Fix IndexOutOfRangeException in Configuration to HOCON adaptor (#632)

Author: Arkatufus

src/Akka.Hosting.Tests/Configuration/ConfigurationHoconAdapterTest.cs

@@ -67,6 +67,13 @@ public ConfigurationHoconAdapterTest()
         Environment.SetEnvironmentVariable("akka__test_value_4__1", "one");
         Environment.SetEnvironmentVariable("akka__actor__serialization_bindings2__\"System.Object\"", "hyperion");
 
+        // Issue #631
+        // Double underscore cases
+        Environment.SetEnvironmentVariable("__MISE_SESSION", "some-random-string");
+        // Quadruple underscore cases
+        Environment.SetEnvironmentVariable("MISE____SESSION", "some-random-string");
+        Environment.SetEnvironmentVariable("____MISE_SESSION", "some-random-string");
+        
         using var stream = new MemoryStream(Encoding.UTF8.GetBytes(ConfigSource));
         _root = new ConfigurationBuilder()
             .AddJsonStream(stream)
@@ -96,6 +103,10 @@ public Task DisposeAsync()
         Environment.SetEnvironmentVariable("akka__test_value_4__1", null);
         Environment.SetEnvironmentVariable("akka__actor__serialization_bindings2__\"System.Object\"", null);
 
+        Environment.SetEnvironmentVariable("__MISE_SESSION", null);
+        Environment.SetEnvironmentVariable("MISE____SESSION", null);
+        Environment.SetEnvironmentVariable("____MISE_SESSION", null);
+        
         return Task.CompletedTask;
     }
 
@@ -139,6 +150,11 @@ public void EnvironmentVariableTest()
         bindings.ContainsKey("System.Object").Should().BeFalse();
         bindings.ContainsKey("system.object").Should().BeTrue();
         bindings["system.object"].GetString().Should().Be("hyperion");
+        
+        // empty string keyed objects should be accessible by using "__"
+        config.GetString("__.mise-session").Should().Be("some-random-string");
+        config.GetString("mise.__.session").Should().Be("some-random-string");
+        config.GetString("__.__.mise-session").Should().Be("some-random-string");
     }
 
     [Fact(DisplayName = "Non-normalized adaptor should read environment variable sourced configuration correctly")]
@@ -179,6 +195,11 @@ public void EnvironmentVariableCaseSensitiveTest()
             .ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
         bindings.ContainsKey("System.Object").Should().BeTrue();
         bindings["System.Object"].GetString().Should().Be("hyperion");
+        
+        // empty string keyed objects should be accessible by using "__"
+        config.GetString("__.MISE-SESSION").Should().Be("some-random-string");
+        config.GetString("MISE.__.SESSION").Should().Be("some-random-string");
+        config.GetString("__.__.MISE-SESSION").Should().Be("some-random-string");
     }
 
     [Fact(DisplayName = "Non-normalized Adaptor should read quote enclosed key inside JSON settings correctly")]

src/Akka.Hosting/Configuration/ConfigurationHoconAdapter.cs

@@ -44,6 +44,12 @@ private static HoconValue ExpandKey(this IConfigurationSection config, HoconObje
             var sanitized = (normalizeKeys ? config.Key.ToLowerInvariant() : config.Key).Replace("_", "-");
             var keys = sanitized.SplitDottedPathHonouringQuotes().ToList();
 
+            // HOCON does not support objects with empty key name.
+            // The most probable source for this is a double underscore prefixed environment variable, which
+            // confuses MS.EXT.Configuration.EnvironmentVariables. Just create a temporary name for it.
+            if(keys.Count == 0)
+                return parent.GetOrCreateKey("__");
+            
             // No need to expand the chain
             if (keys.Count == 1)
             {