Fallback logic for unknown priorities

Author: nickolas-pohilets

stdlib/public/Concurrency/Actor.cpp

@@ -1000,19 +1000,26 @@ static void setNextJob(Job *job, Job* next) {
 enum { NumPriorityBuckets = 5 };
 
 static int getPriorityIndex(JobPriority priority) {
-  switch (priority) {
-    case JobPriority::UserInteractive:
-      return 0;
-    case JobPriority::UserInitiated:
-      return 1;
-    case JobPriority::Unspecified:
-      assert(false);
-    case JobPriority::Default:
-      return 2;
-    case JobPriority::Utility:
-      return 3;
-    case JobPriority::Background:
-      return 4;
+  // Any unknown priorities will be rounded up to a known one.
+  // Priorities higher than UserInteractive are clamped to UserInteractive.
+  // Jobs of unknown priorities will end up in the same bucket as jobs of a
+  // corresponding known priority. Within the bucket they will be sorted in FIFO
+  // order.
+  if (priority > JobPriority::UserInitiated) {
+    // UserInteractive and higher
+    return 0;
+  } else if (priority > JobPriority::Default) {
+    // UserInitiated
+    return 1;
+  } else if (priority > JobPriority::Utility) {
+    // Default
+    return 2;
+  } else if (priority > JobPriority::Background) {
+    // Utility
+    return 3;
+  } else {
+    // Background and lower
+    return 4;
   }
 }