[android] support attachments arriving *before* the message containing them

briar-android/src/main/java/org/briarproject/briar/android/attachment/AttachmentRetriever.java

@@ -12,6 +12,9 @@ import org.briarproject.briar.api.messaging.PrivateMessageHeader;
 import java.io.InputStream;
 import java.util.List;
 
+import androidx.annotation.Nullable;
+
+
 @NotNullByDefault
 public interface AttachmentRetriever {
 
@@ -33,6 +36,14 @@ public interface AttachmentRetriever {
 	 */
 	AttachmentItem createAttachmentItem(Attachment a, boolean needsSize);
 
+	/**
+	 * Load an {@link AttachmentItem} from the database.
+	 *
+	 * @return a pair of the {@link MessageId} of the conversation message
+	 * and the {@link AttachmentItem}
+	 * or {@code null} when the conversation message did not yet arrive.
+	 */
+	@Nullable
 	@DatabaseExecutor
 	Pair<MessageId, AttachmentItem> loadAttachmentItem(MessageId attachmentId)
 			throws DbException;

briar-android/src/main/java/org/briarproject/briar/android/attachment/AttachmentRetrieverImpl.java

@@ -22,6 +22,8 @@ import java.util.logging.Logger;
 
 import javax.inject.Inject;
 
+import androidx.annotation.Nullable;
+
 import static java.util.logging.Level.WARNING;
 import static java.util.logging.Logger.getLogger;
 import static org.briarproject.briar.android.attachment.AttachmentItem.State.AVAILABLE;
@@ -103,11 +105,12 @@ class AttachmentRetrieverImpl implements AttachmentRetriever {
 	}
 
 	@Override
+	@Nullable
 	@DatabaseExecutor
 	public Pair<MessageId, AttachmentItem> loadAttachmentItem(
 			MessageId attachmentId) throws DbException {
 		UnavailableItem unavailableItem = unavailableItems.get(attachmentId);
-		if (unavailableItem == null) throw new AssertionError();
+		if (unavailableItem == null) return null;
 
 		MessageId conversationMessageId =
 				unavailableItem.getConversationMessageId();

briar-android/src/main/java/org/briarproject/briar/android/conversation/ConversationActivity.java

@@ -643,16 +643,21 @@ public class ConversationActivity extends BriarActivity
 		runOnDbThread(() -> {
 			for (AttachmentItem item : items) {
 				if (item.getState() == LOADING)
-					loadMessageAttachment(item.getMessageId());
+					loadMessageAttachment(item.getMessageId(), false);
 			}
 		});
 	}
 
 	@DatabaseExecutor
-	private void loadMessageAttachment(MessageId attachmentId) {
+	private void loadMessageAttachment(MessageId attachmentId,
+			boolean allowedToFail) {
 		try {
-			Pair<MessageId, AttachmentItem> pair =
+			Pair<MessageId, AttachmentItem> pair = attachmentRetriever
-					attachmentRetriever.loadAttachmentItem(attachmentId);
+					.loadAttachmentItem(attachmentId);
+			if (pair == null && allowedToFail) {
+				LOG.warning("Attachment arrived before message");
+				return;
+			} else if (pair == null) throw new AssertionError();
 			MessageId conversationMessageId = pair.getFirst();
 			AttachmentItem item = pair.getSecond();
 			updateMessageAttachment(conversationMessageId, item);
@@ -743,7 +748,9 @@ public class ConversationActivity extends BriarActivity
 
 	@UiThread
 	private void onAttachmentReceived(MessageId attachmentId) {
-		runOnDbThread(() -> loadMessageAttachment(attachmentId));
+		// This is allowed to fail, because the conversation message
+		// might arrive *after* the attachment.
+		runOnDbThread(() -> loadMessageAttachment(attachmentId, true));
 	}
 
 	@UiThread