[Backport 7.7] Fix check for agressive connectionlimit (#4667)

* Fix check for agressive connectionlimit

In dotnet/runtime#22366 we found that if
HttpClient is using the curl handler it will lead to TCP connections
bleeding. Our DefaultConnectionLimit is very restrictive if this is
true. Our check however is too lenient and will default to true always
on .NET core since netcoreapp still ships with CurlHandler as recent as
`3.1.x`

* remove unused reference

* Detect if CurlHandler exists

This commit updates the implementation of determining if
a conservative default connection limit should be set by also checking
if CurlHandler exists when UseSocketsHttpHandler exists and is
disabled.

Co-authored-by: Russ Cam <russ.cam@forloop.co.uk>

Co-authored-by: Martijn Laarman <Mpdreamz@gmail.com>
Co-authored-by: Russ Cam <russ.cam@forloop.co.uk>

Author: 3 people

src/Elasticsearch.Net/Configuration/ConnectionConfiguration.cs

@@ -23,11 +23,42 @@ namespace Elasticsearch.Net
 	/// </summary>
 	public class ConnectionConfiguration : ConnectionConfiguration<ConnectionConfiguration>
 	{
-#if DOTNETCORE
-		private static bool IsCurlHandler { get; } = typeof(HttpClientHandler).Assembly.GetType("System.Net.Http.CurlHandler") != null;
+		/// <summary>
+		/// Detects whether we are running on .NET Core with CurlHandler.
+		/// If this is true, we will set a very restrictive <see cref="DefaultConnectionLimit"/>
+		/// As the old curl based handler is known to bleed TCP connections:
+		/// <para>https://github.com/dotnet/runtime/issues/22366</para>
+		/// </summary>
+        private static bool UsingCurlHandler
+		{
+			get
+			{
+#if !DOTNETCORE
+				return false;
 #else
-		private static bool IsCurlHandler { get; } = false;
+				var curlHandlerExists = typeof(HttpClientHandler).Assembly.GetType("System.Net.Http.CurlHandler") != null;
+				if (!curlHandlerExists) return false;
+
+				var socketsHandlerExists = typeof(HttpClientHandler).Assembly.GetType("System.Net.Http.SocketsHttpHandler") != null;
+				// running on a .NET core version with CurlHandler, before the existence of SocketsHttpHandler.
+				// Must be using CurlHandler.
+				if (!socketsHandlerExists) return true;
+
+				if (AppContext.TryGetSwitch("System.Net.Http.UseSocketsHttpHandler", out var isEnabled))
+					return !isEnabled;
+
+				var environmentVariable =
+					Environment.GetEnvironmentVariable("DOTNET_SYSTEM_NET_HTTP_USESOCKETSHTTPHANDLER");
+
+				// SocketsHandler exists and no environment variable exists to disable it.
+				// Must be using SocketsHandler and not CurlHandler
+				if (environmentVariable == null) return false;
+
+				return environmentVariable.Equals("false", StringComparison.OrdinalIgnoreCase) ||
+					environmentVariable.Equals("0");
 #endif
+			}
+		}
 
 		/// <summary>
 		/// The default ping timeout. Defaults to 2 seconds
@@ -52,7 +83,7 @@ public class ConnectionConfiguration : ConnectionConfiguration<ConnectionConfigu
 		/// <para>Except for <see cref="HttpClientHandler"/> implementations based on curl, which defaults to <see cref="Environment.ProcessorCount"/></para>
 #endif
 		/// </summary>
-		public static readonly int DefaultConnectionLimit = IsCurlHandler ? Environment.ProcessorCount : 80;
+		public static readonly int DefaultConnectionLimit = UsingCurlHandler ? Environment.ProcessorCount : 80;
 
 		/// <summary>
 		/// The default user agent for Elasticsearch.Net