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 <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: Bryan C. Mills

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