go servemux slashes
Signed-off-by: Xe <me@christine.website>
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title: Go net/http.ServeMux and Trailing Slashes
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date: 2021-11-04
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tags:
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- golang
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---
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When you write software, there are two kinds of problems that you run into:
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1. Problems that stretch your fundamental knowledge of how things work and as a
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result of solving them you become one step closer to unlocking the secrets to
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immortality and transcending beyond mere human limitations
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2. Exceedingly stupid typos that static analysis tools can't be taught how to
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catch and thus dooms humans to feel like they wasted so much time on
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something so trivial
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3. Off-by-one errors
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Today I ran into one of these three types of problems.
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[Buckle up, it's story time!](conversation://Cadey/coffee)
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It's a Thursday morning. Everything in this project has been going smoothly.
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Almost too smoothly. Then `go test` is run to make sure that things are working like we expect.
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[Huh, the test is passing, but the debug output says it should be failing.
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What's up with that? What's going on here?](conversation://Mara/hmm)
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The code in question had things that looked like this:
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```go
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func TestKlaDatni(t *testing.T) {
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tru := zbasuTurnis(t)
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ts := httptest.NewServer(tru)
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defer ts.Stop()
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var buf bytes.Buffer
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failOnErr(t, json.NewEncoder(&buf).Encode(Renma{ Judri: "mara@cipra.jbo" }))
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u, _ := url.Parse(ts.BaseURL)
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u.Path = "/api/v2/kla"
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req, err := http.NewRequest(http.MethodPost, u.String(), &buf)
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failOnErr(t, err)
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tru.InjectAuth(req)
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resp, err := http.DefaultClient.Do(req)
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failOnErr(t, err)
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if resp.StatusCode == http.StatusOK {
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t.Fatalf("wanted status code %d, got: %d", http.StatusOK, resp.StatusCode)
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}
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}
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```
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The error message looked like this:
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```
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[INFO] turnis: invalid method GET for path /api/v2/kla
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```
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[I'm not totally sure what's going on, let's dig into Turnis and see what it's
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doing. Surely we're missing something.](conversation://Cadey/coffee)
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Digging deeper into the Turnis code, the API route was declared using
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[net/http.ServeMux](https://pkg.go.dev/net/http#ServeMux) like this:
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```go
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mux.Handle("/api/v2/kla/", logWrap(tru.adminKla))
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```
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[Maybe the `logWrap` middleware is changing it to `GET`
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somehow?](conversation://Cadey/coffee)
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[Nope, it's too trivial for that to happen:](conversation://Mara/hmm)
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```go
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func logWrap(next http.Handler) http.Handler {
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return xsweb.Falible(xsweb.WithLogging(next))
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}
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```
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Then a moment of inspiration hit and part of the [net/http.ServeMux
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documentation](https://pkg.go.dev/net/http#ServeMux)
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came to mind. A ServeMux is basically a type that lets you associate HTTP paths
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with handler functions, kinda like this:
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```
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mux := http.NewServeMux()
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mux.HandleFunc("/", index)
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mux.HandleFunc("/robots.txt", robotsTxt)
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mux.HandleFunc("/blog/", showBlogPost)
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```
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The part of the documentation that stood out was this:
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> Patterns name fixed, rooted paths, like "/favicon.ico", or rooted subtrees,
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> like "/images/" (note the trailing slash). Longer patterns take precedence
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> over shorter ones, so that if there are handlers registered for both
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> "/images/" and "/images/thumbnails/", the latter handler will be called for
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> paths beginning "/images/thumbnails/" and the former will receive requests for
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> any other paths in the "/images/" subtree.
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Based on those rules, here's a small table of inputs and the functions that
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would be called when a request comes in:
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| Path | Handler |
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| :--- | :------ |
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| `/` | `index` |
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| `/robots.txt` | `robotsTxt` |
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| `/blog/` | `showBlogPost` |
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| `/blog/foo` | `showBlogPost` |
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There's a caveat noted in the documentation:
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> If a subtree has been registered and a request is received naming the subtree
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> root without its trailing slash, ServeMux redirects that request to the
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> subtree root (adding the trailing slash). This behavior can be overridden with
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> a separate registration for the path without the trailing slash. For example,
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> registering "/images/" causes ServeMux to redirect a request for "/images" to
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> "/images/", unless "/images" has been registered separately.
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This means that the code from earlier that looked like this:
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```go
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u.Path = "/api/v2/kla"
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```
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wasn't actually going to the `tru.adminKla` function. It was getting redirected.
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This is because HTTP [doesn't allow you to redirect a POST
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request](https://support.postman.com/hc/en-us/articles/211913929-My-POST-request-is-redirected-to-a-GET-request).
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As a result, the POST request is getting downgraded to a GET request and the
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body is just lost forever.
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[Well okay, technically some frameworks _allow you to do this_ and others
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will use a special HTTP status code to automate this, but Go's
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doesn't.](conversation://Cadey/coffee)
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The fix for that part ended up looking like this:
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```diff
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- u.Path = "/api/v2/kla"
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+ u.Path = "/api/v2/kla/"
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```
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Then `go test` was run again and the test started failing even though Turnis was
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reporting that everything was successful. Then the final typo was spotted:
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```diff
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- if resp.StatusCode == http.StatusOK {
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+ if resp.StatusCode != http.StatusOK {
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t.Fatalf("wanted status code %d, got: %d", http.StatusOK, resp.StatusCode)
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}
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```
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<center>
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</center>
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[It took us 6 hours combined to figure this out. Is that okay? It feels like
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that's wasting too much time on a simple problem like
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that.](conversation://Mara/hmm)
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[That's just how some of these kinds of problems are. The dumbest problems
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always take the longest to figure out because they are the ones that tools can't
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really warn you about. I once spent 15 hours of straight effort trying to fix
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something to find out that `ON` is a yaml value for "true" and that what I was
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trying to do needed to be `"ON"` instead. This is our lot in life as software
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people. You are going to make these kinds of mistakes and it is going to make
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you feel like an absolute buffoon every time. That is just how it happens. Let's
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go play Fortnite and forget about all this for
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now.](conversation://Cadey/coffee)
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