Refuse workflows that move cards in a loop

Every enabled workflow is an edge "source stack -> target stack";
LoopDetector finds circles in that graph, a workflow whose source is its
own target being the shortest. Without a check, two workflows A -> B and
B -> A move the same cards back and forth on every run, even within a
single run, with a notice for each move. The trigger date is ignored on
purpose: by start date one way and by due date the other, a card with
both dates passed still circles. Chains stay allowed.

The controller rejects saving an enabled workflow that would close a loop
and names the workflows involved. Workflows stored before this check are
skipped by the runner (logged, not disabled) and flagged in the settings
list via a new `loop` field from index().

lib/Controller/WorkflowController.php

@@ -8,6 +8,7 @@ use OCA\WorkflowDeckAutomation\Db\Workflow;
 use OCA\WorkflowDeckAutomation\Db\WorkflowMapper;
 use OCA\WorkflowDeckAutomation\Service\DeckIntegrationService;
 use OCA\WorkflowDeckAutomation\Service\DeckUnavailableException;
+use OCA\WorkflowDeckAutomation\Service\LoopDetector;
 use OCA\WorkflowDeckAutomation\Service\NotificationPolicy;
 use OCA\WorkflowDeckAutomation\Service\TriggerDate;
 use OCP\AppFramework\Db\DoesNotExistException;
@@ -48,7 +49,17 @@ class WorkflowController extends OCSController {
 	#[NoAdminRequired]
 	public function index(): DataResponse {
 		$workflows = $this->workflowMapper->findAllForBoards($this->ownedBoardIds());
-		return new DataResponse(array_map(static fn (Workflow $w) => $w->jsonSerialize(), $workflows));
+
+		// validate() keeps new loops out, but workflows stored before that
+		// check can still form one; the runner skips them, the list says why.
+		$looping = LoopDetector::findLoopingWorkflows(LoopDetector::edgesOf(
+			array_filter($workflows, static fn (Workflow $w) => (bool)$w->getEnabled()),
+		));
+
+		return new DataResponse(array_map(
+			static fn (Workflow $w) => $w->jsonSerialize() + ['loop' => in_array((int)$w->getId(), $looping, true)],
+			$workflows,
+		));
 	}
 
 	#[NoAdminRequired]
@@ -62,7 +73,7 @@ class WorkflowController extends OCSController {
 		string $notifyChannel = NotificationPolicy::CHANNEL_MAIL,
 		bool $enabled = true,
 	): DataResponse {
-		$this->validate($title, $boardId, $sourceStackId, $targetStackId, $triggerDate, $notifyMode, $notifyChannel);
+		$this->validate(null, $title, $boardId, $sourceStackId, $targetStackId, $triggerDate, $notifyMode, $notifyChannel, $enabled);
 
 		$workflow = new Workflow();
 		$workflow->setUserId($this->currentUserId());
@@ -85,7 +96,7 @@ class WorkflowController extends OCSController {
 		bool $enabled = true,
 	): DataResponse {
 		$workflow = $this->findOwnWorkflow($id);
-		$this->validate($title, $boardId, $sourceStackId, $targetStackId, $triggerDate, $notifyMode, $notifyChannel);
+		$this->validate($id, $title, $boardId, $sourceStackId, $targetStackId, $triggerDate, $notifyMode, $notifyChannel, $enabled);
 
 		// Kept in step with the board owner so the job runs as, and mails,
 		// the right user; validate() made sure the editor owns the board.
@@ -161,7 +172,10 @@ class WorkflowController extends OCSController {
 		}
 	}
 
-	private function validate(string $title, int $boardId, int $sourceStackId, int $targetStackId, string $triggerDate, string $notifyMode, string $notifyChannel): void {
+	/**
+	 * @param ?int $id the workflow being changed, `null` for a new one
+	 */
+	private function validate(?int $id, string $title, int $boardId, int $sourceStackId, int $targetStackId, string $triggerDate, string $notifyMode, string $notifyChannel, bool $enabled): void {
 		if (trim($title) === '') {
 			throw new OCSBadRequestException('Bitte einen Titel angeben.');
 		}
@@ -208,6 +222,37 @@ class WorkflowController extends OCSController {
 		if (!in_array($sourceStackId, $stackIds, true) || !in_array($targetStackId, $stackIds, true)) {
 			throw new OCSBadRequestException('Der gewählte Stapel existiert auf diesem Board nicht.');
 		}
+
+		// A disabled workflow moves nothing and can close no loop; it is
+		// checked again when it gets switched on, which also goes through here.
+		if ($enabled) {
+			$this->assertNoLoop($id, $boardId, $sourceStackId, $targetStackId);
+		}
+	}
+
+	/**
+	 * Rejects a workflow whose target stack leads back to its source through
+	 * the board's other enabled workflows — cards would circle forever. See
+	 * LoopDetector.
+	 */
+	private function assertNoLoop(?int $id, int $boardId, int $sourceStackId, int $targetStackId): void {
+		$others = [];
+		foreach ($this->workflowMapper->findAllForBoards([$boardId]) as $other) {
+			if ($other->getId() !== $id && $other->getEnabled()) {
+				$others[(int)$other->getId()] = $other;
+			}
+		}
+
+		$path = LoopDetector::findPath(LoopDetector::edgesOf($others), $targetStackId, $sourceStackId);
+		if ($path === null) {
+			return;
+		}
+
+		$titles = array_map(static fn (int $otherId) => '„' . $others[$otherId]->getTitle() . '“', $path);
+		throw new OCSBadRequestException(sprintf(
+			'Zusammen mit %s würde dieser Workflow eine Schleife bilden, in der Karten immer wieder hin- und hergeschoben werden. Wähle einen anderen Ziel-Stapel oder deaktiviere einen der Workflows.',
+			implode(', ', $titles),
+		));
 	}
 
 	private function applyFields(