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