Support to retrieve the content of a file from a patch set via REST

By GET on
/changes/<change-id>/revisions/<revision-id>/files/<patch-id>/content
it is now possible to retrieve the content of a certain file from a
patch set. The file content is returned as base64 encoded string so
that also binary files can be handled.

Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
Change-Id: I429ce448c22b7b3b6ffd5503463676a798ea162a

Documentation/rest-api-changes.txt

@@ -1847,6 +1847,30 @@ describes the published comment.
   }
 ----
 
+[[get-content]]
+Get Content
+~~~~~~~~~~~
+[verse]
+'GET /changes/link:#change-id[\{change-id\}]/revisions/link:#revision-id[\{revision-id\}]/files/link:#patch-id[\{patch-id\}]/content'
+
+Gets the content of a file from a certain revision.
+
+.Request
+----
+  GET /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/revisions/674ac754f91e64a0efb8087e59a176484bd534d1/files/gerrit-server%2Fsrc%2Fmain%2Fjava%2Fcom%2Fgoogle%2Fgerrit%2Fserver%2Fproject%2FRefControl.java/content HTTP/1.0
+----
+
+The content is returned as base64 encoded string.
+
+.Response
+----
+  HTTP/1.1 200 OK
+  Content-Disposition: attachment
+  Content-Type: text/plain;charset=UTF-8
+
+  Ly8gQ29weXJpZ2h0IChDKSAyMDEwIFRoZSBBbmRyb2lkIE9wZW4gU291cmNlIFByb2plY...
+----
+
 [[set-reviewed]]
 Set Reviewed
 ~~~~~~~~~~~~

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

@@ -39,4 +39,8 @@ public class FileResource implements RestResource {
   Account.Id getAccountId() {
     return rev.getAccountId();
   }
+
+  RevisionResource getRevision() {
+    return rev;
+  }
 }

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

@@ -0,0 +1,89 @@
+// 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 com.google.common.base.Charsets;
+import com.google.common.io.BaseEncoding;
+import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
+import com.google.gerrit.extensions.restapi.RestReadView;
+import com.google.gerrit.extensions.restapi.StreamingResponse;
+import com.google.gerrit.reviewdb.client.Project;
+import com.google.gerrit.server.git.GitRepositoryManager;
+import com.google.inject.Inject;
+
+import org.eclipse.jgit.lib.ObjectId;
+import org.eclipse.jgit.lib.ObjectLoader;
+import org.eclipse.jgit.lib.Repository;
+import org.eclipse.jgit.revwalk.RevCommit;
+import org.eclipse.jgit.revwalk.RevWalk;
+import org.eclipse.jgit.treewalk.TreeWalk;
+
+import java.io.IOException;
+import java.io.OutputStream;
+import java.io.OutputStreamWriter;
+
+public class GetContent implements RestReadView<FileResource> {
+  private final GitRepositoryManager repoManager;
+
+  @Inject
+  GetContent(GitRepositoryManager repoManager) {
+    this.repoManager = repoManager;
+  }
+
+  @Override
+  public StreamingResponse apply(FileResource rsrc)
+      throws ResourceNotFoundException, IOException {
+    Project.NameKey project =
+        rsrc.getRevision().getControl().getProject().getNameKey();
+    Repository repo = repoManager.openRepository(project);
+    try {
+      RevWalk rw = new RevWalk(repo);
+      try {
+        RevCommit commit =
+            rw.parseCommit(ObjectId.fromString(rsrc.getRevision().getPatchSet()
+                .getRevision().get()));
+        TreeWalk tw =
+            TreeWalk.forPath(rw.getObjectReader(), rsrc.getPatchKey().get(),
+                commit.getTree().getId());
+        if (tw == null) {
+          throw new ResourceNotFoundException();
+        }
+        try {
+          final ObjectLoader loader = repo.open(tw.getObjectId(0));
+          return new StreamingResponse() {
+            @Override
+            public String getContentType() {
+              return "text/plain;charset=UTF-8";
+            }
+
+            @Override
+            public void stream(OutputStream out) throws IOException {
+              OutputStream b64Out = BaseEncoding.base64().encodingStream(
+                  new OutputStreamWriter(out, Charsets.UTF_8));
+              loader.copyTo(b64Out);
+              b64Out.close();
+            }
+          };
+        } finally {
+          tw.release();
+        }
+      } finally {
+        rw.release();
+      }
+    } finally {
+      repo.close();
+    }
+  }
+}

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

@@ -84,6 +84,7 @@ public class Module extends RestApiModule {
     child(REVISION_KIND, "files").to(Files.class);
     put(FILE_KIND, "reviewed").to(PutReviewed.class);
     delete(FILE_KIND, "reviewed").to(DeleteReviewed.class);
+    get(FILE_KIND, "content").to(GetContent.class);
 
     install(new FactoryModule() {
       @Override