x/tools/gopls: change to go.mod file does not trigger recalculation of diagnostics

[myitcv]

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

$ go version
go version devel +6f7b96f6cb Sat Nov 23 11:00:41 2019 +0000 linux/amd64
$ go list -m golang.org/x/tools
golang.org/x/tools v0.0.0-20191113223546-95cb2a1a7eae => github.com/myitcvforks/tools v0.0.0-20191119111301-0222b4b716c6
$ go list -m golang.org/x/tools/gopls
golang.org/x/tools/gopls v0.1.8-0.20191115202509-3a792d9c32b2 => github.com/myitcvforks/tools/gopls v0.0.0-20191119111301-0222b4b716c6

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/myitcv/.cache/go-build"
GOENV="/home/myitcv/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/myitcv/gostuff"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/myitcv/gos"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/myitcv/gos/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/myitcv/gostuff/src/github.com/myitcv/govim/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build615579010=/tmp/go-build -gno-record-gcc-switches"

What did you do?

This is probably related to #35548 but raising as a separate issue because it's specifically about changing go.mod as opposed to configuration.

We have a govim test that verifies the setting of the "env" value GOFLAG=-mod=readonly. It is based on the following setup:

-- go.mod --
module mod.com

go 1.13
-- main.go --
package main

import "example.com/blah"

func main() {
	println(blah.Name)
}

(example.com/blah is a valid module and is accessible FWIW, see later).

We initially verify that the go.mod file is not changed by gopls. This is fine. We also verify that main.go has the following diagnostic:

main.go|3 col 8| could not import example.com/blah (no package for import example.com/blah)

Also fine.

Then we update go.mod from outside the editor using:

go get example.com/blah@v1.0.0

This change gets notified to gopls via didChange but the diagnostics for main.go are not updated.

What did you expect to see?

The diagnostics for main.go to be updated such that there are no errors.

What did you see instead?

The diagnostics for main.go unchanged.

See the interactions via the gopls log file: fail.log

---

cc @stamblerre @matloob

[stamblerre]

This is a consequence of #31999. gopls doesn't yet track modifications to go.mod files. @ridersofrohan will be working on this issue soon. Closing as a duplicate of #31999.