cmd/internal/moddeps: TestAllDependencies requires a writable GOPATH directory

[tamird]

--- FAIL: TestAllDependencies (14.42s)
    --- FAIL: TestAllDependencies/goroot(quick) (0.02s)
        moddeps_test.go:77: /opt/s/w/ir/x/w/go/bin/go list -mod=mod -m all: exit status 1
            go: could not create module cache: mkdir /nonexist-gopath: read-only file system
FAIL
FAIL	cmd/internal/moddeps	14.547s

Seen on go1.17beta1 in https://fxrev.dev/541662. Related to #46269. cc @bcmills

[bcmills]

This needs more logging. It is checking a module in GOROOT that is not supposed to have any dependencies, but trying to create a module cache (which implies that the module does have dependencies, I think?).

[bcmills]

I suspect that this is a bug in cmd/go, in the cmd/go/internal/modfetch.SideLock function:

go/src/cmd/go/internal/modfetch/cache.go

Lines 154 to 156 in 51a47b7

	if err := checkCacheDir(); err != nil {
	base.Fatalf("go: %v", err)
	}

There are places where we invoke modfetch.SideLock as a best-effort call, and those places should not fail if the module cache can't be created at all. (Moreover, if we're using -mod=readonly and the module cache directory doesn't exist, we probably shouldn't try to create it.)

[bcmills]

(But also, the test should be passing -mod=readonly instead of -mod=mod.)

[gopherbot]

Change https://golang.org/cl/328769 mentions this issue: cmd/go: do not require the module cache to exist for 'go mod edit'

[gopherbot]

Change https://golang.org/cl/328771 mentions this issue: cmd/internal/moddeps: use a temporary directory for GOMODCACHE if needed

[gopherbot]

Change https://golang.org/cl/328770 mentions this issue: cmd/internal/moddeps: use -mod=readonly instead of -mod=mod