[Bug] Mixed params and exact paths can cause a 404

[rw-access]

Router1 "/get/test/abc/"
Router2 "/get/:param/abc/"

I tests these two routers with some requests:
/get/test/abc/ -> Router1
/get/xyz/abc/ -> Router2
/get/tt/abc/ -> 404(Expect Router2)
So, if there are some param starts with the prefix 't' in this example, it will not get the right router.

Originally posted by @Tevic in #2663 (comment)

[rw-access]

I don't have the ability to self-assign this, but feel free to assign me.
I've got an idea for a fix.

[g1eny0ung]

@rw-access I'm also facing this problem, here are the example routes I defined:

r.GET("/", xxx)
r.GET("/:foo", xxx)
r.Group("/api", xxx)
r.GET("/static/*any", xxx)
r.GET("/favicon.ico", xxx)

For the realworld project needed, I try to solve this problem. After debugging, I found that it's related to https://github.com/gin-gonic/gin/pull/2663/files#diff-050830ac4334170e8d335e1b95519052a3461c5b57a4ca38a0531e35a8f3d388R410:

// Try all the non-wildcard children first by matching the indices
idxc := path[0]
for i, c := range []byte(n.indices) {
	if c == idxc {
		n = n.children[i]
		continue walk
	}
}

Since the node indices will build as asf to make a fast deep search. When accessing the route like /archives, the above code will jump into /api route group to find the matching route, then it failed (return 404).

Before #2663, the above code wrapped with an if condition if !n.wildChild so it can be executed normally without any other side effect.

I wanted to propose a PR for repair, but I think it’s best to discuss how to do it. Seems to solve this problem, I think it's needed to add an extra condition like if children have a :param node after c == idxc？Here is an example of linking the above code:

// Try all the non-wildcard children first by matching the indices
idxc := path[0]
for i, c := range []byte(n.indices) {
	if c == idxc {
		childWillWalkTo := n.children[i]

		if strings.Split(childWillWalkTo.path, "/")[0] != strings.Split(path, "/")[0] && strings.HasPrefix(n.children[len(n.children)-1].path, ":") {
			// "api" != "archives" && :foo has :
			continue
		} else {
			n = childWillWalkTo
			continue walk
		}
	}
}

I’m not sure if there are other things to pay attention to. Hope to get your feedback.

[appleboy]

@rw-access Can you take a look at the PR #2706?