x/tools/gopls: excessive invocations of "go mod tidy"

[pwaller]

Running gopls from golang/tools@542909f (latest as of writing).

Go 1.14.2; Ubuntu 20.04, amd64.

What did you do?

While working on a Go project whose working directory happened to contain many non-go files (millions of xml files), I discovered that my machine was becoming bogged down by something. In the end I discovered tens of "go mod tidy" processes all competing with each other to stat all of these files. (That cmd/go bug is covered by #38791).

What did you expect to see?

Normal operation of my computer (not progressively slowing down with ever more procesess).

What did you see instead?

CPU spiral of doom, with >30 go mod tidy processes running and the computer becoming progressively more unusable.

Discussion

This highlighted that go mod tidy is being run a lot, and it's not always a cheap command. As I understand it, this command will have to read the directory entries of all directories below the Go module it's run on in order to find Go files, and that alone is still quite expensive in my setup, even if #38791 is addressed.

@mvdan and @bcmills discussed that gopls may be able in principle to do better at minimizing go mod tidy invocations. I think this should be the primary thing which is fixed, since I care about battery life of my laptop.

There is another concern here, which is that gopls allowed so many concurrent invocations of go mod tidy which were stuck competing for the same resources. It seems that these should be gated, and some sort of timeout/warning in place if it takes excessively longer than expected.

/cc @stamblerre @ianthehat

[bcmills]

There is another concern here, which is that gopls allowed so many concurrent invocations of go mod tidy which were stuck competing for the same resources.

Since go mod tidy operates on the whole package graph, I would be surprised if there is any benefit to running it more than once at a time.

Consider guarding it with a semaphore to limit to one invocation at a time, and/or signaling the previous go mod tidy with SIGTERM (and waiting for it to exit) before starting a new run.

[stamblerre]

Thanks for following up on this. I agree that we shouldn't be running go mod tidy concurrently. We have logic in place to handle cancellation, so it sounds like a bug in that code.

@pwaller, if you have a public repro case, that would be helpful, but no worries if not - this sounds simple enough to reproduce.

[pwaller]

No public repro case to hand, sorry. I believe it should be straightforward though. I have around a million files below my directory, FWIW. I don't think it should require this many before go mod tidy becomes noticably slow.

[stamblerre]

@pwaller: Are you able to reproduce this with the latest gopls pre-release (GO111MODULE=on go get golang.org/x/tools/gopls@0.4.2-pre1)? We've made a number of changes to the way that we run the go command, so I'm hoping that fixes the cancellation issues.

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