Adds Skaffold digest downloader. (#6)

Author: coollog

skaffold-plugins-core/src/integration-test/java/com/google/cloud/tools/skaffold/downloader/SkaffoldDownloaderIntegrationTest.java

@@ -17,7 +17,12 @@
 package com.google.cloud.tools.skaffold.downloader;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
 import java.nio.file.Path;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import javax.xml.bind.DatatypeConverter;
 import org.junit.Assert;
 import org.junit.Rule;
 import org.junit.Test;
@@ -29,10 +34,25 @@ public class SkaffoldDownloaderIntegrationTest {
   @Rule public TemporaryFolder temporaryFolder = new TemporaryFolder();
 
   @Test
-  public void testDownloadLatest() throws IOException, InterruptedException {
+  public void testDownloadLatest()
+      throws IOException, InterruptedException, NoSuchAlgorithmException {
     Path temporarySkaffoldExecutable = temporaryFolder.newFile().toPath();
     SkaffoldDownloader.downloadLatest(temporarySkaffoldExecutable);
     Process skaffoldProcess = new ProcessBuilder(temporarySkaffoldExecutable.toString()).start();
     Assert.assertEquals(0, skaffoldProcess.waitFor());
+
+    // Downloads and checks that the digest matches.
+    Path temporarySkaffoldExecutableDigest = temporaryFolder.newFile().toPath();
+    SkaffoldDownloader.downloadLatestDigest(temporarySkaffoldExecutableDigest);
+
+    MessageDigest messageDigest = MessageDigest.getInstance("SHA-256");
+    byte[] expectedDigest = messageDigest.digest(Files.readAllBytes(temporarySkaffoldExecutable));
+
+    String receivedDigestHex =
+        new String(Files.readAllBytes(temporarySkaffoldExecutableDigest), StandardCharsets.UTF_8)
+            .substring(0, 64);
+    byte[] receivedDigest = DatatypeConverter.parseHexBinary(receivedDigestHex);
+
+    Assert.assertArrayEquals(expectedDigest, receivedDigest);
   }
 }

skaffold-plugins-core/src/main/java/com/google/cloud/tools/skaffold/downloader/SkaffoldDownloader.java

@@ -46,8 +46,22 @@ public class SkaffoldDownloader {
    * @throws MalformedURLException if the URL to download from is malformed
    */
   public static void downloadLatest(Path destination) throws IOException {
+    download(getLatestUrl(""), destination);
+  }
+
+  /**
+   * Downloads the latest {@code skaffold} release digest.
+   *
+   * @throws IOException if an I/O exception occurs during download
+   * @throws MalformedURLException if the URL to download from is malformed
+   */
+  public static void downloadLatestDigest(Path destination) throws IOException {
+    download(getLatestUrl(".sha256"), destination);
+  }
+
+  private static URL getLatestUrl(String suffix) throws MalformedURLException {
     // Skaffold publishes the latest release with version "latest".
-    download(new URL(getUrl("latest", OperatingSystem.resolve())), destination);
+    return new URL(getUrl("latest", OperatingSystem.resolve()) + suffix);
   }
 
   /**