Avoid returning incomplete TypeAdapters in concurrent calls

[mwalkerr]

Resolves #1829
Resolves #625 partially

In the current implementation, a TypeAdapter may be added to the typeTokenCache despite having a FutureTypeAdapter with a null delegate as part of its definition. This would occasionally cause us to see an IllegalStateException from GSON. This would happen when bringing up an application in a docker container and then running parallelized API tests against it. Some of the objects the application deserializes are recursively defined such as A -> B -> A. I believe what was happening was that one thread would start to deserialize A and call to getAdapter(A) which would call to getAdapter(B) which would call getAdapter(A), which would return the FutureTypeAdapter for A. Before the first thread is able to update the delegate for the FutureTypeAdapter for A, another thread tries to deserialize B, calls getAdapter(B), and retrieves the entry added by the first thread which contains a TypeAdapter for A which is a FutureTypeAdapter with a null delegate

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[mwalkerr]

I used the following test to verify the issue and the solution:

package com.google.gson.regression;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import junit.framework.TestCase;

import java.lang.reflect.Field;
import java.util.Map;
import java.util.concurrent.CyclicBarrier;

public class RecursiveTypeRaceConditionTest extends TestCase {
  private final Gson gson = new Gson();

  public void testNullSafeBugSerialize() throws Exception {
    for (int i=0;i<100000;i++){
      Field f = gson.getClass().getDeclaredField("typeTokenCache");
      f.setAccessible(true);
      ((Map) f.get(gson)).clear();
      final CyclicBarrier gate = new CyclicBarrier(3);
      final boolean[] exception = {false};
      Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
          try {
            gate.await();
            gson.getAdapter(TypeToken.get(A.class)).fromJson("{\"b\": {\"a\":{}}}");
          } catch (Exception e) {
            exception[0] = true;
            e.printStackTrace();
          }
        }
      });
      Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
          try {
            gate.await();
            gson.getAdapter(TypeToken.get(B.class)).fromJson("{\"a\": {\"b\":{}}}");
          } catch (Exception e) {
            exception[0] = true;
            e.printStackTrace();
          }
        }
      });
      t1.start();
      t2.start();
      gate.await();
      t1.join();
      t2.join();
      assertFalse(exception[0]);
    }
  }

  private static final class A {
    B b;
  }
  private static final class B {
    A a;
  }
}

[Marcono1234]

Thanks a lot for you work on this! This has now be fixed by #1832, but thank you nonetheless!