Better thread safety for session building <-> use.

libaxolotl/src/main/java/org/whispersystems/libaxolotl/SessionBuilder.java

@@ -205,7 +205,7 @@ public class SessionBuilder {
    *                                                                  trusted.
    */
   public void process(PreKeyBundle preKey) throws InvalidKeyException, UntrustedIdentityException {
-
+    synchronized (SessionCipher.SESSION_LOCK) {
       if (!identityKeyStore.isTrustedIdentity(recipientId, preKey.getIdentityKey())) {
         throw new UntrustedIdentityException();
       }
@@ -252,6 +252,7 @@ public class SessionBuilder {
       sessionStore.storeSession(recipientId, deviceId, sessionRecord);
       identityKeyStore.saveIdentity(recipientId, preKey.getIdentityKey());
     }
+  }
 
   /**
    * Build a new session from a {@link org.whispersystems.libaxolotl.protocol.KeyExchangeMessage}
@@ -264,7 +265,7 @@ public class SessionBuilder {
   public KeyExchangeMessage process(KeyExchangeMessage message)
       throws InvalidKeyException, UntrustedIdentityException, StaleKeyExchangeException
   {
-
+    synchronized (SessionCipher.SESSION_LOCK) {
       if (!identityKeyStore.isTrustedIdentity(recipientId, message.getIdentityKey())) {
         throw new UntrustedIdentityException();
       }
@@ -276,6 +277,7 @@ public class SessionBuilder {
 
       return responseMessage;
     }
+  }
 
   private KeyExchangeMessage processInitiate(KeyExchangeMessage message) throws InvalidKeyException {
     int           flags         = KeyExchangeMessage.RESPONSE_FLAG;
@@ -375,6 +377,7 @@ public class SessionBuilder {
    * @return the KeyExchangeMessage to deliver.
    */
   public KeyExchangeMessage process() {
+    synchronized (SessionCipher.SESSION_LOCK) {
       try {
         int             sequence         = KeyHelper.getRandomSequence(65534) + 1;
         int             flags            = KeyExchangeMessage.INITIATE_FLAG;
@@ -393,6 +396,7 @@ public class SessionBuilder {
         throw new AssertionError(e);
       }
     }
+  }
 
 
 }

libaxolotl/src/main/java/org/whispersystems/libaxolotl/SessionCipher.java

@@ -59,7 +59,7 @@ import static org.whispersystems.libaxolotl.state.SessionState.UnacknowledgedPre
  */
 public class SessionCipher {
 
-  private static final Object SESSION_LOCK = new Object();
+  public static final Object SESSION_LOCK = new Object();
 
   private final SessionStore   sessionStore;
   private final SessionBuilder sessionBuilder;