fix error with DNS resolution breaking endpoint iterator (#1393)

* re-implement GetEnumerator in EndPointCollection to be a bit more forgiving to concurrent changes during iteration

* poke release notes

docs/ReleaseNotes.md

@@ -23,6 +23,7 @@
 - fix: `ScriptEvaluateAsync` keyspace isolation (#1377 via gliljas)
 - fix: F# compatibility enhancements (#1386)
 - fix: improved `ScriptResult` null support (#1392)
+- fix: error with DNS resolution breaking endpoint iterator (#1393)
 - tests: better docker support for tests (#1389 via ejsmith; #1391)
 - tests: general test improvements (#1183, #1385, #1384)
 

src/StackExchange.Redis/EndPointCollection.cs

 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using System.Net;
@@ -8,7 +9,7 @@ namespace StackExchange.Redis
     /// <summary>
     /// A list of endpoints
     /// </summary>
-    public sealed class EndPointCollection : Collection<EndPoint>
+    public sealed class EndPointCollection : Collection<EndPoint>, IEnumerable, IEnumerable<EndPoint>
     {
         /// <summary>
         /// Create a new EndPointCollection
@@ -109,5 +110,21 @@ internal void SetDefaultPorts(int defaultPort)
                 }
             }
         }
+
+        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        IEnumerator<EndPoint> IEnumerable<EndPoint>.GetEnumerator() => GetEnumerator();
+
+        /// <inheritdoc/>
+        public new IEnumerator<EndPoint> GetEnumerator()
+        {
+            // this does *not* need to handle all threading scenarios; but we do
+            // want it to at least allow overwrites of existing endpoints without
+            // breaking the enumerator; in particular, this avoids a problem where
+            // ResolveEndPointsAsync swaps the addresses on us
+            for (int i = 0; i < Count; i++)
+            {
+                yield return this[i];
+            }
+        }
     }
 }

tests/StackExchange.Redis.Tests/Config.cs

@@ -385,5 +385,23 @@ public async Task TestAutomaticHeartbeat()
                 }
             }
         }
+
+        [Fact]
+        public void EndpointIteratorIsReliableOverChanges()
+        {
+            var eps = new EndPointCollection
+            {
+                { IPAddress.Loopback, 7999 },
+                { IPAddress.Loopback, 8000 },
+            };
+
+            using var iter = eps.GetEnumerator();
+            Assert.True(iter.MoveNext());
+            Assert.Equal(7999, ((IPEndPoint)iter.Current).Port);
+            eps[1] = new IPEndPoint(IPAddress.Loopback, 8001); // boom
+            Assert.True(iter.MoveNext());
+            Assert.Equal(8001, ((IPEndPoint)iter.Current).Port);
+            Assert.False(iter.MoveNext());
+        }
     }
 }