feat(ai-context-limit): add context window limit wasm plugin (#4000)

Signed-off-by: Cai Rui <cairui@U-7VTK6WQN-2207.local>

plugins/wasm-go/extensions/ai-context-limit/config.go

@@ -0,0 +1,73 @@
+// Copyright (c) 2026 Alibaba Group Holding Ltd.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/tidwall/gjson"
+)
+
+// Config 上下文限制插件配置
+type Config struct {
+	// MaxContextTokens 必填，输入侧 token 上限（用户阈值）
+	MaxContextTokens int `json:"max_context_tokens"`
+	// ErrorStatusCode 超限响应码，默认 400
+	ErrorStatusCode int `json:"error_status_code"`
+	// BufferRatio token 预估值放大系数，默认 1.10
+	BufferRatio float64 `json:"buffer_ratio"`
+}
+
+const (
+	defaultErrorStatusCode = 400
+	defaultBufferRatio     = 1.10
+	// MaxRequestBodyBytes 强制调大的 envoy 请求体 buffer 上限
+	// 上下文限制仅需要读取请求体中的文本输入，8MB 可覆盖常见长上下文请求。
+	MaxRequestBodyBytes uint32 = 8 * 1024 * 1024
+)
+
+// parseConfig 解析 WasmPlugin defaultConfig 字段
+func parseConfig(jsonConfig gjson.Result, cfg *Config) error {
+	if err := json.Unmarshal([]byte(jsonConfig.Raw), cfg); err != nil {
+		return fmt.Errorf("parse config failed: %w", err)
+	}
+	if cfg.MaxContextTokens < 0 {
+		return fmt.Errorf("max_context_tokens must be non-negative, got %d", cfg.MaxContextTokens)
+	}
+	if cfg.MaxContextTokens == 0 {
+		// 阈值为 0 视为未启用，不拦截请求（防止误配置导致全量 5xx）
+		return nil
+	}
+	if cfg.ErrorStatusCode == 0 {
+		cfg.ErrorStatusCode = defaultErrorStatusCode
+	} else if cfg.ErrorStatusCode < 400 || cfg.ErrorStatusCode > 599 {
+		return fmt.Errorf("error_status_code must be between 400 and 599, got %d", cfg.ErrorStatusCode)
+	}
+	if cfg.BufferRatio < 0 {
+		return fmt.Errorf("buffer_ratio must be non-negative, got %f", cfg.BufferRatio)
+	}
+	if cfg.BufferRatio == 0 {
+		cfg.BufferRatio = defaultBufferRatio
+	} else if cfg.BufferRatio > 10 {
+		return fmt.Errorf("buffer_ratio must not exceed 10, got %f", cfg.BufferRatio)
+	}
+	return nil
+}
+
+// IsEnabled 判断当前配置是否需要执行拦截
+func (c *Config) IsEnabled() bool {
+	return c.MaxContextTokens > 0
+}