fix(export): 为函数图像与导出产物增加资源上限

针对 PR 审阅「函数数量没有限制，可能生成数百 MB 的 SVG」：

- parser: 单块 function-plot 表达式上限 _MAX_EXPRESSIONS=16，超限整块回退
- html: 单篇文档函数图像上限 _MAX_FUNCTION_PLOTS=16，超出回退源码占位
- service: 输入源 MAX_MARKDOWN_CHARS、产物 MAX_EXPORT_BYTES，超限分别
  拒绝创建或标记 failed（EXPORT_OUTPUT_TOO_LARGE）
- 补充 4 条回归测试与文档说明

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/export/exporters/html.py

@@ -22,6 +22,9 @@ _RAW_HTML_WARNING = "原始 HTML 已按纯文本转义保留"
 # 链接/图片地址允许的协议；无 scheme 的相对地址视为安全，其余协议一律降级
 _ALLOWED_URL_SCHEMES = frozenset({"http", "https", "mailto"})
 
+# 单篇文档允许的函数图像数量上限，超出部分回退占位，防止多图块并发采样耗尽内存/线程
+_MAX_FUNCTION_PLOTS = 16
+
 
 def _safe_url(url: str) -> str | None:
     """校验 URL 协议；安全返回原串，不安全返回 None。"""
@@ -69,6 +72,7 @@ class HtmlExporter:
     def render(self, document: Document, options: ExportOptions) -> ExportResult:
         """同步渲染；CPU 密集，调用方应放入线程执行，避免阻塞事件循环。"""
         self._options = options
+        self._plot_count = 0
         warnings: list[str] = []
         body = self._render_children(document.children, warnings)
         content = self._assemble(document, options, body, warnings)
@@ -203,6 +207,13 @@ class HtmlExporter:
         return f"函数图像：{diag.message}{loc}"
 
     def _render_function_plot(self, node: DocumentNode, warnings: list[str]) -> str:
+        # 文档级数量上限：超出部分直接回退占位，不解析不采样，防止海量图像耗尽资源
+        self._plot_count += 1
+        if self._plot_count > _MAX_FUNCTION_PLOTS:
+            warnings.append(
+                f"函数图像：文档内函数图像数量超过上限 {_MAX_FUNCTION_PLOTS}，已回退为源码占位"
+            )
+            return f'<pre class="function-plot">{html.escape(node.text)}</pre>'
         # 解析与渲染共同纳入局部异常回退：单个图像失败只回退占位 + warning，
         # 绝不阻断整篇导出（含复杂表达式触发的 RecursionError 等异常）。
         try:

backend/app/export/service.py

@@ -39,8 +39,10 @@ _jobs: dict[str, ExportJob] = {}
 _tasks: dict[str, asyncio.Task] = {}
 _cancel_flags: dict[str, asyncio.Event] = {}
 MAX_JOBS = 100
-# markdown 源大小上限，防止未保存预览塞爆内存/产物
+# 输入源（note / markdown）统一大小上限，防止未保存预览或超长笔记塞爆内存/产物
 MAX_MARKDOWN_CHARS = 200_000
+# 最终导出产物大小上限，防止超大 HTML 耗尽内存/磁盘
+MAX_EXPORT_BYTES = 20 * 1024 * 1024  # 20 MB
 # 产物有效期
 FILE_TTL = timedelta(hours=24)
 
@@ -51,6 +53,10 @@ class ExportCancelled(Exception):
     """导出在渲染前被取消时抛出，用于标记 cancelled。"""
 
 
+class ExportTooLarge(Exception):
+    """导出产物超过大小上限时抛出，用于标记 failed 并携带专用错误码。"""
+
+
 def _now() -> datetime:
     return datetime.now(timezone.utc)
 
@@ -123,6 +129,13 @@ async def _resolve_source(source: ExportSource) -> tuple[str, str, dict | None]:
                 "note not found",
                 {"note_id": source.note_id},
             )
+        if len(note.markdown) > MAX_MARKDOWN_CHARS:
+            raise ApiError(
+                400,
+                "EXPORT_OPTIONS_INVALID",
+                f"note source exceeds {MAX_MARKDOWN_CHARS} characters",
+                {"size": len(note.markdown), "limit": MAX_MARKDOWN_CHARS},
+            )
         metadata = {
             "file_path": note.file_path,
             "tags": note.tags,
@@ -210,6 +223,8 @@ async def _execute(
         result = await asyncio.to_thread(_render_document, document, options)
         if cancel_event.is_set():
             raise ExportCancelled()
+        if len(result.content) > MAX_EXPORT_BYTES:
+            raise ExportTooLarge()
 
         out_dir = get_settings().exports_path
         out_dir.mkdir(parents=True, exist_ok=True)
@@ -241,6 +256,15 @@ async def _execute(
                 "completed_at": _now(),
             }
         )
+    except ExportTooLarge:
+        _jobs[job_id] = _jobs[job_id].model_copy(
+            update={
+                "status": ExportStatus.failed,
+                "error": "Export output exceeds size limit.",
+                "error_code": "EXPORT_OUTPUT_TOO_LARGE",
+                "completed_at": _now(),
+            }
+        )
     except Exception as exc:  # 渲染失败不拖垮服务，只记日志与项目错误码
         logger.exception("Export failed: job_id=%s", job_id)
         _jobs[job_id] = _jobs[job_id].model_copy(

backend/app/plot/parser.py

@@ -51,6 +51,8 @@ _NUMBER_RE = re.compile(r"^(\d+\.?\d*|\.\d+)([eE][+-]?\d+)?$")
 # 用白名单校验提前拦截，保证失败走正常诊断路径而不是异常逃逸出导出链路。
 _MAX_AST_DEPTH = 200
 _MAX_AST_NODES = 1000
+# 单块 function-plot 允许的表达式数量上限，防止海量表达式导致超大 SVG 与海量采样求值
+_MAX_EXPRESSIONS = 16
 
 
 class PlotParseError(Exception):
@@ -368,6 +370,16 @@ def parse_source(source: str) -> FunctionPlotParseResult:
             has_error = True
             continue
         expressions.append(FunctionPlotExpression(expression=expr_text))
+        # 表达式数量超限：整块回退并提前终止，避免对海量表达式做采样求值
+        if len(expressions) > _MAX_EXPRESSIONS:
+            diagnostics.append(
+                PlotDiagnostic(
+                    severity="error",
+                    code="FUNCTION_PLOT_TOO_MANY_EXPRESSIONS",
+                    message=f"表达式数量超过上限 {_MAX_EXPRESSIONS}，已回退为源码占位",
+                )
+            )
+            return FunctionPlotParseResult(plot=None, diagnostics=diagnostics)
 
     if has_error:
         return FunctionPlotParseResult(plot=None, diagnostics=diagnostics)

backend/tests/test_export.py

@@ -412,3 +412,40 @@ def test_export_source_requires_matching_field() -> None:
         ExportSource(type=ExportSourceType.note, note_id=None)
     with pytest.raises(ValidationError):
         ExportSource(type=ExportSourceType.markdown, markdown=None)
+
+
+# --------------------------------------------------------------------------- #
+# 审阅回归：资源上限
+# --------------------------------------------------------------------------- #
+def test_export_note_source_size_limit(monkeypatch) -> None:
+    # P1：note 源超出 MAX_MARKDOWN_CHARS 应在创建期拒绝，不进入后台渲染
+    from app.services import note_service
+
+    monkeypatch.setattr(export_service, "MAX_MARKDOWN_CHARS", 10)
+
+    async def _go():
+        note = await note_service.create_note(
+            title="超长笔记", markdown="a" * 20, folder="导出", tags=[]
+        )
+        return await export_service.create_export(
+            ExportRequest(
+                source=ExportSource(type=ExportSourceType.note, note_id=note.note_id),
+                format=ExportFormat.html,
+            )
+        )
+
+    with pytest.raises(ApiError) as exc:
+        asyncio.run(_go())
+    assert exc.value.status_code == 400
+    assert exc.value.code == "EXPORT_OPTIONS_INVALID"
+
+
+def test_export_output_too_large(monkeypatch) -> None:
+    # P1：产物超出 MAX_EXPORT_BYTES 应标记 failed 且不落盘
+    monkeypatch.setattr(export_service, "MAX_EXPORT_BYTES", 10)
+
+    finished = _create_and_wait(_markdown_request("# 产物超限"))
+    assert finished.status == ExportStatus.failed
+    assert finished.error_code == "EXPORT_OUTPUT_TOO_LARGE"
+    assert finished.file is None
+    assert not (get_settings().exports_path / f"{finished.job_id}.html").exists()

backend/tests/test_plot.py

@@ -247,3 +247,25 @@ def test_render_svg_extreme_range_no_nan() -> None:
     assert "nan" not in rendered.content
     assert "inf" not in rendered.content
     assert any("range" in w for w in rendered.warnings)
+
+
+# --------------------------------------------------------------------------- #
+# 审阅回归：函数/图像数量上限
+# --------------------------------------------------------------------------- #
+def test_parse_source_rejects_too_many_expressions() -> None:
+    # P1：单块表达式数量超限应整块回退，避免海量采样求值
+    source = "\n".join(f"y = x + {i}" for i in range(50))
+    result = parse_source(source)
+    assert result.plot is None
+    assert any(d.code == "FUNCTION_PLOT_TOO_MANY_EXPRESSIONS" for d in result.diagnostics)
+
+
+def test_html_exporter_limits_function_plot_count() -> None:
+    # P1：文档内函数图像数量超限，超出部分回退占位，不耗尽资源
+    blocks = "\n\n".join("```function-plot\ny = x\n```" for _ in range(20))
+    result = asyncio.run(HtmlExporter().export(parse_document(blocks), ExportOptions()))
+    html = result.content.decode("utf-8")
+    # 上限 16：前 16 个渲染为 SVG，其余 4 个回退占位
+    assert html.count('<figure class="function-plot">') == 16
+    assert html.count('<pre class="function-plot">') == 4
+    assert any("函数图像数量超过上限" in w for w in result.warnings)