Add --path parameter to GetPatch

Adds path parameter to GetPatch.java for fetching a diff of a specific
file.

Also includes a unit test for the path parameter.

Feature: Issue 4669
Change-Id: Id211fff3b6d9ba6ae16835253f2d328ccf7b5020

Documentation/rest-api-changes.txt

@@ -3477,6 +3477,9 @@ Query parameter `download` (e.g. `/changes/.../patch?download`)
 will suggest the browser save the patch as `commitsha1.diff.base64`,
 for later processing by command line tools.
 
+If the `path` parameter is set, the returned content is a diff of the single
+file that the path refers to.
+
 [[submit-preview]]
 === Submit Preview
 --

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/PushOneCommit.java

@@ -52,15 +52,7 @@ public class PushOneCommit {
   public static final String SUBJECT = "test commit";
   public static final String FILE_NAME = "a.txt";
   public static final String FILE_CONTENT = "some content";
-  public static final String PATCH =
+  public static final String PATCH_FILE_ONLY =
-      "From %s Mon Sep 17 00:00:00 2001\n" +
-      "From: Administrator <admin@example.com>\n" +
-      "Date: %s\n" +
-      "Subject: [PATCH] test commit\n" +
-      "\n" +
-      "Change-Id: %s\n" +
-      "---\n" +
-      "\n" +
       "diff --git a/a.txt b/a.txt\n" +
       "new file mode 100644\n" +
       "index 0000000..f0eec86\n" +
@@ -69,6 +61,15 @@ public class PushOneCommit {
       "@@ -0,0 +1 @@\n" +
       "+some content\n" +
       "\\ No newline at end of file\n";
+  public static final String PATCH =
+      "From %s Mon Sep 17 00:00:00 2001\n" +
+      "From: Administrator <admin@example.com>\n" +
+      "Date: %s\n" +
+      "Subject: [PATCH] test commit\n" +
+      "\n" +
+      "Change-Id: %s\n" +
+      "---\n" +
+      "\n" + PATCH_FILE_ONLY;
 
   public interface Factory {
     PushOneCommit create(

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/api/revision/RevisionIT.java

@@ -18,6 +18,7 @@ import static com.google.common.truth.Truth.assertThat;
 import static com.google.gerrit.acceptance.PushOneCommit.FILE_CONTENT;
 import static com.google.gerrit.acceptance.PushOneCommit.FILE_NAME;
 import static com.google.gerrit.acceptance.PushOneCommit.PATCH;
+import static com.google.gerrit.acceptance.PushOneCommit.PATCH_FILE_ONLY;
 import static com.google.gerrit.acceptance.PushOneCommit.SUBJECT;
 import static com.google.gerrit.reviewdb.client.Patch.COMMIT_MSG;
 import static com.google.gerrit.reviewdb.client.Patch.MERGE_LIST;
@@ -58,6 +59,7 @@ import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.BinaryResult;
 import com.google.gerrit.extensions.restapi.ETagView;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.gerrit.reviewdb.client.Branch;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.reviewdb.client.PatchSetApproval;
@@ -922,6 +924,24 @@ public class RevisionIT extends AbstractDaemonTest {
         String.format(PATCH, r.getCommit().name(), date, r.getChangeId()));
   }
 
+  @Test
+  public void patchWithPath() throws Exception {
+    PushOneCommit.Result r = createChange();
+    ChangeApi changeApi = gApi.changes()
+        .id(r.getChangeId());
+    BinaryResult bin = changeApi
+        .revision(r.getCommit().name())
+        .patch(FILE_NAME);
+    ByteArrayOutputStream os = new ByteArrayOutputStream();
+    bin.writeTo(os);
+    String res = new String(os.toByteArray(), UTF_8);
+    assertThat(res).isEqualTo(PATCH_FILE_ONLY);
+
+    exception.expect(ResourceNotFoundException.class);
+    exception.expectMessage("File not found: nonexistent-file.");
+    changeApi.revision(r.getCommit().name()).patch("nonexistent-file");
+  }
+
   @Test
   public void actions() throws Exception {
     PushOneCommit.Result r = createChange();

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/RevisionApi.java

@@ -72,6 +72,7 @@ public interface RevisionApi {
    * Returns patch of revision.
    */
   BinaryResult patch() throws RestApiException;
+  BinaryResult patch(String path) throws RestApiException;
 
   Map<String, ActionInfo> actions() throws RestApiException;
 
@@ -252,6 +253,11 @@ public interface RevisionApi {
       throw new NotImplementedException();
     }
 
+    @Override
+    public BinaryResult patch(String path) throws RestApiException {
+      throw new NotImplementedException();
+    }
+
     @Override
     public Map<String, ActionInfo> actions() throws RestApiException {
       throw new NotImplementedException();

gerrit-server/src/main/java/com/google/gerrit/server/api/changes/RevisionApiImpl.java

@@ -462,6 +462,15 @@ class RevisionApiImpl implements RevisionApi {
     }
   }
 
+  @Override
+  public BinaryResult patch(String path) throws RestApiException {
+    try {
+      return getPatch.setPath(path).apply(revision);
+    } catch (IOException e) {
+      throw new RestApiException("Cannot get patch", e);
+    }
+  }
+
   @Override
   public Map<String, ActionInfo> actions() throws RestApiException {
     return revisionActions.apply(revision).value();

gerrit-server/src/main/java/com/google/gerrit/server/change/GetPatch.java

@@ -18,6 +18,7 @@ import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.gerrit.extensions.restapi.BinaryResult;
 import com.google.gerrit.extensions.restapi.ResourceConflictException;
+import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.gerrit.extensions.restapi.RestReadView;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.server.git.GitRepositoryManager;
@@ -30,6 +31,7 @@ import org.eclipse.jgit.lib.PersonIdent;
 import org.eclipse.jgit.lib.Repository;
 import org.eclipse.jgit.revwalk.RevCommit;
 import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.treewalk.filter.PathFilter;
 import org.kohsuke.args4j.Option;
 
 import java.io.IOException;
@@ -43,20 +45,25 @@ import java.util.zip.ZipOutputStream;
 public class GetPatch implements RestReadView<RevisionResource> {
   private final GitRepositoryManager repoManager;
 
+  private final String FILE_NOT_FOUND = "File not found: %s.";
+
   @Option(name = "--zip")
   private boolean zip;
 
   @Option(name = "--download")
   private boolean download;
 
+  @Option(name = "--path")
+  private String path;
+
   @Inject
   GetPatch(GitRepositoryManager repoManager) {
     this.repoManager = repoManager;
   }
 
   @Override
-  public BinaryResult apply(RevisionResource rsrc)
+  public BinaryResult apply(RevisionResource rsrc) throws
-      throws ResourceConflictException, IOException {
+      ResourceConflictException, IOException, ResourceNotFoundException {
     Project.NameKey project = rsrc.getControl().getProject().getNameKey();
     final Repository repo = repoManager.openRepository(project);
     boolean close = true;
@@ -93,9 +100,15 @@ public class GetPatch implements RestReadView<RevisionResource> {
           }
 
           private void format(OutputStream out) throws IOException {
+            // Only add header if no path is specified
+            if (path == null) {
               out.write(formatEmailHeader(commit).getBytes(UTF_8));
+            }
             try (DiffFormatter fmt = new DiffFormatter(out)) {
               fmt.setRepository(repo);
+              if (path != null) {
+                fmt.setPathFilter(PathFilter.create(path));
+              }
               fmt.format(base.getTree(), commit.getTree());
               fmt.flush();
             }
@@ -108,6 +121,11 @@ public class GetPatch implements RestReadView<RevisionResource> {
           }
         };
 
+        if (path != null && bin.asString().isEmpty()) {
+          throw new ResourceNotFoundException(
+               String.format(FILE_NOT_FOUND, path));
+        }
+
         if (zip) {
           bin.disableGzip()
              .setContentType("application/zip")
@@ -134,6 +152,11 @@ public class GetPatch implements RestReadView<RevisionResource> {
     }
   }
 
+  public GetPatch setPath(String path) {
+    this.path = path;
+    return this;
+  }
+
   private static String formatEmailHeader(RevCommit commit) {
     StringBuilder b = new StringBuilder();
     PersonIdent author = commit.getAuthorIdent();