Fix PriorityUnboundedBlockingQueue priority translation

Summary: [Folly] Fix `PriorityUnboundedBlockingQueue` priority translation from `Executor` priorities to offsets as used by `PriorityUnboundedBlockingQueueSet`.

Reviewed By: andriigrynenko

Differential Revision: D15619435

fbshipit-source-id: 1c5d4d68dfde7f9db07d64f40e73717b48a05ed1

folly/executors/task_queue/PriorityUnboundedBlockingQueue.h

@@ -76,11 +76,11 @@ class PriorityUnboundedBlockingQueue : public BlockingQueue<T> {
 
  private:
   size_t translatePriority(int8_t const priority) {
-    int8_t const priorities = queue_.priorities();
-    int8_t const value = (priorities - 1) / 2 + priority;
-    int8_t const lo = 0;
-    int8_t const hi = priorities - 1;
-    return hi - constexpr_clamp(value, lo, hi);
+    size_t const priorities = queue_.priorities();
+    assert(priorities <= 255);
+    int8_t const hi = (priorities + 1) / 2 - 1;
+    int8_t const lo = hi - (priorities - 1);
+    return hi - constexpr_clamp(priority, lo, hi);
   }
 
   T dequeue() {

folly/executors/task_queue/test/PriorityUnboundedBlockingQueueTest.cpp

@@ -79,7 +79,7 @@ TEST_F(PriorityUnboundedBlockingQueueTest, priority_order) {
   EXPECT_EQ(0, q.size());
 }
 
-// Since PrioritizedBlockingQueueSetTest implements folly::BlockingQueue<T>,
+// Since PriorityUnboundedBlockingQueue implements folly::BlockingQueue<T>,
 // addWithPriority method has to accept priority as int_8. This means invalid
 // values for priority (such as negative or very large numbers) might get
 // passed. Verify this behavior.
@@ -91,3 +91,12 @@ TEST_F(PriorityUnboundedBlockingQueueTest, invalid_priorities) {
   EXPECT_EQ(q.take(), 2);
   EXPECT_EQ(q.take(), 1);
 }
+
+TEST_F(PriorityUnboundedBlockingQueueTest, invalid_priorities_edge) {
+  PriorityUnboundedBlockingQueue<int> q(2);
+  q.addWithPriority(1, Executor::LO_PRI);
+  q.addWithPriority(2, Executor::HI_PRI);
+
+  EXPECT_EQ(q.take(), 2);
+  EXPECT_EQ(q.take(), 1);
+}