Fixed: Support for Mono 5.x with the newer BoringTLS provider.

src/NzbDrone.Common/Security/SecurityProtocolPolicy.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Net;
 using NLog;
+using NzbDrone.Common.EnvironmentInfo;
 using NzbDrone.Common.Instrumentation;
 
 namespace NzbDrone.Common.Security
@@ -14,6 +15,12 @@ namespace NzbDrone.Common.Security
 
         public static void Register()
         {
+            if (OsInfo.IsNotWindows)
+            {
+                // This was never meant to be used on mono, and will cause issues with mono 5 and higher if btls is enabled.
+                return;
+            }
+
             try
             {
                 // TODO: In v3 we should drop support for SSL3 because its very insecure. Only leaving it enabled because some people might rely on it.

src/NzbDrone.Core/HealthCheck/Checks/MonoTlsCheck.cs

@@ -0,0 +1,69 @@
+using System;
+using System.Linq;
+using System.Reflection;
+using NLog;
+using NzbDrone.Common.EnvironmentInfo;
+
+namespace NzbDrone.Core.HealthCheck.Checks
+{
+    public class MonoTlsCheck : HealthCheckBase
+    {
+        private readonly IPlatformInfo _platformInfo;
+        private readonly Logger _logger;
+
+        public MonoTlsCheck(IPlatformInfo platformInfo, Logger logger)
+        {
+            _platformInfo = platformInfo;
+            _logger = logger;
+        }
+
+        public override HealthCheck Check()
+        {
+            if (!PlatformInfo.IsMono)
+            {
+                return new HealthCheck(GetType());
+            }
+
+            var monoVersion = _platformInfo.Version;
+
+            if (monoVersion >= new Version("5.0.0") && Environment.GetEnvironmentVariable("MONO_TLS_PROVIDER") == "legacy")
+            {
+                _logger.Debug("Mono version 5.0.0 or higher and legacy TLS provider is selected, recommending user to switch to btls.");
+                return new HealthCheck(GetType(), HealthCheckResult.Warning, "Sonarr now supports Mono 5.x with btls enabled, consider removing MONO_TLS_PROVIDER=legacy option");
+            }
+
+            return new HealthCheck(GetType());
+        }
+
+        public override bool CheckOnSchedule => false;
+
+        private bool HasMonoBug18599()
+        {
+            _logger.Debug("mono version 3.4.0, checking for mono bug #18599.");
+            var numberFormatterType = Type.GetType("System.NumberFormatter");
+
+            if (numberFormatterType == null)
+            {
+                _logger.Debug("Couldn't find System.NumberFormatter. Aborting test.");
+                return false;
+            }
+
+            var fieldInfo = numberFormatterType.GetField("userFormatProvider",
+                BindingFlags.Static | BindingFlags.NonPublic);
+
+            if (fieldInfo == null)
+            {
+                _logger.Debug("userFormatProvider field not found, version likely preceeds the official v3.4.0.");
+                return false;
+            }
+
+            if (fieldInfo.GetCustomAttributes(false).Any(v => v is ThreadStaticAttribute))
+            {
+                _logger.Debug("userFormatProvider field doesn't contain the ThreadStatic Attribute, version is affected by the critical bug #18599.");
+                return true;
+            }
+
+            return false;
+        }
+    }
+}

src/NzbDrone.Core/NzbDrone.Core.csproj

@@ -567,6 +567,7 @@
     <Compile Include="HealthCheck\Checks\DownloadClientCheck.cs" />
     <Compile Include="HealthCheck\Checks\DownloadClientStatusCheck.cs" />
     <Compile Include="HealthCheck\Checks\DeprecatedDroneFactoryCheck.cs" />
+    <Compile Include="HealthCheck\Checks\MonoTlsCheck.cs" />
     <Compile Include="HealthCheck\Checks\MountCheck.cs" />
     <Compile Include="HealthCheck\Checks\DroneFactoryCheck.cs" />
     <Compile Include="HealthCheck\Checks\ImportMechanismCheck.cs" />