion on Slack with @findleyr, probably enough to pinpoint the problem.

What did you expect to see?

Requests processed as usual after a cancel.

What did you see instead?

The client hangs.