Support to get a revision as formatted git patch via REST

Bug: issue 302
Change-Id: Iccdb5990683ad911259a79e977ec4c057078e4e1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

Documentation/rest-api-changes.txt

@@ -1268,6 +1268,34 @@ message is contained in the response body.
   "revision 674ac754f91e64a0efb8087e59a176484bd534d1 is not current revision"
 ----
 
+[[get-patch]]
+Get Patch
+~~~~~~~~~
+[verse]
+'GET /changes/link:#change-id[\{change-id\}]/revisions/link:#revision-id[\{revision-id\}]/patch'
+
+Gets the formatted patch for one revision.
+
+.Request
+----
+  GET /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/revisions/current/patch HTTP/1.0
+----
+
+The formatted patch is returned as text wrapped in JSON.
+
+.Response
+----
+  HTTP/1.1 200 OK
+  Content-Disposition: attachment
+  Content-Type: application/json;charset=UTF-8
+
+  )]}'
+  "From 4cf8380e526b96de9a0ad50f745028fe416c4639 Tue, 02 Apr 2013 10:26:14 +0200\nFrom: John Doe \
+  u003cjohn.doe@example.com\u003e\nDate: Thu, 28 Mar 2013 15:23:52 +0200\nSubject: [PATCH] Test Commit\n\n\n
+  diff--git a/a.txt b/a.txt\nindex bb0b141..e613f23 100644\n--- a/a.txt\n+++ b/a.txt\n@@ -1,5 +1,6 @@
+  \n line1\n-line2\n line3\n line4\n+line4.1\n+line4.2\n line5\n"
+----
+
 [[get-submit-type]]
 Get Submit Type
 ~~~~~~~~~~~~~~~

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

@@ -0,0 +1,104 @@
+// Copyright (C) 2013 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.change;
+
+import static com.google.common.base.Charsets.UTF_8;
+
+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.GerritPersonIdent;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.inject.Inject;
+
+import org.eclipse.jgit.diff.DiffFormatter;
+import org.eclipse.jgit.lib.ObjectId;
+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 java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.text.SimpleDateFormat;
+import java.util.Calendar;
+import java.util.Locale;
+
+public class GetPatch implements RestReadView<RevisionResource> {
+  private final GitRepositoryManager repoManager;
+  private final SimpleDateFormat df;
+
+  @Inject
+  GetPatch(@GerritPersonIdent final PersonIdent gerritIdent,
+      GitRepositoryManager repoManager) {
+    this.repoManager = repoManager;
+    this.df = new SimpleDateFormat("EEE, dd MMM yyyy HH:mm:ss Z", Locale.US);
+    this.df.setCalendar(Calendar.getInstance(gerritIdent.getTimeZone(), Locale.US));
+  }
+
+  @Override
+  public String apply(RevisionResource rsrc)
+      throws ResourceNotFoundException, ResourceConflictException {
+    Project.NameKey project = rsrc.getControl().getProject().getNameKey();
+    try {
+      Repository repo = repoManager.openRepository(project);
+      try {
+        RevWalk rw = new RevWalk(repo);
+        try {
+          RevCommit commit =
+              rw.parseCommit(ObjectId.fromString(rsrc.getPatchSet()
+                  .getRevision().get()));
+          RevCommit[] parents = commit.getParents();
+          if (parents.length > 1) {
+            throw new ResourceConflictException(
+                "Revision has more than 1 parent.");
+          } else if (parents.length == 0) {
+            throw new ResourceConflictException("Revision has no parent.");
+          }
+          rw.parseBody(parents[0]);
+
+          StringBuilder b = new StringBuilder();
+          appendHeader(b, commit);
+          ByteArrayOutputStream out = new ByteArrayOutputStream();
+          DiffFormatter f = new DiffFormatter(out);
+          f.setRepository(repo);
+          f.format(parents[0].getTree(), commit.getTree());
+          f.flush();
+          b.append(out.toString(UTF_8.name()));
+          return b.toString();
+        } finally {
+          rw.release();
+        }
+      } finally {
+        repo.close();
+      }
+    } catch (IOException e) {
+      throw new ResourceNotFoundException();
+    }
+  }
+
+  private void appendHeader(StringBuilder b, RevCommit commit) {
+    PersonIdent author = commit.getAuthorIdent();
+    b.append("From ").append(commit.getId().getName()).append(" ")
+        .append(df.format(Long.valueOf(System.currentTimeMillis()))).append("\n");
+    b.append("From: ").append(author.getName()).append(" <").append(author.getEmailAddress()).append(">\n");
+    b.append("Date: ").append(df.format(author.getWhen())).append("\n");
+    b.append("Subject: [PATCH] ").append(commit.getShortMessage());
+    String message = commit.getFullMessage().substring(
+        commit.getShortMessage().length());
+    b.append(message).append("\n\n");
+  }
+}

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

@@ -64,6 +64,7 @@ public class Module extends RestApiModule {
     post(REVISION_KIND, "review").to(PostReview.class);
     post(REVISION_KIND, "submit").to(Submit.class);
     get(REVISION_KIND, "submit_type").to(TestSubmitType.Get.class);
+    get(REVISION_KIND, "patch").to(GetPatch.class);
     post(REVISION_KIND, "test.submit_rule").to(TestSubmitRule.class);
     post(REVISION_KIND, "test.submit_type").to(TestSubmitType.class);