Panic runtime error: index out of range in context.go:108

[Collapsik]

After commit b97ccf3 I catch panic that never been before. In my case length of c.handlers equal 1 and c.index equal 1.

[thinkerou]

@Collapsik can you post your test code? I use it reproduce the issue, thanks!

[kirides]

I get the same / a simmilar issue. It appears to happen after my handler returned

Did not happen before, but i cannot pin in down to a specific version, was, about August when i first installed gin-gonic, and just updated all my stuff on monday using go get -u all

Code: https://pastebin.com/kM6UwyB3

Stacktrace:

2018/12/07 09:34:55 http: panic serving 192.168.50.34:63110: runtime error: index out of range
goroutine 23 [running]:
net/http.(*conn).serve.func1(0xc00016b180)
        C:/Go/src/net/http/server.go:1746 +0xd7
panic(0xa254e0, 0xf9e740)
        C:/Go/src/runtime/panic.go:513 +0x1c7
github.com/gin-gonic/gin.(*Context).Next(0xc0001ec210)
        C:/Users/USERNAME/go/src/github.com/gin-gonic/gin/context.go:109 +0x87
github.com/gin-gonic/gin.(*Engine).handleHTTPRequest(0xc0000a9680, 0xc0001ec210)
        C:/Users/USERNAME/go/src/github.com/gin-gonic/gin/gin.go:383 +0x5b2
github.com/gin-gonic/gin.(*Engine).ServeHTTP(0xc0000a9680, 0xbb20a0, 0xc0001e8380, 0xc000100900)
        C:/Users/USERNAME/go/src/github.com/gin-gonic/gin/gin.go:349 +0x139
net/http.serverHandler.ServeHTTP(0xc00006be10, 0xbb20a0, 0xc0001e8380, 0xc000100900)
        C:/Go/src/net/http/server.go:2741 +0xb2
net/http.(*conn).serve(0xc00016b180, 0xbb2aa0, 0xc0001b83c0)
        C:/Go/src/net/http/server.go:1847 +0x64d
created by net/http.(*Server).Serve
        C:/Go/src/net/http/server.go:2851 +0x2fc
2018/12/07 09:34:55 http: panic serving 192.168.50.34:63113: runtime error: index out of range
goroutine 24 [running]:
net/http.(*conn).serve.func1(0xc00016b220)
        C:/Go/src/net/http/server.go:1746 +0xd7
panic(0xa254e0, 0xf9e740)
        C:/Go/src/runtime/panic.go:513 +0x1c7
github.com/gin-gonic/gin.(*Context).Next(0xc0000c04d0)
        C:/Users/USERNAME/go/src/github.com/gin-gonic/gin/context.go:109 +0x87
github.com/gin-gonic/gin.(*Engine).handleHTTPRequest(0xc0000a9680, 0xc0000c04d0)
        C:/Users/USERNAME/go/src/github.com/gin-gonic/gin/gin.go:383 +0x5b2
github.com/gin-gonic/gin.(*Engine).ServeHTTP(0xc0000a9680, 0xbb20a0, 0xc0001d82a0, 0xc000100a00)
        C:/Users/USERNAME/go/src/github.com/gin-gonic/gin/gin.go:349 +0x139
net/http.serverHandler.ServeHTTP(0xc00006be10, 0xbb20a0, 0xc0001d82a0, 0xc000100a00)
        C:/Go/src/net/http/server.go:2741 +0xb2
net/http.(*conn).serve(0xc00016b220, 0xbb2aa0, 0xc000178980)
        C:/Go/src/net/http/server.go:1847 +0x64d
created by net/http.(*Server).Serve
        C:/Go/src/net/http/server.go:2851 +0x2fc

[thinkerou]

Sorry, I still can't reproduce it.

[vkd]

I have reproduced it.

It happens because during func (c *Context) Next() { method it iterates over c.handlers with precalculated size of c.handlers.

func (c *Context) Next() {
	c.index++
	for s := int8(len(c.handlers)); c.index < s; c.index++ {
		c.handlers[c.index](c)
	}
}

But func (group *RouterGroup) StaticFS(...) ... { rewrite c.handlers to slice with other size than we calculated in c.Next(...) (https://github.com/gin-gonic/gin/blob/master/routergroup.go#L198)

func (group *RouterGroup) createStaticHandler(relativePath string, fs http.FileSystem) HandlerFunc {
	...
	return func(c *Context) {
		...
		// Check if file exists and/or if we have permission to access it
		if _, err := fs.Open(file); err != nil {
			c.Writer.WriteHeader(http.StatusNotFound)
			c.handlers = group.engine.allNoRoute
			// Reset index
			c.index = -1
			return
		}

		fileServer.ServeHTTP(c.Writer, c.Request)
	}
}

[thinkerou]

#1744