Fix file upload HTTP client not handling DNS changes (#3266)

By setting the connection lease timeout to 5 minutes (defaults to infinite), HTTP operations will perform DNS lookups once every 5 minutes. This allows a client to continue performing file upload operations even if the service changes DNS due to failover

iothub/device/src/Transport/Http/HttpClientHelper.cs

@@ -43,6 +43,22 @@ internal sealed class HttpClientHelper : IHttpClientHelper
         private readonly ProductInfo _productInfo;
         private readonly bool _isClientPrimaryTransportHandler;
 
+        // These default values are consistent with Azure.Core default values:
+        // https://github.com/Azure/azure-sdk-for-net/blob/7e3cf643977591e9041f4c628fd4d28237398e0b/sdk/core/Azure.Core/src/Pipeline/ServicePointHelpers.cs#L28
+        private const int DefaultMaxConnectionsPerServer = 50;
+
+        // How long, in milliseconds, a given cached TCP connection created by this client's HTTP layer will live before being closed.
+        // If this value is set to any negative value, the connection lease will be infinite. If this value is set to 0, then the TCP connection will close after
+        // each HTTP request and a new TCP connection will be opened upon the next request.
+        //
+        // By closing cached TCP connections and opening a new one upon the next request, the underlying HTTP client has a chance to do a DNS lookup
+        // to validate that it will send the requests to the correct IP address. While it is atypical for a given IoT hub to change its IP address, it does
+        // happen when a given IoT hub fails over into a different region.
+        //
+        // This default value is consistent with the default value used in Azure.Core
+        // https://github.com/Azure/azure-sdk-for-net/blob/7e3cf643977591e9041f4c628fd4d28237398e0b/sdk/core/Azure.Core/src/Pipeline/ServicePointHelpers.cs#L29
+        private static readonly TimeSpan s_defaultConnectionLeaseTimeout = TimeSpan.FromMinutes(5);
+
         public HttpClientHelper(
             Uri baseAddress,
             IAuthorizationProvider authenticationHeaderProvider,
@@ -59,6 +75,9 @@ public HttpClientHelper(
             _authenticationHeaderProvider = authenticationHeaderProvider;
             _defaultErrorMapping = new ReadOnlyDictionary<HttpStatusCode, Func<HttpResponseMessage, Task<Exception>>>(defaultErrorMapping);
 
+            ServicePoint servicePoint = ServicePointManager.FindServicePoint(_baseAddress);
+            servicePoint.ConnectionLeaseTimeout = s_defaultConnectionLeaseTimeout.Milliseconds;
+
 #if NET451
             TlsVersions.Instance.SetLegacyAcceptableVersions();
 
@@ -105,6 +124,8 @@ public HttpClientHelper(
                 _httpClientHandler.Proxy = proxy;
             }
 
+            _httpClientHandler.MaxConnectionsPerServer = DefaultMaxConnectionsPerServer;
+
             _httpClientObj = new HttpClient(_httpClientHandler);
 #endif
 
@@ -594,5 +615,5 @@ private static async Task<T> ReadAsAsync<T>(HttpContent content, CancellationTok
         }
 
 #endif
-        }
+    }
 }