Fiber: 🐛 ctx.IPs() returns wrong result when there is no X-Forwarded-For in request header

[abowloflrf]

Fiber version
v1.13.3

Issue description
fiber.Ctx.IPs() returns slice of string with one empty string ([]string{""}) in it when there is no X-Forwarded-For header. But what I espect is an empty slice of string. I think it's cased by a recent change in #612

https://github.com/gofiber/fiber/pull/612/files#diff-8500aba9f2dd08390bcbe6a506b66564R501

ips = make([]string, bytes.Count(header, []byte(","))+1)

In line L501. I think it's not a good idea to calculate the forward ips' length by the counting the comma becase there could be no X-Forwarded-For field in header.

BTW, should we consider the non-standard HTTP headers? For example in the X-Forwarded-For header:

a,b,  c # no space or extra space after comma
,a,b    # comma in wrong place
a,,b    # and this
,       # only comma

What should IPs() return? I assume []string{a,b,c} []string{a,b} []string{a,b} []string{} is what we expected, right?

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[abowloflrf]

What about switching back to the strings.Split way, with some improvement:

func (ctx *Ctx) IPs() []string {
    ips := []string{}
    for _, ip := range strings.Split(ctx.Get(HeaderXForwardedFor), ",") {
        trimmedIP := strings.TrimSpace(ip)
        if len(trimmedIP) != 0 {
            ips = append(ips, trimmedIP)
        }
    }
    return ips
}

In most cases, there could be only one ip or empty in the X-Forwarded-For header. I think this part is only slight impact on performance 🤔

[kiyonlin]

Thanks for your reporting! If the value of X-Forwarded-For header is empty, the answer should be an empty slice too(or nil).

And we don’t consider non-standard Http headers until now.

[Fenny]

@abowloflrf, nice find 👍 should be tagged next week