Cleaning up SslStream, MobileAuthenticatedStream and TaskToApm. (#17393)

### Overview

After the removal of the Legacy TLS Provider, we can make some cleanups and improvements to the `SslStream` class.  Since both the remaining providers use the internal `MobileAuthenticatedStream` and support Task-async code, we can get rid of some of the Begin/End async logic and the the underlying
`Stream` class handle it instead.

This also fixes a nasty issue with some of those task objects leaking unobserved exceptions.  The problem was that `SslStream.Dispose()` needs to clear out all resources, but in doing so also nulls out the `Impl` handle to the `MobileAuthenticatedStream` - so a subsequent `End*()` call will not reach it's corresponding `TaskToApm.End()` method.

There are also some cleanups in the internal APIs that were only ever used internally and not intended for public consumption.

### Mono.Security.dll

* `Mono.Security.Interface`: removed internal `ICertificateValidator2` interface.

* `Mono.Security.Interface.IMonoSslStream`: this internal interface is only used by the web-tests, which only use it as a way to get the `SslStream` instance, so all those `Begin/End` async methods are now gone.

* `Mono.Security.Interface.IMonoSslStream2`: removed another internal interface.

* `Mono.Security.Interface.MonoTlsProvider`: removed some `internal abstract` methods; these have been moved into a new `Mono.Net.Security.MobileTlsProvider` inside `System.dll`.

### System.dll

* `Mono.Net.Security.MobileTlsProvider`:
  New abstract internal base class, which received the aforementioned abstract internal methods.  The naming is such to make it consistent with the already existing `MobileTlsContext` and `MobileAuthenticatedStream`.

Since both `AppleTlsProvider` and `BtlsProvider` now have an abstract internal base class inside `System.dll`, they can use the `ChainValidationHelper` class directly and we can also avoid some `Mono.Security` dances like for instance that `MonoSslPolicyErrors` enum.

This will also help make the code easier to read and maintainer.
  
* `Mono.AppleTls.AppleTlsProvider` and `Mono.Btls.MonoBtlsProvider`: change base class into `MobileTlsProvider` and reflect above mentioned overload changes.

* `Mono.AppleTls.AppleCertificateHelper`: use `ChainValidationHelper` as well.

* `Mono.Net.Security.ChainValidationHelper`: only implement `ICertificateValidator`.
  The `ICertificateValidator` interface is still required because it is in use by the web-tests, but the previous `ICertificateValidator2` was an internal dance.

* `Mono.Net.Security.MobileAuthenticatedStream`: largely cleaned up this class.
  - we now implement the current slimmer version of `IMonoSslStream`.
  - moved all those overloads into `SslStream` that only fill in default values for some of their arguments and then call another overloaded version of their own.
  - removed lots of `Begin/End` methods that are not used anymore.

* `Mono.Net.Security.MonoTlsProviderFactory`: use `MobileTlsStream` everywhere.

* `Mono.Net.Security.MonoTlsStream`: cleanup `SslStream` creation logic; instead of going throw `provider.CreateSslStream()`, we can now use an internal constructor directly, thus avoiding the `IMonoSslStream` interface while also making the code a lot easier to read and understand.

* `System.Net.Mail.SmtpClient`: same here.

* `System.Net.HttpWebRequest`: use `MobileTlsProvider`.

* `System.Net.Security.SslStream`: largely cleaned up this class.
  - we now use `MobileAuthenticatedStream` directly instead of the `IMonoSslStream` interface and therefore could also get rid of `IMonoSslStream2`.
  - cleanup constructor logic to allow the internal constructor being used with a `MobileTlsProvider` instead of going through `provider.CreateSslStream()`.
  - all those overloads that are only filling in default values for some of their arguments before calling another overloaded version of their own now live here instead of in the `MobileAuthenticatedStream` class.  The reason is to simply that `MobileAuthenticatedStream` is already quite a complex and difficult class, and removing some of that complexity out of it will make it easier to understand.
  - handle the `TaskToApm.Begin()` / `TaskToApm.End()` here to ensure that we are not leaking any task objects with unobserved exceptions.
  - removed `BeginRead()` and `BeginWrite()` overloads to let the underlying `Stream` class handle these.


* Bump API snapshot submodule

* [csproj] Update project files

3 files changed, 1 insertions, 96 deletions

diff --git a/mcs/class/Mono.Security/Mono.Security.Interface/CertificateValidationHelper.cs b/mcs/class/Mono.Security/Mono.Security.Interface/CertificateValidationHelper.cs
index 9b55f6cc91a..a40f5e64339 100644
--- a/mcs/class/Mono.Security/Mono.Security.Interface/CertificateValidationHelper.cs
+++ b/mcs/class/Mono.Security/Mono.Security.Interface/CertificateValidationHelper.cs
@@ -96,14 +96,6 @@ namespace Mono.Security.Interface
 		ValidationResult ValidateCertificate (string targetHost, bool serverMode, X509CertificateCollection certificates);
 	}
 
-	internal interface ICertificateValidator2 : ICertificateValidator
-	{
-		/*
-		 * Internal use only.
-		 */
-		ValidationResult ValidateCertificate (string targetHost, bool serverMode, X509Certificate leaf, X509Chain chain);
-	}
-
 	public static class CertificateValidationHelper
 	{
 		const string SecurityLibrary = "/System/Library/Frameworks/Security.framework/Security";
diff --git a/mcs/class/Mono.Security/Mono.Security.Interface/IMonoSslStream.cs b/mcs/class/Mono.Security/Mono.Security.Interface/IMonoSslStream.cs
index fd8b7da41bb..84989904faa 100644
--- a/mcs/class/Mono.Security/Mono.Security.Interface/IMonoSslStream.cs
+++ b/mcs/class/Mono.Security/Mono.Security.Interface/IMonoSslStream.cs
@@ -42,59 +42,11 @@ namespace Mono.Security.Interface
 			get;
 		}
 
-		void AuthenticateAsClient (string targetHost);
-
-		void AuthenticateAsClient (string targetHost, X509CertificateCollection clientCertificates, bool checkCertificateRevocation);
-
-		void AuthenticateAsClient (string targetHost, X509CertificateCollection clientCertificates, SSA.SslProtocols enabledSslProtocols, bool checkCertificateRevocation);
-
-		IAsyncResult BeginAuthenticateAsClient (string targetHost, AsyncCallback asyncCallback, object asyncState);
-
-		IAsyncResult BeginAuthenticateAsClient (string targetHost, X509CertificateCollection clientCertificates, bool checkCertificateRevocation, AsyncCallback asyncCallback, object asyncState);
-
-		IAsyncResult BeginAuthenticateAsClient (string targetHost, X509CertificateCollection clientCertificates, SSA.SslProtocols enabledSslProtocols, bool checkCertificateRevocation, AsyncCallback asyncCallback, object asyncState);
-
-		void EndAuthenticateAsClient (IAsyncResult asyncResult);
-
-		void AuthenticateAsServer (X509Certificate serverCertificate);
-
-		void AuthenticateAsServer (X509Certificate serverCertificate, bool clientCertificateRequired, bool checkCertificateRevocation);
-
-		void AuthenticateAsServer (X509Certificate serverCertificate, bool clientCertificateRequired, SSA.SslProtocols enabledSslProtocols, bool checkCertificateRevocation);
-
-		IAsyncResult BeginAuthenticateAsServer (X509Certificate serverCertificate, AsyncCallback asyncCallback, object asyncState);
-
-		IAsyncResult BeginAuthenticateAsServer (X509Certificate serverCertificate, bool clientCertificateRequired, bool checkCertificateRevocation, AsyncCallback asyncCallback, object asyncState);
-
-		IAsyncResult BeginAuthenticateAsServer (X509Certificate serverCertificate, bool clientCertificateRequired, SSA.SslProtocols enabledSslProtocols, bool checkCertificateRevocation, AsyncCallback asyncCallback, object asyncState);
-
-		void EndAuthenticateAsServer (IAsyncResult asyncResult);
-
-		Task AuthenticateAsClientAsync (string targetHost);
-
-		Task AuthenticateAsClientAsync (string targetHost, X509CertificateCollection clientCertificates, bool checkCertificateRevocation);
-
 		Task AuthenticateAsClientAsync (string targetHost, X509CertificateCollection clientCertificates, SSA.SslProtocols enabledSslProtocols, bool checkCertificateRevocation);
 
-		Task AuthenticateAsServerAsync (X509Certificate serverCertificate);
-
-		Task AuthenticateAsServerAsync (X509Certificate serverCertificate, bool clientCertificateRequired, bool checkCertificateRevocation);
-
 		Task AuthenticateAsServerAsync (X509Certificate serverCertificate, bool clientCertificateRequired, SSA.SslProtocols enabledSslProtocols, bool checkCertificateRevocation);
 
-		int Read (byte[] buffer, int offset, int count);
-
-		void Write (byte[] buffer);
-
-		void Write (byte[] buffer, int offset, int count);
-
-		IAsyncResult BeginRead (byte[] buffer, int offset, int count, AsyncCallback asyncCallback, object asyncState);
-
-		int EndRead (IAsyncResult asyncResult);
-
-		IAsyncResult BeginWrite (byte[] buffer, int offset, int count, AsyncCallback asyncCallback, object asyncState);
-
-		void EndWrite (IAsyncResult asyncResult);
+		Task<int> ReadAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken);
 
 		Task WriteAsync (byte[] buffer, int offset, int count, CancellationToken cancellationToken);
 
@@ -215,12 +167,5 @@ namespace Mono.Security.Interface
 
 		Task RenegotiateAsync (CancellationToken cancellationToken);
 	}
-
-	interface IMonoSslStream2 : IMonoSslStream
-	{
-		Task AuthenticateAsClientAsync (IMonoSslClientAuthenticationOptions sslClientAuthenticationOptions, CancellationToken cancellationToken);
-
-		Task AuthenticateAsServerAsync (IMonoSslServerAuthenticationOptions sslServerAuthenticationOptions, CancellationToken cancellationToken);
-	}
 }
 
diff --git a/mcs/class/Mono.Security/Mono.Security.Interface/MonoTlsProvider.cs b/mcs/class/Mono.Security/Mono.Security.Interface/MonoTlsProvider.cs
index 2f040e0f613..f61ccbacbbd 100644
--- a/mcs/class/Mono.Security/Mono.Security.Interface/MonoTlsProvider.cs
+++ b/mcs/class/Mono.Security/Mono.Security.Interface/MonoTlsProvider.cs
@@ -121,10 +121,6 @@ namespace Mono.Security.Interface
 			Stream innerStream, bool leaveInnerStreamOpen,
 			MonoTlsSettings settings = null);
 
-		internal abstract IMonoSslStream CreateSslStreamInternal (
-			SslStream sslStream, Stream innerStream, bool leaveInnerStreamOpen,
-			MonoTlsSettings settings);
-
 #endregion
 
 #region Native Certificate Implementation
@@ -133,34 +129,6 @@ namespace Mono.Security.Interface
 			get { return false; }
 		}
 
-		internal virtual X509Certificate2Impl GetNativeCertificate (
-			byte[] data, string password, X509KeyStorageFlags flags)
-		{
-			throw new InvalidOperationException ();
-		}
-
-		internal virtual X509Certificate2Impl GetNativeCertificate (
-			X509Certificate certificate)
-		{
-			throw new InvalidOperationException ();
-		}
-
-#endregion
-
-#region Certificate Validation
-		/*
-		 * If @serverMode is true, then we're a server and want to validate a certificate
-		 * that we received from a client.
-		 *
-		 * On OS X and Mobile, the @chain will be initialized with the @certificates, but not actually built.
-		 *
-		 * Returns `true` if certificate validation has been performed and `false` to invoke the
-		 * default system validator.
-		 */
-		internal abstract bool ValidateCertificate (
-			ICertificateValidator2 validator, string targetHost, bool serverMode,
-			X509CertificateCollection certificates, bool wantsChain, ref X509Chain chain,
-			ref MonoSslPolicyErrors errors, ref int status11);
 #endregion
 
 #region Misc