Test interleaved enqueue/dequeue.

Author: sharpjs

DependencyQueue.Tests/DependencyQueueTests.cs

@@ -825,6 +825,46 @@ public void Complete_NotEnqueued()
         queue.ShouldHaveTopic("y", providedBy: [itemY]);
     }
 
+    [Test]
+    public void DequeueThenEnqueue()
+    {
+        using var queue = new Queue();
+
+        var itemA = queue.Enqueue("a", value: new());
+        var itemB = queue.Enqueue("b", value: new(), requires: ["a"]);
+
+        queue.Dequeue().ShouldBeSameAs(itemA);
+
+        // Pretend that processing of itemA enqueues itemC
+        var itemC = queue.Enqueue("c", value: new(), requires: ["b"]);
+
+        // Dequeueing should still work
+        queue.Complete(itemA);
+        queue.Dequeue().ShouldBeSameAs(itemB);
+        queue.Complete(itemB);
+        queue.Dequeue().ShouldBeSameAs(itemC);
+
+        // Pretend that processing of itemC enqueues itemD, which is invalid
+        //                                            unprovided ↓
+        var itemD = queue.Enqueue("d", value: new(), requires: ["k"]);
+
+        // Dequeueing should report the invalidity
+        queue.Complete(itemC);
+        Should.Throw<InvalidDependencyQueueException>(() => queue.Dequeue())
+            .Errors
+            .ShouldHaveSingleItem()
+            .ShouldBeOfType<DependencyQueueUnprovidedTopicError<Value>>()
+            .Topic.Name.ShouldBe("k");
+
+        // Now enqueue itemK, which resolves the invalidity
+        var itemK = queue.Enqueue("k", value: new());
+
+        // Dequeueing should now work
+        queue.Dequeue().ShouldBeSameAs(itemK); queue.Complete(itemK);
+        queue.Dequeue().ShouldBeSameAs(itemD); queue.Complete(itemD);
+        queue.Dequeue().ShouldBeNull();
+    }
+
     [Test]
     public void Clear_Ok()
     {

README.md

@@ -152,6 +152,9 @@ By default, topic names use case-sensitive ordinal comparison.  To use
 different comparison rules, pass a `StringComparer` instance to the queue
 constructor.
 
+While typical uses of DependencyQueue involve separate enqueue and dequeue
+phases, the queue supports interleaved enqueue and dequeue operations.
+
 ### Dequeueing Items
 
 To dequeue (remove) an item, call `Dequeue()` or `DequeueAsync()`.  Both