在中间件测试（Golang）中需要帮助查明零引用错误的原因

I've run up against an "invalid memory address or nil pointer dereference" error while writing unit tests for my http handlers--specifically, authentication middleware.

Here is the function to be tested:

func Authenticate(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
    var err error
    ck, err := req.Cookie("session")
    if err != nil {
        w.Header().Add("Authorization", "expired/invalid")
        next.ServeHTTP(w, req)
        return
    }

    r, err := env.Db.VerifySession(ck.Value)
    if err != nil {
        http.Error(w, http.StatusText(500), http.StatusInternalServerError)
        return
    }
    if r == "expired/invalid" {
        ck.MaxAge = -1
        http.SetCookie(w, ck)
        w.Header().Add("Authorization", r)
        next.ServeHTTP(w, req)
        return
    }

    w.Header().Add("Authorization", r)
    next.ServeHTTP(w, req)
})
}

The specific test case on which the error is thrown:

rec = httptest.NewRecorder()
req, _ = http.NewRequest("GET", "/", nil)
c := &http.Cookie{
    Name:  "session",
    Value: "success",
}
req.AddCookie(c)
routes.Authenticate(http.NotFoundHandler()).ServeHTTP(rec, req)
assert.Equal("admin", rec.Header().Get("Authorization"))

The mocked version of env.DB.VerifySession used for testing:

func (mdb *mockdb) VerifySession(val string) (string, error) {
switch val {
case "success":
    return "admin", nil
case "failure":
    return "expired/invalid", nil
default:
    return "", errors.New("500")
}
}

I have written a passing test for the the first part of Authenticate(): if no "session" cookie is found, the test confirms that the response header's been appropriately updated. However, when I pass in a "session" cookie, as with the test case above, Authenticate() will run up tor, err := env.Db.VerifySession(ck.Value). Attempting to step over or into that function call in debug mode results in the nil pointer error being thrown.

I don't think the problem is the env.Db variable, as I've successfully run a suite of tests on my non-middleware handlers using the same variable without issue.

I can also confirm through debugging that the "session" cookie does actually exist.

Any guidance on what's going on here would be greatly appreciated.

Nevermind, I realized what the issue is. For my other routes I pass the env variable in as an argument. I need to find a way to provide the middleware with access to it as well.

I suppose I'll define the middleware as a method on env.