Fixed cleanup of propagated SshScopes

The RequestScopePropagator handles RequestCleanup independently of
the original context. However, SshScope.Context.subContext() was
chaining the cleanup in the parent. This would cause an exception
if the parent scope happened to finish before the sub-context. Since
cleanup is already handled correctly by the RequestScopePropagator,
don't register the continued contect with the original context cleanup.

Change-Id: Ie508d1921050887b95dd18bc5806c17e4894391f

gerrit-sshd/src/main/java/com/google/gerrit/sshd/SshScope.java

@@ -39,7 +39,7 @@ class SshScope {
     volatile long started;
     volatile long finished;
 
-    Context(final SshSession s, final String c) {
+    private Context(final SshSession s, final String c, final long at) {
       cleanup = new RequestCleanup();
       session = s;
       commandLine = c;
@@ -47,25 +47,17 @@ class SshScope {
       map = new HashMap<Key<?>, Object>();
       map.put(RC_KEY, cleanup);
 
-      final long now = System.currentTimeMillis();
-      created = now;
-      started = now;
-      finished = now;
+      created = started = finished = at;
     }
 
     private Context(Context p, SshSession s, String c) {
-      cleanup = new RequestCleanup();
-      session = s;
-      commandLine = c;
-
-      map = new HashMap<Key<?>, Object>();
-      map.put(RC_KEY, cleanup);
-
-      created = p.created;
+      this(s, c, p.created);
       started = p.started;
       finished = p.finished;
+    }
 
-      p.cleanup.add(cleanup);
+    Context(final SshSession s, final String c) {
+      this(s, c, System.currentTimeMillis());
     }
 
     String getCommandLine() {
@@ -87,7 +79,9 @@ class SshScope {
     }
 
     synchronized Context subContext(SshSession newSession, String newCommandLine) {
-      return new Context(this, newSession, newCommandLine);
+      Context ctx = new Context(this, newSession, newCommandLine);
+      cleanup.add(ctx.cleanup);
+      return ctx;
     }
   }
 
@@ -112,7 +106,9 @@ class SshScope {
 
     @Override
     protected Context continuingContext(Context ctx) {
-      return ctx.subContext(ctx.getSession(), ctx.getCommandLine());
+      // The cleanup is not chained, since the RequestScopePropagator executors
+      // the Context's cleanup when finished executing.
+      return new Context(ctx, ctx.getSession(), ctx.getCommandLine());
     }
   }