cmd/go: repeated, unhelpful errors when module cache can't be created

[jayconrod]

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

$ go version
go1.16.2

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=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jayconrod/Library/Caches/go-build"
GOENV="/Users/jayconrod/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/does/not/exist/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/does/not/exist"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/go/installed"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/go/installed/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/rq/x0692kqj6ml8cvrhcqh5bswc008xj1/T/go-build3704992266=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Set environment variables such that the module cache directory can't be created as a directory. In this example below, I've explicitly set GOMODCACHE to a path I don't have access to.

export GOMODCACHE=/does/not/exist
go install golang.org/x/tools/gopls@latest

This came up in this golang-nuts thread. In that example, neither GOPATH nor GOMODCACHE was set, so GOMODCACHE defaulted to $HOME/go/pkg/mod, but $HOME/go is not a directory.

What did you expect to see?

An error message that explains what the problem is and how to fix it, printed once.

What did you see instead?

An unhelpful message, printed multiple times.

$ go install golang.org/x/tools/gopls@latest
go: writing stat cache: mkdir /does: read-only file system
go: writing stat cache: mkdir /does: read-only file system
go: writing go.mod cache: mkdir /does: read-only file system
go: downloading golang.org/x/tools/gopls v0.6.8
go: writing go.mod cache: mkdir /does: read-only file system
go: downloading golang.org/x/tools v0.1.0
go install golang.org/x/tools/gopls@latest: mkdir /does: read-only file system

[gopherbot]

Change https://golang.org/cl/304571 mentions this issue: cmd/go: display helpful error when module cache can't be created