| 1 | package main |
| 2 | |
| 3 | // ECB reference rates for the pricing picker's local-currency ≈ display |
| 4 | // (docs/pricing-bands.md § Local-currency display). Billing stays EUR; the |
| 5 | // local figure is a labelled estimate. This file is the whole third-party |
| 6 | // surface: home fetches the ECB's daily XML (free, no key, ~30 currencies) |
| 7 | // on a timer and serves a tiny cached JSON — the BROWSER never talks to a |
| 8 | // third party (its CSP stays pinned to this origin). |
| 9 | // |
| 10 | // Availability beats freshness: a fetch failure keeps the last-good snapshot |
| 11 | // (reference rates move ~daily; a stale ≈ is still honest, and the label |
| 12 | // says "at today's rate" of the snapshot's own date). Before the first |
| 13 | // successful fetch the endpoint serves an empty rate set and the client |
| 14 | // simply shows no ≈ figure. |
| 15 | |
| 16 | import ( |
| 17 | "encoding/xml" |
| 18 | "io" |
| 19 | "log" |
| 20 | "net/http" |
| 21 | "sync" |
| 22 | "time" |
| 23 | ) |
| 24 | |
| 25 | const ecbRatesURL = "https://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml" |
| 26 | |
| 27 | // fxCache is the last-good snapshot. Zero value ready (empty until fetched). |
| 28 | type fxCache struct { |
| 29 | mu sync.Mutex |
| 30 | date string // the ECB's own quote date, e.g. "2026-07-17" |
| 31 | rates map[string]float64 // currency code → units per EUR |
| 32 | } |
| 33 | |
| 34 | // ecbEnvelope is just enough of the ECB feed's XML to read the rate cubes. |
| 35 | type ecbEnvelope struct { |
| 36 | Cube struct { |
| 37 | Day struct { |
| 38 | Time string `xml:"time,attr"` |
| 39 | Rates []struct { |
| 40 | Currency string `xml:"currency,attr"` |
| 41 | Rate float64 `xml:"rate,attr"` |
| 42 | } `xml:"Cube"` |
| 43 | } `xml:"Cube"` |
| 44 | } `xml:"Cube"` |
| 45 | } |
| 46 | |
| 47 | // refresh fetches the feed once, replacing the snapshot only on success. |
| 48 | func (c *fxCache) refresh(client *http.Client, feedURL string) error { |
| 49 | resp, err := client.Get(feedURL) |
| 50 | if err != nil { |
| 51 | return err |
| 52 | } |
| 53 | defer resp.Body.Close() |
| 54 | if resp.StatusCode != http.StatusOK { |
| 55 | return errHTTP(resp.StatusCode) |
| 56 | } |
| 57 | var env ecbEnvelope |
| 58 | if err := xml.NewDecoder(io.LimitReader(resp.Body, 1<<20)).Decode(&env); err != nil { |
| 59 | return err |
| 60 | } |
| 61 | rates := map[string]float64{} |
| 62 | for _, r := range env.Cube.Day.Rates { |
| 63 | if r.Currency != "" && r.Rate > 0 { |
| 64 | rates[r.Currency] = r.Rate |
| 65 | } |
| 66 | } |
| 67 | if len(rates) == 0 { |
| 68 | return errEmptyFeed |
| 69 | } |
| 70 | c.mu.Lock() |
| 71 | c.date, c.rates = env.Cube.Day.Time, rates |
| 72 | c.mu.Unlock() |
| 73 | return nil |
| 74 | } |
| 75 | |
| 76 | type fxErr string |
| 77 | |
| 78 | func (e fxErr) Error() string { return string(e) } |
| 79 | |
| 80 | const errEmptyFeed = fxErr("fx: feed parsed but held no rates") |
| 81 | |
| 82 | func errHTTP(code int) error { return fxErr("fx: feed answered HTTP " + http.StatusText(code)) } |
| 83 | |
| 84 | // fxLoop refreshes the snapshot at boot and then twice a day (the feed |
| 85 | // updates once each business day; a missed pass self-heals on the next). |
| 86 | func (s *Server) fxLoop(feedURL string) { |
| 87 | client := &http.Client{Timeout: 30 * time.Second} |
| 88 | for { |
| 89 | if err := s.fx.refresh(client, feedURL); err != nil { |
| 90 | log.Printf("fx: refresh: %v (serving the last-good snapshot)", err) |
| 91 | } |
| 92 | time.Sleep(12 * time.Hour) |
| 93 | } |
| 94 | } |
| 95 | |
| 96 | // fxHandler: GET /api/fx — the cached snapshot, verbatim. Public: reference |
| 97 | // rates are public data and the response identifies nobody. |
| 98 | func (s *Server) fxHandler(w http.ResponseWriter, r *http.Request) { |
| 99 | s.fx.mu.Lock() |
| 100 | date, rates := s.fx.date, s.fx.rates |
| 101 | s.fx.mu.Unlock() |
| 102 | if rates == nil { |
| 103 | rates = map[string]float64{} |
| 104 | } |
| 105 | // A day's cache: clients re-check on load; the snapshot moves ~daily. |
| 106 | w.Header().Set("Cache-Control", "public, max-age=21600") |
| 107 | writeJSON(w, http.StatusOK, map[string]any{"base": "EUR", "date": date, "rates": rates}) |
| 108 | } |
| 109 | |
amadan