Golang 错误处理错误

Question

以下程序是来自this page 官方教程的自定义版本的网络服务器。只有我修改了 save() 和 loadPage() 函数，使用 jmcvetta 的包 neoism 将我的页面保存到 neo4j。

// web1 project main.go
package main

import (
    "errors"
    "fmt"
    "github.com/jmcvetta/neoism"
    "html/template"
    "io/ioutil"
    "net/http"
    "regexp"
)

type Page struct {
    Title string
    Body  string
}

func (p *Page) save() error {
    db, err := neoism.Connect("http://localhost:7474/db/data")
    if err != nil {
        return err
    }
    res := []struct {
        N neoism.Node
    }{}
    cq := neoism.CypherQuery{
        Statement:  "MERGE (n:Page {title: {title}}) ON MATCH SET n.body = {body} RETURN n",
        Parameters: neoism.Props{"title": p.Title, "body": p.Body},
        Result:     &res,
    }
    db.Cypher(&cq)
    return nil
}

func loadPage(title string) (*Page, error) {
    db, err := neoism.Connect("http://localhost:7474/db/data")
    if err != nil {
        return nil, err
    }
    res := []struct {
        Title string `json:"a.title"` // `json` tag matches column name in query
        Body  string `json:"a.body"`
    }{}
    cq := neoism.CypherQuery{
        Statement: `
            MATCH (a:Page)
            WHERE a.title = {name}
            RETURN a.title, a.body
        `,
        Parameters: neoism.Props{"name": title},
        Result:     &res,
    }
    db.Cypher(&cq)
    r := res[0]
    if r.Title == "" || r.Body == "" {
        return nil, errors.New("Page doesn't exist")
    }
    return &Page{Title: r.Title, Body: r.Body}, nil
}

func viewHandler(w http.ResponseWriter, r *http.Request, title string) {
    p, err := loadPage(title)
    if err != nil {
        http.Redirect(w, r, "/edit/"+title, http.StatusFound)
        return
    }
    renderTemplate(w, "view", p)
}

func editHandler(w http.ResponseWriter, r *http.Request, title string) {
    p, err := loadPage(title)
    if err != nil {
        p = &Page{Title: title}
    }
    renderTemplate(w, "edit", p)
}

func saveHandler(w http.ResponseWriter, r *http.Request, title string) {
    body := r.FormValue("body")
    p := &Page{Title: title, Body: body}
    err := p.save()
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    http.Redirect(w, r, "/view/"+title, http.StatusFound)
}

func homeHandler(w http.ResponseWriter, r *http.Request) {
    body, err := ioutil.ReadFile("home.html")
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    fmt.Fprintf(w, "%s", body)
}

func rootHandler(w http.ResponseWriter, r *http.Request) {
    http.Redirect(w, r, "/home", http.StatusFound)
}

var templates = template.Must(template.ParseFiles("edit.html", "view.html"))

func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) {
    err := templates.ExecuteTemplate(w, tmpl+".html", p)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
    }
}

var validPath = regexp.MustCompile("^/(edit|save|view)/([a-zA-Z0-9]+)$")

func makeHandler(fn func(http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        m := validPath.FindStringSubmatch(r.URL.Path)
        if m == nil {
            http.NotFound(w, r)
            return
        }
        fn(w, r, m[2])
    }
}

func main() {
    http.HandleFunc("/view/", makeHandler(viewHandler))
    http.HandleFunc("/edit/", makeHandler(editHandler))
    http.HandleFunc("/save/", makeHandler(saveHandler))
    http.HandleFunc("/home", homeHandler)
    http.HandleFunc("/", rootHandler)
    http.ListenAndServe(":8082", nil)
}

我相信，我的问题是在 loadPage() 函数中，当返回的页面为空时，它似乎没有返回任何错误，导致处理程序尝试执行模板带有空结构（至少我倾向于认为就是这样）。 有人知道我哪里做错了吗？

编辑：这是我尝试加载尚不存在的页面时的输出：

2014/06/19 23:13:35 http: panic serving 127.0.0.1:60326: runtime error: index out of range
goroutine 5 [running]:
net/http.func·009()
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/net/http/server.go:1093 +0xb1
runtime.panic(0x6d1f20, 0xa977d7)
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/runtime/panic.c:248 +0x11b
main.loadPage(0xc08407a12a, 0x6, 0x332978, 0xaa7560, 0x3)
    C:/Users/Nicolas/Go/src/web1/main.go:55 +0x369
main.editHandler(0x332940, 0xc08400b140, 0xc084019750, 0xc08407a12a, 0x6)
    C:/Users/Nicolas/Go/src/web1/main.go:72 +0x34
main.func·001(0x332940, 0xc08400b140, 0xc084019750)
    C:/Users/Nicolas/Go/src/web1/main.go:121 +0xc9
net/http.HandlerFunc.ServeHTTP(0xc084024ae0, 0x332940, 0xc08400b140, 0xc084019750)
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/net/http/server.go:1220 +0x43
net/http.(*ServeMux).ServeHTTP(0xc0840381b0, 0x332940, 0xc08400b140, 0xc084019750)
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/net/http/server.go:1496 +0x166
net/http.serverHandler.ServeHTTP(0xc084005e10, 0x332940, 0xc08400b140, 0xc084019750)
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/net/http/server.go:1597 +0x171
net/http.(*conn).serve(0xc08403f580)
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/net/http/server.go:1167 +0x7ba
created by net/http.(*Server).Serve
    C:/Users/ADMINI~1/AppData/Local/Temp/2/makerelease250988475/go/src/pkg/net/http/server.go:1644 +0x28e

精度：对于已经存在的页面，程序按预期工作，我可以编辑页面并稍后加载它们。

Answer 1

如果不运行代码，我怀疑错误在这里：

db.Cypher(&cq)
r := res[0]
if r.Title == "" || r.Body == "" {
    return nil, errors.New("Page doesn't exist")
}

代码假定存在 res[0] 值。你应该检查

   if len(res) == 0 {
      return nil, errors.New("Page does not exists")
   }

您的错误出现在您的 loadPage 代码中，并且是索引错误。