Work around buggy MyersDiff by killing threads

The JGIt MyersDiff class contains a bug that triggers an infinite loop
on only certain input files.  Most source code is able to be processed
in a reasonable time bound, but some just steal a thread and never
return to the caller.

Implement a custom thread pool that is used to invoke MyersDiff for
the intraline difference data.  If the worker thread doesn't end
within the configured time bound (default of 5 seconds), Gerrit
removes the worker from the pool and tries to kill the worker with the
unsafe Thread.stop() method.

A custom thread pool is used to try and make Thread.stop() safe by
having the amount of data accessed by each worker thread be limited to
only the "constant" inputs supplied by the cache lookup request, and
the result that the thread would return.

If any locks are released early as a result of ThreadDeath going up
the worker thread stack at worst only the incoming or outgoing queues
that are private to that worker will be corrupted.  Since these queues
are private to the worker, and to the thread that is currently
borrowing this worker from the pool (and who is also now killing it),
we can safely ensure that the queues won't be touched after the
Thread.stop() request is made.  This wouldn't be true if we reused any
of the java.util.concurrent thread pool utilities.

This change doesn't actually fix the MyersDiff bug, so we're leaving
issue 487 open.  It does however reduce the impact by trying to abort
the runaway thread, and still show the file with intraline difference
support disabled on just that one file.

Bug: issue 487
Change-Id: I6cbfdd0acc6f7e612a29ed789efe9da591a45273
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-gwtui/src/main/java/com/google/gerrit/client/patches/PatchConstants.java

@@ -31,6 +31,7 @@ public interface PatchConstants extends Constants {
 
   String patchHistoryTitle();
   String disabledOnLargeFiles();
+  String intralineFailure();
 
   String upToChange();
   String linePrev();

gerrit-gwtui/src/main/java/com/google/gerrit/client/patches/PatchConstants.properties

@@ -13,6 +13,7 @@ patchHeaderOld = Old Version
 patchHeaderNew = New Version
 patchHistoryTitle = Patch History
 disabledOnLargeFiles = Disabled on very large source files.
+intralineFailure = Intraline difference not available due to server error.
 
 upToChange = Up to change
 linePrev = Previous line

gerrit-gwtui/src/main/java/com/google/gerrit/client/patches/PatchScreen.java

@@ -15,6 +15,7 @@
 package com.google.gerrit.client.patches;
 
 import com.google.gerrit.client.Dispatcher;
+import com.google.gerrit.client.ErrorDialog;
 import com.google.gerrit.client.Gerrit;
 import com.google.gerrit.client.RpcStatus;
 import com.google.gerrit.client.changes.CommitMessageBlock;
@@ -140,6 +141,7 @@ public abstract class PatchScreen extends Screen implements
   /** Keys that cause an action on this screen */
   private KeyCommandSet keysNavigation;
   private HandlerRegistration regNavigation;
+  private boolean intralineFailure;
 
   /**
    * How this patch should be displayed in the patch screen.
@@ -461,6 +463,17 @@ public abstract class PatchScreen extends Screen implements
       settingsPanel.getReviewedCheckBox().setValue(true);
       setReviewedByCurrentUser(true /* reviewed */);
     }
+
+    intralineFailure = isFirst && script.hasIntralineFailure();
+  }
+
+  @Override
+  public void onShowView() {
+    super.onShowView();
+    if (intralineFailure) {
+      intralineFailure = false;
+      new ErrorDialog(PatchUtil.C.intralineFailure()).show();
+    }
   }
 
   private void showPatch(final boolean showPatch) {