Use ascii decoding routine that disallows null chars (#1445)

* Use ascii decoding routine that disallows null chars
- GetAsciiString() in newer corefxlab builds allows 0 (which is a valid
ascii char). To avoid future regressions, GetAsciiStringNonNullCharacters()
was added and used in place of GetAsciiString() when interpreting the
request line and headers
- Make GetAsciiStringNonNullCharacters return empty instead of null

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/Frame.cs

@@ -1228,7 +1228,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
             if (needDecode)
             {
                 // Read raw target before mutating memory.
-                rawTarget = target.GetAsciiString() ?? string.Empty;
+                rawTarget = target.GetAsciiStringNonNullCharacters();
 
                 // URI was encoded, unescape and then parse as utf8
                 int pathLength = UrlEncoder.Decode(path, path);
@@ -1237,7 +1237,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
             else
             {
                 // URI wasn't encoded, parse as ASCII
-                requestUrlPath = path.GetAsciiString() ?? string.Empty;
+                requestUrlPath = path.GetAsciiStringNonNullCharacters();
 
                 if (query.Length == 0)
                 {
@@ -1247,21 +1247,21 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                 }
                 else
                 {
-                    rawTarget = target.GetAsciiString() ?? string.Empty;
+                    rawTarget = target.GetAsciiStringNonNullCharacters();
                 }
             }
 
             var normalizedTarget = PathNormalizer.RemoveDotSegments(requestUrlPath);
             if (method != HttpMethod.Custom)
             {
-                Method = HttpUtilities.MethodToString(method) ?? String.Empty;
+                Method = HttpUtilities.MethodToString(method) ?? string.Empty;
             }
             else
             {
-                Method = customMethod.GetAsciiString() ?? string.Empty;
+                Method = customMethod.GetAsciiStringNonNullCharacters();
             }
 
-            QueryString = query.GetAsciiString() ?? string.Empty;
+            QueryString = query.GetAsciiStringNonNullCharacters();
             RawTarget = rawTarget;
             HttpVersion = HttpUtilities.VersionToString(version);
 
@@ -1289,7 +1289,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
             {
                 RejectRequest(RequestRejectionReason.TooManyHeaders);
             }
-            var valueString = value.GetAsciiString() ?? string.Empty;
+            var valueString = value.GetAsciiStringNonNullCharacters();
 
             FrameRequestHeaders.Append(name, valueString);
         }

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Infrastructure/HttpUtilities.cs

@@ -94,6 +94,28 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure
             }
         }
 
+        public unsafe static string GetAsciiStringNonNullCharacters(this Span<byte> span)
+        {
+            if (span.IsEmpty)
+            {
+                return string.Empty;
+            }
+
+            var asciiString = new string('\0', span.Length);
+
+            fixed (char* output = asciiString)
+            fixed (byte* buffer = &span.DangerousGetPinnableReference())
+            {
+                // This version if AsciiUtilities returns null if there are any null (0 byte) characters
+                // in the string
+                if (!AsciiUtilities.TryGetAsciiString(buffer, output, span.Length))
+                {
+                    throw new InvalidOperationException();
+                }
+            }
+            return asciiString;
+        }
+
         public static string GetAsciiStringEscaped(this Span<byte> span, int maxChars)
         {
             var sb = new StringBuilder();