cmd/go: clean the directory path containing the packages

The heuristic for determining if the packages or commands are stale
fails as the mtime comparison happens even though the GOROOT and
current package paths are the same, since the path name isn't
canonicalized before the comparison (GOROOT is).

Fixes: #12690

Change-Id: Ia7d142fbbed8aac2bd2f71d1db4efd1f3ff5aece
Reviewed-on: https://go-review.googlesource.com/16483Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -1191,6 +1191,17 @@ func TestPackageMainTestImportsArchiveNotBinary(t *testing.T) {
 	tg.run("test", "main_test")
 }
 
+// Issue 12690
+func TestPackageNotStaleWithTrailingSlash(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	goroot := runtime.GOROOT()
+	tg.setenv("GOROOT", goroot+"/")
+	tg.wantNotStale("runtime", "with trailing slash in GOROOT, runtime listed as stale")
+	tg.wantNotStale("os", "with trailing slash in GOROOT, os listed as stale")
+	tg.wantNotStale("io", "with trailing slash in GOROOT, io listed as stale")
+}
+
 // With $GOBIN set, binaries get installed to $GOBIN.
 func TestInstallIntoGOBIN(t *testing.T) {
 	tg := testgo(t)

src/go/build/build.go

@@ -289,7 +289,7 @@ func defaultContext() Context {
 
 	c.GOARCH = envOr("GOARCH", runtime.GOARCH)
 	c.GOOS = envOr("GOOS", runtime.GOOS)
-	c.GOROOT = runtime.GOROOT()
+	c.GOROOT = pathpkg.Clean(runtime.GOROOT())
 	c.GOPATH = envOr("GOPATH", "")
 	c.Compiler = runtime.Compiler