Fixed MaxMimeDepth logic to still use MimePart subclasses

When the MaxMimeDepth is reached by the parser, before this fix, the
parser would only instantiate MimePart (and not Multipart or MessagePart
subclasses) in order to avoid recursing even further.

With this fix, it will instaniate any/all MimePart subclasses as well
(such as TextPart).

Fixes issue #1006

Author: jstedfast

MimeKit/ParserOptions.cs

@@ -306,27 +306,31 @@ static bool EqualsAny (string value, params string[] values)
 		internal MimeEntity CreateEntity (ContentType contentType, IList<Header> headers, bool toplevel, int depth)
 		{
 			var args = new MimeEntityConstructorArgs (this, contentType, headers, toplevel);
-
-			if (depth >= MaxMimeDepth)
-				return new MimePart (args);
-
 			var subtype = contentType.MediaSubtype;
 			var type = contentType.MediaType;
 
 			if (mimeTypes.Count > 0) {
 				var mimeType = $"{type}/{subtype}";
 
-				if (mimeTypes.TryGetValue (mimeType, out var ctor))
-					return (MimeEntity) ctor.Invoke (new object[] { args });
+				if (mimeTypes.TryGetValue (mimeType, out var ctor)) {
+					// Instantiate the custom type if-and-only-if the current parser depth is < MaxMimeDepth -or- the custom type is a MimePart subclass.
+					if (depth < MaxMimeDepth || typeof (MimePart).IsAssignableFrom (ctor.DeclaringType))
+						return (MimeEntity) ctor.Invoke (new object[] { args });
+				}
 			}
 
 			if (type.Equals ("text", StringComparison.OrdinalIgnoreCase)) {
 				// text/rfc822-headers
-				if (subtype.Equals ("rfc822-headers", StringComparison.OrdinalIgnoreCase) && !IsEncoded (headers))
+				if (depth < MaxMimeDepth && subtype.Equals ("rfc822-headers", StringComparison.OrdinalIgnoreCase) && !IsEncoded (headers))
 					return new TextRfc822Headers (args);
 
 				return new TextPart (args);
 			} else if (type.Equals ("multipart", StringComparison.OrdinalIgnoreCase)) {
+				if (depth >= MaxMimeDepth) {
+					// We don't want to recurse any further, so treat this as a leaf node.
+					return new MimePart (args);
+				}
+
 				// multipart/alternative
 				if (subtype.Equals ("alternative", StringComparison.OrdinalIgnoreCase))
 					return new MultipartAlternative (args);
@@ -375,13 +379,13 @@ internal MimeEntity CreateEntity (ContentType contentType, IList<Header> headers
 
 				// message/rfc822
 				if (EqualsAny (subtype, "rfc822", "global", "news", "external-body", "rfc2822")) {
-					if (!IsEncoded (headers))
+					if (depth < MaxMimeDepth && !IsEncoded (headers))
 						return new MessagePart (args);
 				} else if (subtype.Equals ("partial", StringComparison.OrdinalIgnoreCase)) {
 					if (!IsEncoded (headers))
 						return new MessagePartial (args);
 				} else if (subtype.Equals ("global-headers", StringComparison.OrdinalIgnoreCase)) {
-					if (!IsEncoded (headers))
+					if (depth < MaxMimeDepth && !IsEncoded (headers))
 						return new TextRfc822Headers (args);
 				}
 			} else if (type.Equals ("application", StringComparison.OrdinalIgnoreCase)) {