# M6.7 — Single-binary embed verification

## Context

M6.7 is the final task in M6 (Go-native HTML frontend). Per the
[progress tracker](2026-05-03-2349-go-backend-rewrite-progress.md): "Wire
`embed.FS` into handlers; verify single-binary deployment includes all
assets."

The wiring is **already in place** from M6.1 onward:

- [go/internal/web/assets.go](../../go/internal/web/assets.go) declares
  `//go:embed templates` → `templateFS` and `//go:embed static` → `staticFS`.
- [go/internal/web/render.go:66](../../go/internal/web/render.go#L66)
  parses every page template via `template.New(...).ParseFS(templateFS, ...)`.
- [go/internal/web/server.go:48-68](../../go/internal/web/server.go#L48-L68)
  serves `/static/*` via `http.FileServerFS(fs.Sub(staticFS, "static"))`.
- [go/build/Dockerfile](../../go/build/Dockerfile) copies only the compiled
  binary into the `alpine:3` runtime image — no `templates/` or `static/`
  directory ever lands beside it.

What is missing is **proof** the embed is complete and stays complete:

1. Nothing fails the build/test if a contributor adds a new file under
   `internal/web/templates/` or `internal/web/static/` that isn't matched
   by an `//go:embed` glob (or, more realistically, adds a sibling
   directory like `static/img/` and the glob still picks it up — but a
   typo'd directive would silently drop it).
2. No automated test exercises the `/static/*` route against the embedded
   FS — current tests in
   [html_handler_test.go](../../go/internal/web/html_handler_test.go)
   render templates (which proves `templateFS` is good) but never hit a
   static URL through the mux.
3. The "single binary, no working-dir assets" property is undocumented —
   if it ever broke, no one would notice until the Docker image started
   500'ing in prod.

The intended outcome: a small test file plus a documented manual
verification step, after which M6.7 can be ticked and M6 closed.

## Plan

### 1. Add `go/internal/web/assets_test.go`

One new file, two tests, no production code changes.

**Test A — embed completeness regression guard.** Walks the on-disk
`templates/` and `static/` directories and asserts every regular file is
also present in the corresponding embedded FS. Catches:

- A new template added without updating the `//go:embed` directive
  (current globs are `templates` and `static` — recursive by default for
  directories, so this is a low-probability regression, but the test
  doubles as living documentation of the contract).
- A typo in the directive (e.g. someone renames `static` → `assets` in
  one place but not the other).

Implementation sketch:

```go
func TestEmbedCompleteness(t *testing.T) {
    cases := []struct {
        name   string
        diskFS fs.FS         // os.DirFS("templates") / os.DirFS("static")
        embed  fs.FS         // exported helper or via internal test in package web
        root   string
    }{...}
    for _, tc := range cases {
        _ = fs.WalkDir(tc.diskFS, ".", func(path string, d fs.DirEntry, err error) error {
            if err != nil || d.IsDir() { return err }
            embPath := tc.root + "/" + path
            if _, err := fs.Stat(tc.embed, embPath); err != nil {
                t.Errorf("file %q on disk but missing in embed.FS: %v", embPath, err)
            }
            return nil
        })
    }
}
```

Because `templateFS` and `staticFS` are unexported package vars, this
test lives in `package web` (not `web_test`) — sibling to
[assets.go](../../go/internal/web/assets.go). All the existing handler
tests are in `package web_test`; that's fine, this one is internal.

**Test B — `/static/*` end-to-end via the mux.** Builds an `http.ServeMux`
with the same wiring as
[server.go:68](../../go/internal/web/server.go#L68), fires httptest
requests, asserts:

- `GET /static/css/app.css` → 200, `Content-Type: text/css; charset=utf-8`,
  body contains a known string from app.css (e.g. a CSS selector).
- `GET /static/js/member-detail.js` → 200, `Content-Type` starts with
  `text/javascript` or `application/javascript`, body non-empty.
- `GET /static/js/payment-qr.js` → 200, body non-empty.
- `GET /static/css/missing.css` → 404 (sanity: the file server actually
  rejects unknown paths instead of returning some default).

Rather than duplicate the mux assembly, factor a tiny helper (or test the
existing mux). The cleanest move: extract `staticHandler()` from
[server.go:48-50,68](../../go/internal/web/server.go#L48-L68) into a small
exported-from-package function or just `staticFS` / `fs.Sub` helper, and
have the test call it. Smallest delta: keep production code unchanged and
replicate the two-line wiring inside the test file (acceptable — it's
two lines and the test exists precisely to lock that contract).

### 2. Manual / one-shot verification (no code; documented in plan only)

Run once locally and tick M6.7. Command transcript:

```bash
make go-build                  # → ./bin/fuj
cp bin/fuj /tmp/fuj-standalone
cd /tmp                        # working dir has no templates/ or static/
./fuj-standalone server &
SERVER_PID=$!
sleep 1
curl -sf http://localhost:8080/adults | grep -q "Adults Dashboard"
curl -sf http://localhost:8080/juniors | grep -q "Juniors"
curl -sf http://localhost:8080/payments | grep -q "Payments Ledger"
curl -sf -o /tmp/app.css http://localhost:8080/static/css/app.css \
  && test -s /tmp/app.css
curl -sf -o /tmp/qr.js http://localhost:8080/static/js/payment-qr.js \
  && test -s /tmp/qr.js
kill $SERVER_PID
```

`fuj server` will fail to talk to Sheets without credentials, so the
`/adults` etc. pages will render with the `Error` field set — that's
fine; the assertion is that the **template + static asset pipeline** is
self-contained, not that data loads. Each curl above only checks for
markup present in every render path (header text and stylesheet body).

### 3. Tracker + changelog

- Tick `M6.7` in
  [docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md:120](2026-05-03-2349-go-backend-rewrite-progress.md#L120),
  append the merge SHA on the line.
- Mark "Last updated" date and bump milestone status: M6 complete, next is
  M7.
- Append a `CHANGELOG.md` entry per CLAUDE.md convention (`date "+%Y-%m-%d %H:%M %Z"`).

## Files touched

| File | Change |
| --- | --- |
| [go/internal/web/assets_test.go](../../go/internal/web/assets_test.go) | **new** — two tests (embed completeness + `/static/*` mux) |
| [docs/plans/2026-05-03-2349-go-backend-rewrite-progress.md](2026-05-03-2349-go-backend-rewrite-progress.md) | tick M6.7, bump "last updated" |
| [CHANGELOG.md](../../CHANGELOG.md) | new top entry |

No production source files change. (If extracting the static handler
reads cleaner, a 4-line refactor in
[server.go](../../go/internal/web/server.go) is acceptable but optional.)

## Branch + MR

Per project convention this is a feature, so:

```bash
git checkout -b feat/go-m6-7-embed-verify
# … commits …
git push -u origin feat/go-m6-7-embed-verify
tea pr create --title "feat(go): M6.7 — single-binary embed verification" \
  --description "<short body referencing M6.7>" --base main \
  --head feat/go-m6-7-embed-verify
```

## Verification

After implementation:

1. `make go-test` → green (new `TestEmbedCompleteness` and `TestStaticAssetsServed` pass).
2. `make go-lint` → clean.
3. Run the manual transcript in §2 above — all curls succeed, no
   "template not found" or 404 on static assets.
4. `make go-build && docker build -f go/build/Dockerfile -t fuj-go:m6-7 go/` →
   succeeds; `docker run --rm -p 8080:8080 fuj-go:m6-7` serves `/adults`
   with stylesheet attached (visual smoke test in browser).

## Out of scope

- Re-architecting how templates are parsed or served.
- Compressing / fingerprinting static assets (a separate concern).
- Live integration test with real Sheets data — covered later in M7.