Postpone writing to output stream in diff command

Without this, the gzip filter would write the first bytes to the
response output stream, before the diff command is triggered and
potential exceptions may be thrown. When exceptions are thrown too late,
filters like the GZip filter may already have sent bytes to the response
stream. Afterwards this cannot be undone and the response created by an
exception mapper may not be valid anymore.

scm-plugins/scm-hg-plugin/src/main/java/sonia/scm/repository/spi/HgDiffCommand.java

@@ -39,13 +39,12 @@ import com.google.common.base.Strings;
 import com.google.common.io.ByteStreams;
 import com.google.common.io.Closeables;
 import sonia.scm.repository.Repository;
+import sonia.scm.repository.api.DiffCommandBuilder;
 import sonia.scm.repository.api.DiffFormat;
 import sonia.scm.repository.spi.javahg.HgDiffInternalCommand;
 import sonia.scm.web.HgUtil;
 
-import java.io.IOException;
 import java.io.InputStream;
-import java.io.OutputStream;
 
 //~--- JDK imports ------------------------------------------------------------
 
@@ -71,8 +70,7 @@ public class HgDiffCommand extends AbstractCommand implements DiffCommand
   //~--- get methods ----------------------------------------------------------
 
   @Override
-  public void getDiffResult(DiffCommandRequest request, OutputStream output)
-    throws IOException
+  public DiffCommandBuilder.OutputStreamConsumer getDiffResult(DiffCommandRequest request)
   {
     com.aragost.javahg.Repository hgRepo = open();
 
@@ -86,26 +84,22 @@ public class HgDiffCommand extends AbstractCommand implements DiffCommand
 
     cmd.change(HgUtil.getRevision(request.getRevision()));
 
-    InputStream inputStream = null;
+    return output -> {
+      InputStream inputStream = null;
 
-    try
-    {
+      try {
 
-      if (!Strings.isNullOrEmpty(request.getPath()))
-      {
-        inputStream = cmd.stream(hgRepo.file(request.getPath()));
+        if (!Strings.isNullOrEmpty(request.getPath())) {
+          inputStream = cmd.stream(hgRepo.file(request.getPath()));
+        } else {
+          inputStream = cmd.stream();
+        }
+
+        ByteStreams.copy(inputStream, output);
+
+      } finally {
+        Closeables.close(inputStream, true);
       }
-      else
-      {
-        inputStream = cmd.stream();
-      }
-
-      ByteStreams.copy(inputStream, output);
-
-    }
-    finally
-    {
-      Closeables.close(inputStream, true);
-    }
+    };
   }
 }