cmd/dist: restore the original GOCACHE before building std and cmd

The user is likely to run other commands that need these libraries
immediately after they are built.

For #57734.
Updates #56889.

Change-Id: I2a1a234e6031d85f017ee692ea1ace8c6e0e7355
Reviewed-on: https://go-review.googlesource.com/c/go/+/452679
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/dist/build.go

@@ -46,6 +46,7 @@ var (
  tooldir string
  oldgoos string
  oldgoarch string
+ oldgocache string
  exe string
  defaultcc map[string]string
  defaultcxx map[string]string
@@ -234,11 +235,6 @@ func xinit() {
  os.Setenv("GOROOT", goroot)
  os.Setenv("GOROOT_FINAL", goroot_final)
 
- // Use a build cache separate from the default user one.
- // Also one that will be wiped out during startup, so that
- // make.bash really does start from a clean slate.
- os.Setenv("GOCACHE", pathf("%s/pkg/obj/go-build", goroot))
-
  // Set GOBIN to GOROOT/bin. The meaning of GOBIN has drifted over time
  // (see https://go.dev/issue/3269, https://go.dev/cl/183058,
  // https://go.dev/issue/31576). Since we want binaries installed by 'dist' to
@@ -1211,7 +1207,6 @@ func cmdenv() {
  xprintf(format, "GO111MODULE", "")
  xprintf(format, "GOARCH", goarch)
  xprintf(format, "GOBIN", gorootBin)
- xprintf(format, "GOCACHE", os.Getenv("GOCACHE"))
  xprintf(format, "GODEBUG", os.Getenv("GODEBUG"))
  xprintf(format, "GOENV", "off")
  xprintf(format, "GOFLAGS", "")
@@ -1340,6 +1335,12 @@ func cmdbootstrap() {
  // go tool may complain.
  os.Setenv("GOPATH", pathf("%s/pkg/obj/gopath", goroot))
 
+ // Use a build cache separate from the default user one.
+ // Also one that will be wiped out during startup, so that
+ // make.bash really does start from a clean slate.
+ oldgocache = os.Getenv("GOCACHE")
+ os.Setenv("GOCACHE", pathf("%s/pkg/obj/go-build", goroot))
+
  // Disable GOEXPERIMENT when building toolchain1 and
  // go_bootstrap. We don't need any experiments for the
  // bootstrap toolchain, and this lets us avoid duplicating the
@@ -1467,6 +1468,13 @@ func cmdbootstrap() {
  copyfile(pathf("%s/compile3", tooldir), pathf("%s/compile", tooldir), writeExec)
  }
 
+ // Now that toolchain3 has been built from scratch, its compiler and linker
+ // should have accurate build IDs suitable for caching.
+ // Now prime the build cache with the rest of the standard library for
+ // testing, and so that the user can run 'go install std cmd' to quickly
+ // iterate on local changes without waiting for a full rebuild.
+ os.Setenv("GOCACHE", oldgocache)
+
  if goos == oldgoos && goarch == oldgoarch {
  // Common case - not setting up for cross-compilation.
  timelog("build", "toolchain")

src/cmd/dist/test.go

@@ -773,7 +773,6 @@ func (t *tester) registerTests() {
  pkg: "fmt",
  }).command(t)
  unsetEnv(cmd, "GOROOT")
- unsetEnv(cmd, "GOCACHE") // TODO(bcmills): ...why‽
  err := cmd.Run()
 
  if rerr := os.Rename(moved, goroot); rerr != nil {
@@ -1699,18 +1698,8 @@ func (t *tester) makeGOROOTUnwritable() (undo func()) {
  }
  }
 
- gocache := os.Getenv("GOCACHE")
- if gocache == "" {
- panic("GOCACHE not set")
- }
- gocacheSubdir, _ := filepath.Rel(dir, gocache)
-
  filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
  if suffix := strings.TrimPrefix(path, dir+string(filepath.Separator)); suffix != "" {
- if suffix == gocacheSubdir {
- // Leave GOCACHE writable: we may need to write test binaries into it.
- return filepath.SkipDir
- }
  if suffix == ".git" {
  // Leave Git metadata in whatever state it was in. It may contain a lot
  // of files, and it is highly unlikely that a test will try to modify