Prevent infinite loop when a subject from the trigger is missing

Given the trigger provides two subjects but only one subject was given via parameter and no
transformer was able to generate the second from the first, this method runs into an
infinite loop. This commit prevents this infinite loop.

[MAILPOET-4935]

mailpoet/lib/Automation/Engine/Control/SubjectTransformerHandler.php

@@ -96,12 +96,17 @@ class SubjectTransformerHandler {
    */
   public function provideAllSubjects(Trigger $trigger, Subject ...$subjects): array {
     $allSubjectsKeys = $this->subjectKeysForTrigger($trigger);
-    $allSubjectKeyTargets = array_diff($allSubjectsKeys, array_map(
+    $allSubjectKeyTargets = array_filter(
+      array_diff($allSubjectsKeys, array_map(
       function(Subject $subject): string {
         return $subject->getKey();
       },
       $subjects
-    ));
+      )),
+    function(string $target) use ($subjects): bool {
+      return $this->canSubjectsTransformTo($target, ...$subjects);
+    }
+    );
 
     $allSubjects = [];
     foreach ($subjects as $existingSubject) {
@@ -118,9 +123,17 @@ class SubjectTransformerHandler {
         }
       }
     }
-    while (count($allSubjects) < count($allSubjectsKeys)) {
+
-      $allSubjects = $this->provideAllSubjects($trigger, ...array_values($allSubjects));
+    $allSubjectsTransformed = true;
+    do {
+      foreach ($allSubjectKeyTargets as $key) {
+        if (!isset($allSubjects[$key])) {
+          $allSubjectsTransformed = false;
+          break;
         }
+      }
+      $allSubjects = $allSubjectsTransformed ? $allSubjects : $this->provideAllSubjects($trigger, ...array_values($allSubjects));
+    } while (!$allSubjectsTransformed);
     return array_values($allSubjects);
   }
 
@@ -164,4 +177,25 @@ class SubjectTransformerHandler {
 
     return $transformerChain;
   }
+
+  private function canSubjectsTransformTo(string $target, Subject ...$subjects): bool {
+    if (
+      in_array($target, array_map(
+      function(Subject $subject): string {
+        return $subject->getKey();
+      },
+      $subjects
+      ))
+    ) {
+      return true;
+    }
+
+    foreach ($subjects as $subject) {
+      $possibleTransformations = $this->getPossibleTransformations($subject->getKey());
+      if (in_array($target, $possibleTransformations)) {
+        return true;
+      }
+    }
+    return false;
+  }
 }