Fix path length 255 (dotnet/coreclr#21838)

* Fix path length 255

* Fix assert

* Dead code

Signed-off-by: dotnet-bot <[email protected]>

Author: danmoseley

src/Common/src/CoreLib/Interop/Unix/System.Native/Interop.ReadDir.cs

@@ -30,21 +30,22 @@ internal unsafe struct DirectoryEntry
             internal byte* Name;
             internal int NameLength;
             internal NodeType InodeType;
-            internal const int NameBufferSize = 256;
+            internal const int NameBufferSize = 256; // sizeof(dirent->d_name) == NAME_MAX + 1
 
             internal ReadOnlySpan<char> GetName(Span<char> buffer)
             {
-                Debug.Assert(buffer.Length >= Encoding.UTF8.GetMaxCharCount(NameBufferSize - 1), "should have enough space for the max file name");
+                // -1 for null terminator (buffer will not include one),
+                //  and -1 because GetMaxCharCount pessimistically assumes the buffer may start with a partial surrogate
+                Debug.Assert(buffer.Length >= Encoding.UTF8.GetMaxCharCount(NameBufferSize - 1 - 1));
+
                 Debug.Assert(Name != null, "should not have a null name");
 
                 ReadOnlySpan<byte> nameBytes = (NameLength == -1)
                     // In this case the struct was allocated via struct dirent *readdir(DIR *dirp);
-                    ? new ReadOnlySpan<byte>(Name, new ReadOnlySpan<byte>(Name, NameBufferSize - 1).IndexOf<byte>(0))
+                    ? new ReadOnlySpan<byte>(Name, new ReadOnlySpan<byte>(Name, NameBufferSize).IndexOf<byte>(0))
                     : new ReadOnlySpan<byte>(Name, NameLength);
 
                 Debug.Assert(nameBytes.Length > 0, "we shouldn't have gotten a garbage value from the OS");
-                if (nameBytes.Length == 0)
-                    return buffer.Slice(0, 0);
 
                 int charCount = Encoding.UTF8.GetChars(nameBytes, buffer);
                 ReadOnlySpan<char> value = buffer.Slice(0, charCount);