enh: reply to yjs sync step 1 as the server

The yjs sync protocol has three types of messages:
* Sync step 1 (Question)
* Sync step 2 (Answer)
* Update

The different parties will exchange questions and answers to sync.
Once all parties are synced they exchange updates containing the latest changes.

We used to relay all questions to all parties
which resulted in a lot of unneccessary traffic and db entries.

Instead only store the answers and updates on the server.
Whenever any question comes in reply with the log of all the other messages.
This way questions are only answered once (by the server)
and we can still stick to a "stupid" server
that does not need to understand state vectors and so on.

Signed-off-by: Max <max@nextcloud.com>

lib/Service/ApiService.php

@@ -177,11 +177,11 @@ public function push($documentId, $sessionId, $sessionToken, $version, $steps, $
 		$file = $this->documentService->getFileForSession($session, $token);
 		if (!$this->documentService->isReadOnly($file, $token)) {
 			try {
-				$steps = $this->documentService->addStep($documentId, $sessionId, $steps, $version);
+				$result = $this->documentService->addStep($documentId, $sessionId, $steps, $version);
 			} catch (InvalidArgumentException $e) {
 				return new DataResponse($e->getMessage(), 422);
 			}
-			return new DataResponse($steps);
+			return new DataResponse($result);
 		}
 		return new DataResponse([], 403);
 	}

lib/Service/DocumentService.php

@@ -183,9 +183,39 @@ public function get($documentId) {
 	 * @throws InvalidArgumentException
 	 */
 	public function addStep($documentId, $sessionId, $steps, $version): array {
+		$stepsToInsert = [];
+		$querySteps = [];
+		$getStepsSinceVersion = null;
+		$newVersion = $version;
+		foreach ($steps as $step) {
+			if ($step > "AAE") {
+				array_push($stepsToInsert, $step);
+			} else {
+				array_push($querySteps, $step);
+			}
+		}
+		if (sizeof($stepsToInsert) > 0) {
+			$newVersion = $this->insertSteps($documentId, $sessionId, $stepsToInsert, $version);
+		}
+		$getStepsSinceVersion = sizeof($querySteps) > 0 ? 0 : $version;
+		return [
+			'steps' => $this->getSteps($documentId, $getStepsSinceVersion),
+			'version' => $newVersion
+		];
+	}
+
+	/**
+	 * @param $documentId
+	 * @param $sessionId
+	 * @param $steps
+	 * @param $version
+	 * @return string
+	 * @throws DoesNotExistException
+	 * @throws InvalidArgumentException
+	 */
+	private function insertSteps($documentId, $sessionId, $steps, $version): string {
 		$document = null;
 		$stepsVersion = null;
-
 		try {
 			$document = $this->documentMapper->find($documentId);
 			$stepsJson = json_encode($steps);
@@ -202,7 +232,7 @@ public function addStep($documentId, $sessionId, $steps, $version): array {
 			$step->setVersion($newVersion);
 			$this->stepMapper->insert($step);
 			// TODO write steps to cache for quicker reading
-			return $steps;
+			return $newVersion;
 		} catch (DoesNotExistException $e) {
 			throw $e;
 		} catch (\Throwable $e) {