Initial commit: WpywMail 自建邮件系统：.NET 8 原生 SMTP/IMAP 服务端（DKIM 签名、SPF/DKIM/DMARC 入站校验、SQLite 存储、完整账号体系）、Node 服务端、WinUI 3 客户端与 Web 前端

server-native-v2/SmtpReader.cs

@@ -0,0 +1,116 @@
+using System.Text;
+
+namespace WpywMail.Native;
+
+/// <summary>
+/// 字节级 SMTP 行读取器。
+///
+/// 为什么不用 StreamReader：
+///   1. StreamReader 只能返回字符串，8bit 正文会被字符集转换破坏；
+///   2. StreamReader 会预读缓冲，若之后改从原始流直接读 DATA，属于正文的字节
+///      可能已经被吞进它的缓冲区，造成命令/正文错位。
+/// 这里让命令与 DATA 共用同一份缓冲区，从根本上避免这两个问题。
+/// </summary>
+internal sealed class SmtpReader
+{
+    private const int MaxCommandLine = 8192;
+    private static readonly byte[] Crlf = [13, 10];
+
+    private readonly Stream stream;
+    private readonly byte[] buffer = new byte[8192];
+    private readonly MemoryStream line = new(MaxCommandLine);
+    private int start;
+    private int end;
+
+    public SmtpReader(Stream stream) => this.stream = stream;
+
+    /// <summary>读一行命令（不含 CRLF），连接关闭返回 null。</summary>
+    public async Task<string?> ReadLineAsync(CancellationToken token)
+    {
+        while (true)
+        {
+            if (start >= end)
+            {
+                end = await stream.ReadAsync(buffer.AsMemory(0, buffer.Length), token);
+                start = 0;
+                if (end <= 0) return null;
+            }
+
+            var current = buffer[start++];
+            if (current == (byte)'\n')
+            {
+                var bytes = line.ToArray();
+                line.SetLength(0);
+                var length = bytes.Length;
+                if (length > 0 && bytes[length - 1] == (byte)'\r') length--;
+                return Encoding.ASCII.GetString(bytes, 0, length);
+            }
+
+            line.WriteByte(current);
+            if (line.Length > MaxCommandLine) throw new InvalidOperationException("命令行过长，已断开。");
+        }
+    }
+
+    /// <summary>从同一缓冲区读取恰好 length 个字节（IMAP 的 literal 需要）。</summary>
+    public async Task<byte[]> ReadExactlyAsync(int length, CancellationToken token)
+    {
+        if (length < 0) throw new InvalidOperationException("长度非法。");
+        var result = new byte[length];
+        var written = 0;
+        while (written < length)
+        {
+            if (start >= end)
+            {
+                end = await stream.ReadAsync(buffer.AsMemory(0, buffer.Length), token);
+                start = 0;
+                if (end <= 0) throw new IOException("连接在读取 literals 途中关闭。");
+            }
+            var take = Math.Min(length - written, end - start);
+            Buffer.BlockCopy(buffer, start, result, written, take);
+            start += take;
+            written += take;
+        }
+        return result;
+    }
+
+    /// <summary>
+    /// 读取 DATA 段直到单独一行的 "."。按字节忠实处理并做 dot-unstuffing，
+    /// 行尾统一为 CRLF。超过 maxBytes 抛 InvalidOperationException。
+    /// </summary>
+    public async Task<byte[]> ReadDataAsync(int maxBytes, CancellationToken token)
+    {
+        var message = new MemoryStream();
+        line.SetLength(0);
+
+        while (true)
+        {
+            if (start >= end)
+            {
+                end = await stream.ReadAsync(buffer.AsMemory(0, buffer.Length), token);
+                start = 0;
+                if (end <= 0) throw new IOException("连接在 DATA 中途关闭。");
+            }
+
+            var current = buffer[start++];
+            if (current != (byte)'\n')
+            {
+                line.WriteByte(current);
+                if (line.Length > maxBytes) throw new InvalidOperationException("单行过长，已拒绝。");
+                continue;
+            }
+
+            var text = line.ToArray();
+            line.SetLength(0);
+            if (text.Length > 0 && text[^1] == (byte)'\r') text = text[..^1];
+            if (text.Length == 1 && text[0] == (byte)'.') break;           // DATA 结束
+            if (text.Length > 1 && text[0] == (byte)'.') text = text[1..];  // dot-unstuffing
+
+            message.Write(text);
+            message.Write(Crlf);
+            if (message.Length > maxBytes)
+                throw new InvalidOperationException($"邮件超过 {maxBytes / 1024 / 1024} MB 上限，已拒绝。");
+        }
+
+        return message.ToArray();
+    }
+}