Revert "gotooltest: proxy the new GOMODCACHE env var too" (#136)

We started making gotooltest share the host's module download cache with
test scripts, since we did it with GOCACHE and it initially made sense.

However, the upside is significantly smaller for GOMODCACHE compared to
GOCACHE. The build cache can save a significant amount of time, since
many tools have to load or build Go packages as part of their tests.
In contrast, few tests download modules, and those which do tend to
download those modules from a local proxy like goproxytest, which is
very fast already.

The downsides of sharing the module download cache are a few:

* We don't share GOPATH, and since the default GOMODCACHE is
  GOPATH/pkg/mod, sharing one and not the other is unexpected and
  inconsistent.

* Upstream testscript shares GOCACHE, but does not share GOMODCACHE.
  See golang/go#42017.

* Losing a degree of isolation in the tests is a downside in itself,
  especially given that sharing GOMODCACHE isn't crucial.

This reverts commit c5fd45a.

Note that we keep the env.txt test in place, just flipping the
expectation that GOMODCACHE does not contain ${WORK}.

cmd/testscript/testdata/simple.txt

@@ -1,7 +1,5 @@
 # With .gomodproxy supporting files, any GOPROXY from the
 # environment should be overridden by the test proxy.
-# Note that we want an empty GOMODCACHE, to ensure we have to download modules
-# from our proxy.
 env GOPROXY=0.1.2.3
 
 unquote file.txt
@@ -10,7 +8,6 @@ stdout 'example.com/mod'
 ! stderr .+
 
 -- file.txt --
->env GOMODCACHE=$WORK
 >go get -d fruit.com
 >go list -m
 >stdout 'example.com/mod'

cmd/txtar-addmod/addmod.go

@@ -20,7 +20,6 @@ package main
 
 import (
 	"bytes"
-	"encoding/json"
 	"flag"
 	"fmt"
 	"io/ioutil"
@@ -101,17 +100,9 @@ func main1() int {
 		return string(out)
 	}
 
-	var goEnv struct {
-		GOPATH     string
-		GOMODCACHE string
-	}
-	if err := json.Unmarshal([]byte(run("go", "env", "-json", "GOPATH", "GOMODCACHE")), &goEnv); err != nil {
-		fatalf("cannot unmarshal 'go env -json': %v", err)
-	}
-	modCache := goEnv.GOMODCACHE
-	if modCache == "" {
-		// For Go 1.14 and older.
-		modCache = filepath.Join(goEnv.GOPATH, "pkg", "mod")
+	gopath := strings.TrimSpace(run("go", "env", "GOPATH"))
+	if gopath == "" {
+		fatalf("cannot find GOPATH")
 	}
 
 	exitCode := 0
@@ -140,13 +131,13 @@ func main1() int {
 		}
 		path = encpath
 
-		mod, err := ioutil.ReadFile(filepath.Join(modCache, "cache", "download", path, "@v", vers+".mod"))
+		mod, err := ioutil.ReadFile(filepath.Join(gopath, "pkg/mod/cache/download", path, "@v", vers+".mod"))
 		if err != nil {
 			log.Printf("%s: %v", arg, err)
 			exitCode = 1
 			continue
 		}
-		info, err := ioutil.ReadFile(filepath.Join(modCache, "cache", "download", path, "@v", vers+".info"))
+		info, err := ioutil.ReadFile(filepath.Join(gopath, "pkg/mod/cache/download", path, "@v", vers+".info"))
 		if err != nil {
 			log.Printf("%s: %v", arg, err)
 			exitCode = 1

gotooltest/setup.go

@@ -27,7 +27,6 @@ var (
 	goEnv struct {
 		GOROOT      string
 		GOCACHE     string
-		GOMODCACHE  string
 		GOPROXY     string
 		goversion   string
 		releaseTags []string
@@ -60,14 +59,13 @@ func initGoEnv() error {
 	eout, stderr, err := run("go", "env", "-json",
 		"GOROOT",
 		"GOCACHE",
-		"GOMODCACHE",
 		"GOPROXY",
 	)
 	if err != nil {
 		return fmt.Errorf("failed to determine environment from go command: %v\n%v", err, stderr)
 	}
 	if err := json.Unmarshal(eout.Bytes(), &goEnv); err != nil {
-		return fmt.Errorf("failed to unmarshal 'go env -json' output: %v\n%v", err, eout)
+		return fmt.Errorf("failed to unmarshal GOROOT and GOCACHE tags from go command out: %v\n%v", err, eout)
 	}
 
 	version := goEnv.releaseTags[len(goEnv.releaseTags)-1]
@@ -140,7 +138,6 @@ func goEnviron(env0 []string) []string {
 		"GOOS=" + runtime.GOOS,
 		"GOROOT=" + goEnv.GOROOT,
 		"GOCACHE=" + goEnv.GOCACHE,
-		"GOMODCACHE=" + goEnv.GOMODCACHE,
 		"GOPROXY=" + goEnv.GOPROXY,
 		"goversion=" + goEnv.goversion,
 	}...)

gotooltest/testdata/env.txt

@@ -1,6 +1,6 @@
-# GOPATH should point inside the temporary work directory, but GOCACHE and
-# GOMODCACHE should not, as they should reuse the host's.
+# GOPATH and GOMODCACHE are not shared with the host,
+# but GOCACHE is.
 go env
 stdout GOPATH=.*${WORK@R}
+stdout GOMODCACHE=.*${WORK@R}
 ! stdout GOCACHE=.*${WORK@R}
-! stdout GOMODCACHE=.*${WORK@R}