15.5 确保网页应用健壮

当网页应用的处理函数发生 panic，服务器会简单地终止运行。这可不妙：网页服务器必须是足够健壮的程序，能够承受任何可能的突发问题。

首先能想到的是在每个处理函数中使用 defer/recover，不过这样会产生太多的重复代码。13.5节 使用闭包的错误处理模式是更优雅的方案。我们把这种机制应用到前一章的简单网页服务器上。实际上，它可以被简单地应用到任何网页服务器程序中。

为增强代码可读性，我们为页面处理函数创建一个类型：

type HandleFnc func(http.ResponseWriter, *http.Request)

我们的错误处理函数应用了13.5节 的模式，成为 logPanics 函数：

func logPanics(function HandleFnc) HandleFnc {
    return func(writer http.ResponseWriter, request *http.Request) {
        defer func() {
            if x := recover(); x != nil {
                log.Printf("[%v] caught panic: %v", request.RemoteAddr, x)
            }
        }()
        function(writer, request)
    }
}

然后我们用 logPanics 来包装对处理函数的调用：

http.HandleFunc("/test1", logPanics(SimpleServer))
http.HandleFunc("/test2", logPanics(FormServer))

处理函数现在可以恢复 panic 调用，类似13.5节 中的错误检测函数。完整代码如下：

示例 15.11 robust_webserver.go

package main

import (
    "io"
    "log"
    "net/http"
)

const form = `<html><body><form action="#" method="post" name="bar">
        <input type="text" name="in"/>
        <input type="submit" value="Submit"/>
    </form></html></body>`

type HandleFnc func(http.ResponseWriter, *http.Request)

/* handle a simple get request */
func SimpleServer(w http.ResponseWriter, request *http.Request) {
    io.WriteString(w, "<h1>hello, world</h1>")
}

/* handle a form, both the GET which displays the form
   and the POST which processes it.*/
func FormServer(w http.ResponseWriter, request *http.Request) {
    w.Header().Set("Content-Type", "text/html")
    switch request.Method {
    case "GET":
        /* display the form to the user */
        io.WriteString(w, form)
    case "POST":
        /* handle the form data, note that ParseForm must
           be called before we can extract form data*/
        //request.ParseForm();
        //io.WriteString(w, request.Form["in"][0])
        io.WriteString(w, request.FormValue("in"))
    }
}

func main() {
    http.HandleFunc("/test1", logPanics(SimpleServer))
    http.HandleFunc("/test2", logPanics(FormServer))
    if err := http.ListenAndServe(":8088", nil); err != nil {
        panic(err)
    }
}

func logPanics(function HandleFnc) HandleFnc {
    return func(writer http.ResponseWriter, request *http.Request) {
        defer func() {
            if x := recover(); x != nil {
                log.Printf("[%v] caught panic: %v", request.RemoteAddr, x)
            }
        }()
        function(writer, request)
    }
}

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