Check hashes of files in download cache for validity

The gomobile init command will use files cached in a 'dl' folder
in lieu of downloading a new file from scratch. If a user of gomobile
does not upgrade for a substantial amount of time, the cached files
will be out of date and possibly cause the init command to fail because
they are missing something the init command now expects.

The init command has a list of expectad sha256 message digests of files
it downloads, which can be utilized to check a file that exists in the 'dl'
folder before proceeeding to utilize it to continue with the init process.

This fixes the problem cited in issue #14503, which is not platform-specific:
golang/go#14503

Some files detailing debugging of the original problem and asserting a solution
are at the following URL:
https://gist.github.com/aoeu/657386c35131b71893153efe34ec07b4

cmd/gomobile/init.go

@@ -520,7 +520,11 @@ func fetch(url string) (dst string, err error) {
 			printcmd("stat %s", dst)
 		}
 		if _, err = os.Stat(dst); err == nil {
-			return dst, nil
+			if err := checkFetchHash(name, dst); err != nil {
+				fmt.Fprintf(os.Stderr, "A correct message disgest could not be calculated for existing download of '%v'.\n%v\n", dst, err)
+			} else {
+				return dst, nil
+			}
 		}
 	}
 	if buildX {
@@ -574,6 +578,30 @@ func fetch(url string) (dst string, err error) {
 	return dst, nil
 }
 
+func checkFetchHash(name string, filepath string) error {
+	f, err := os.Open(filepath)
+	if err != nil {
+		return err
+	}
+	h, err := calcHash(f)
+	switch {
+	case err != nil:
+		return fmt.Errorf("could not calculate sha256 for %v: %v", name, err)
+	case fetchHashes[name] != h:
+		return fmt.Errorf("sha256 for %q: %v, want %v", name, h, fetchHashes[name])
+	default:
+		return nil
+	}
+}
+
+func calcHash(r io.Reader) (string, error) {
+	w := sha256.New()
+	if _, err := io.Copy(w, r); err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(w.Sum(nil)), nil
+}
+
 func doCopyAll(dst, src string) error {
 	return filepath.Walk(src, func(path string, info os.FileInfo, errin error) (err error) {
 		if errin != nil {