cmd/go: disable ccache when setting HOME to non-existent dir

This fixes tests on systems where ccache is the default compiler.

Also simplify a prior workaround for this fault.

Fixed #26789

Change-Id: I031ff0b65ace7fc5e284393298e004aa2ad3b6f5
Reviewed-on: https://go-review.googlesource.com/127775Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -224,13 +224,11 @@ func TestMain(m *testing.M) {
 	os.Unsetenv("GOBIN")
 	os.Unsetenv("GOPATH")
 	os.Unsetenv("GIT_ALLOW_PROTOCOL")
-	if home, ccacheDir := os.Getenv("HOME"), os.Getenv("CCACHE_DIR"); home != "" && ccacheDir == "" {
-		// On some systems the default C compiler is ccache.
-		// Setting HOME to a non-existent directory will break
-		// those systems. Set CCACHE_DIR to cope. Issue 17668.
-		os.Setenv("CCACHE_DIR", filepath.Join(home, ".ccache"))
-	}
 	os.Setenv("HOME", "/test-go-home-does-not-exist")
+	// On some systems the default C compiler is ccache.
+	// Setting HOME to a non-existent directory will break
+	// those systems. Disable ccache and use real compiler. Issue 17668.
+	os.Setenv("CCACHE_DISABLE", "1")
 	if os.Getenv("GOCACHE") == "" {
 		os.Setenv("GOCACHE", testGOCACHE) // because $HOME is gone
 	}

src/cmd/go/script_test.go

@@ -87,6 +87,7 @@ func (ts *testScript) setup() {
 		"WORK=" + ts.workdir, // must be first for ts.abbrev
 		"PATH=" + testBin + string(filepath.ListSeparator) + os.Getenv("PATH"),
 		homeEnvName() + "=/no-home",
+		"CCACHE_DISABLE=1", // ccache breaks with non-existent HOME
 		"GOARCH=" + runtime.GOARCH,
 		"GOCACHE=" + testGOCACHE,
 		"GOOS=" + runtime.GOOS,