dqyin0101 2018-11-04 21:55
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优雅地处理gin-gonic中的模板渲染错误

I'm learning Go and using gin-gonic for a web application. I'm trying to recover gracefully from template errors and haven't been able to figure out how to buffer output or redirect properly to achieve this.

With this code:

package main

import (
    "net/http"

    "github.com/gin-gonic/gin"
)

func main() {
    g := gin.New()
    g.LoadHTMLGlob("templates/*")
    g.Use(func(c *gin.Context) {
        defer func() {
            if err := recover(); err != nil {
                c.HTML(http.StatusInternalServerError, "error.tmpl", nil)
            }
        }()

        c.Next()
    })
    g.GET("/", func(c *gin.Context) {
        c.HTML(http.StatusOK, "index.tmpl", gin.H{"var": 4})
    })
    g.Run(":80")
}

where templates/index.tmpl is

Before
<br>
Bad: {{.var.x}}
<br>
After

and templates/error.tmpl is

Oops! We encountered an error.

when I load my page, I see

Before
Bad: Oops! We encountered an error.

and the response code ends up being 200. I'd prefer to cleanly catch the error so that the only thing displayed to the user is

Oops! We encountered an error.

the response code comes out as 500, and the error gets logged on the server for later investigation.

What's the best way within gin to catch template errors without showing partial output to the user? I've seen several examples of accomplishing this with buffering using the built-in net/http stuff, but haven't been able to find anything for a good way to handle it in gin.

Edited with solution

Building off of @big pigeon's comments on the accepted answer, I ended up executing the template myself into a buffer and using c.Data() to display it if there were no errors. This still seems less than ideal since it bypasses features like multitemplate's ability to dynamically reload a parsed template at runtime in dev builds, but it works. The updated proof-of-concept code looks like:

package main

import (
    "bytes"
    "html/template"
    "net/http"

    "github.com/gin-gonic/gin"
)

func main() {
    g := gin.New()
    g.LoadHTMLGlob("templates/*")
    g.Use(func(c *gin.Context) {
        defer func() {
            if err := recover(); err != nil {
                c.HTML(http.StatusInternalServerError, "error.tmpl", nil)
            }
        }()

        c.Next()
    })
    g.GET("/", func(c *gin.Context) {
        if tmpl, err := template.ParseFiles("templates/index.tmpl"); err != nil {
            panic(err)
        } else {
            buf := &bytes.Buffer{}
            if err = tmpl.Execute(buf, gin.H{"var": 4}); err != nil {
                panic(err)
            } else {
                c.Data(http.StatusOK, "text/html; charset=utf-8", buf.Bytes())
            }
        }
    })
    g.Run(":80")
}

Using buffer pooling, pre-parsing templates, and other such niceties are left as an exercise to any future readers.

If anyone knows of a better way to handle this without bypassing gin's parsing/rendering features, I'm very open to it.

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