Fix StackOverflowException on StringUtil.Create (#259)

* Fix StackOverflowException on StringUtil.Create

`Span<byte> buffer = stackalloc byte[(int)sequence.Length];`

* Refactor StringUtil.Create to remove unsafe code

Replaces unsafe pointer usage in StringUtil.Create with safe Encoding.GetString overloads. Also changes length check from ushort.MaxValue to short.MaxValue for consistency.

Encoding.GetString(ReadOnlySpan<byte>) available with .NET Standard 2.1

Author: tinohager

Src/SmtpServer/Protocol/SmtpParser.cs

@@ -1099,23 +1099,35 @@ public bool TryMakeMailbox(ref TokenReader reader, out IMailbox mailbox)
 
             if (reader.TryMake(TryMakeDomain, out var domain))
             {
-                mailbox = CreateMailbox(localpart, domain);
+                mailbox = CreateMailbox(localpart, domain) ?? throw new SmtpResponseException(SmtpResponse.MailboxNameNotAllowed);
                 return true;
             }
 
             if (reader.TryMake(TryMakeAddressLiteral, out var address))
             {
-                mailbox = CreateMailbox(localpart, address);
+                mailbox = CreateMailbox(localpart, address) ?? throw new SmtpResponseException(SmtpResponse.MailboxNameNotAllowed);
                 return true;
             }
 
             return false;
 
             static Mailbox CreateMailbox(ReadOnlySequence<byte> localpart, ReadOnlySequence<byte> domainOrAddress)
             {
-                var user = Regex.Unescape(StringUtil.Create(localpart, Encoding.UTF8).Trim('"'));
+                var tempLocalpart = StringUtil.Create(localpart, Encoding.UTF8)?.Trim('"');
+                if (tempLocalpart == null)
+                {
+                    return null;
+                }
+
+                var tempDomain = StringUtil.Create(domainOrAddress);
+                if (tempDomain == null)
+                {
+                    return null;
+                }
+
+                var unescapedLocalpart = Regex.Unescape(tempLocalpart);
 
-                return new Mailbox(user, StringUtil.Create(domainOrAddress));
+                return new Mailbox(unescapedLocalpart, tempDomain);
             }
         }
 

Src/SmtpServer/Text/StringUtil.cs

@@ -11,21 +11,21 @@ internal static string Create(ReadOnlySequence<byte> sequence)
             return Create(sequence, Encoding.ASCII);
         }
 
-        internal static unsafe string Create(ReadOnlySequence<byte> sequence, Encoding encoding)
+        internal static string Create(ReadOnlySequence<byte> sequence, Encoding encoding)
         {
             if (sequence.Length == 0)
             {
                 return null;
             }
 
-            if (sequence.IsSingleSegment)
+            if (sequence.Length > short.MaxValue)
             {
-                var span = sequence.First.Span;
+                return null;
+            }
 
-                fixed (byte* ptr = span)
-                {
-                    return encoding.GetString(ptr, span.Length);
-                }
+            if (sequence.IsSingleSegment)
+            {
+                return encoding.GetString(sequence.First.Span);
             }
             else
             {
@@ -43,10 +43,7 @@ internal static unsafe string Create(ReadOnlySequence<byte> sequence, Encoding e
                     }
                 }
 
-                fixed (byte* ptr = buffer)
-                {
-                    return encoding.GetString(ptr, buffer.Length);
-                }
+                return encoding.GetString(buffer);
             }
         }