frontend/server/internal/content/content_test.go

@@ -19,6 +19,7 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"thermograph/frontend/internal/config"
 	"thermograph/frontend/internal/contentapi"
@@ -474,6 +475,116 @@ func TestMonthCtx(t *testing.T) {
 	}
 }
 
+// --- fetchWithCity -----------------------------------------------------------
+
+func TestFetchWithCityHappyPath(t *testing.T) {
+	api := &fakeAPI{
+		city: func(slug, _ string) (*contentapi.CityPayload, error) {
+			return &contentapi.CityPayload{City: contentapi.CityInfo{Slug: slug, Name: "Testville"}}, nil
+		},
+	}
+	primary, city, err := fetchWithCity(api, "testville", func() (string, error) { return "primary-value", nil })
+	if err != nil {
+		t.Fatal(err)
+	}
+	if primary != "primary-value" {
+		t.Errorf("primary = %q", primary)
+	}
+	if city.Slug != "testville" || city.Name != "Testville" {
+		t.Errorf("city = %+v", city)
+	}
+}
+
+// When only primary fails, its error must win even though City() also ran
+// (and, here, succeeded) -- matching the old sequential code's behavior,
+// where City() was never even called once primary had already failed.
+func TestFetchWithCityPrimaryErrorWins(t *testing.T) {
+	primaryErr := notFound("no such month")
+	api := &fakeAPI{
+		city: func(slug, _ string) (*contentapi.CityPayload, error) {
+			return &contentapi.CityPayload{City: contentapi.CityInfo{Slug: slug}}, nil
+		},
+	}
+	_, _, err := fetchWithCity(api, "testville", func() (string, error) { return "", primaryErr })
+	if err != primaryErr {
+		t.Errorf("err = %v, want the primary error", err)
+	}
+}
+
+// When primary succeeds but City() fails, City's error must surface -- same
+// as the old sequential code (it called City() second and returned whatever
+// that call did).
+func TestFetchWithCityCityErrorSurfaces(t *testing.T) {
+	cityErr := notFound("no such city")
+	api := &fakeAPI{
+		city: func(_, _ string) (*contentapi.CityPayload, error) { return nil, cityErr },
+	}
+	_, _, err := fetchWithCity(api, "testville", func() (string, error) { return "primary-value", nil })
+	if err != cityErr {
+		t.Errorf("err = %v, want the city error", err)
+	}
+}
+
+// When BOTH fail, primary's error still wins -- the same priority as the
+// single-failure case above, just with both goroutines erroring.
+func TestFetchWithCityBothErrorPrimaryWins(t *testing.T) {
+	primaryErr, cityErr := notFound("primary"), notFound("city")
+	api := &fakeAPI{
+		city: func(_, _ string) (*contentapi.CityPayload, error) { return nil, cityErr },
+	}
+	_, _, err := fetchWithCity(api, "testville", func() (string, error) { return "", primaryErr })
+	if err != primaryErr {
+		t.Errorf("err = %v, want the primary error (city error must not win)", err)
+	}
+}
+
+// Proves the two calls actually run CONCURRENTLY rather than sequentially --
+// deterministically, via a rendezvous, not by timing (which would be flaky
+// under CI load). Each fake blocks until BOTH have started; if fetchWithCity
+// ran them sequentially, the second would never start until the first
+// returned, and this would deadlock and fail on the test's own timeout.
+func TestFetchWithCityRunsConcurrently(t *testing.T) {
+	started := make(chan struct{}, 2)
+	release := make(chan struct{})
+	rendezvous := func() {
+		started <- struct{}{}
+		<-release
+	}
+
+	api := &fakeAPI{
+		city: func(_, _ string) (*contentapi.CityPayload, error) {
+			rendezvous()
+			return &contentapi.CityPayload{}, nil
+		},
+	}
+
+	done := make(chan struct{})
+	go func() {
+		fetchWithCity(api, "testville", func() (string, error) {
+			rendezvous()
+			return "", nil
+		})
+		close(done)
+	}()
+
+	// Both goroutines must reach the rendezvous before either can proceed --
+	// only possible if they were launched concurrently.
+	for range 2 {
+		select {
+		case <-started:
+		case <-time.After(2 * time.Second):
+			t.Fatal("timed out waiting for both calls to start -- they are not running concurrently")
+		}
+	}
+	close(release)
+
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("fetchWithCity did not return after release")
+	}
+}
+
 func TestMonthCtxUnknownLabelIsError(t *testing.T) {
 	h := newHandlers(t, fixtureAPI(t))
 	j := loadFixture[contentapi.MonthPayload](t, "month")

frontend/server/internal/content/pages.go

@@ -7,6 +7,7 @@ import (
 	"html/template"
 	"net/http"
 	"strings"
+	"sync"
 
 	"thermograph/frontend/internal/contentapi"
 	"thermograph/frontend/internal/contentdata"
@@ -165,6 +166,49 @@ func (h *Handlers) cityCtx(j *contentapi.CityPayload) (map[string]any, format.Un
 	return ctx, unit, nil
 }
 
+// fetchWithCity runs a page's primary API call and the shared City lookup
+// concurrently instead of sequentially: both are independent single-slug
+// fetches (City never depends on primary's result), so waiting for one to
+// fully round-trip before even starting the other was pure latency with
+// nothing to show for it. Concurrent instead cuts that leg to roughly
+// whichever call is slower.
+//
+// If primary fails, its error wins even when City also fails or hasn't
+// finished — matching the old sequential code's behavior exactly (it never
+// even called City() once primary had already failed). The one real
+// trade-off: City() is now ALWAYS launched, even on a request that's about
+// to 404 from primary, so an invalid slug costs one extra (wasted, cheap)
+// backend lookup it previously skipped. Worth it: a bad slug is the rare
+// path; a good one is the common path this speeds up.
+func fetchWithCity[T any](api API, slug string, primary func() (T, error)) (T, contentapi.CityInfo, error) {
+	var (
+		pVal  T
+		pErr  error
+		cPay  *contentapi.CityPayload
+		cErr  error
+		group sync.WaitGroup
+	)
+	group.Add(2)
+	go func() {
+		defer group.Done()
+		pVal, pErr = primary()
+	}()
+	go func() {
+		defer group.Done()
+		cPay, cErr = api.City(slug, "") // no origin: the Python called city(slug) bare here too
+	}()
+	group.Wait()
+
+	if pErr != nil {
+		return pVal, contentapi.CityInfo{}, pErr
+	}
+	if cErr != nil {
+		var zero T
+		return zero, contentapi.CityInfo{}, cErr
+	}
+	return pVal, cPay.City, nil
+}
+
 // --- month page --------------------------------------------------------------
 
 // MonthPage renders /climate/{slug}/{month} (content.py's month_page).
@@ -176,17 +220,14 @@ func (h *Handlers) cityCtx(j *contentapi.CityPayload) (map[string]any, format.Un
 // Jinja iterated (label, value) tuples).
 func (h *Handlers) MonthPage(w http.ResponseWriter, r *http.Request) {
 	slug, month := r.PathValue("slug"), r.PathValue("month")
-	j, err := h.api.CityMonth(slug, month)
+	j, city, err := fetchWithCity(h.api, slug, func() (*contentapi.MonthPayload, error) {
+		return h.api.CityMonth(slug, month)
+	})
 	if err != nil {
 		h.apiError(w, err)
 		return
 	}
-	cp, err := h.api.City(slug, "") // no origin: the Python called city(slug) bare here
-	if err != nil {
-		h.apiError(w, err)
-		return
-	}
-	ctx, unit, err := h.monthCtx(j, cp.City)
+	ctx, unit, err := h.monthCtx(j, city)
 	if err != nil {
 		h.serverError(w, err)
 		return
@@ -308,17 +349,15 @@ func fmtPeriodSide(unit format.Unit, s contentapi.PeriodSide) map[string]any {
 // temperature metrics, nil otherwise, as the Python set them.
 func (h *Handlers) RecordsPage(w http.ResponseWriter, r *http.Request) {
 	slug := r.PathValue("slug")
-	j, err := h.api.CityRecords(slug, origin(r))
+	o := origin(r)
+	j, city, err := fetchWithCity(h.api, slug, func() (*contentapi.RecordsPayload, error) {
+		return h.api.CityRecords(slug, o)
+	})
 	if err != nil {
 		h.apiError(w, err)
 		return
 	}
-	cp, err := h.api.City(slug, "") // no origin, same as month_page
-	if err != nil {
-		h.apiError(w, err)
-		return
-	}
-	ctx, unit, err := h.recordsCtx(j, cp.City)
+	ctx, unit, err := h.recordsCtx(j, city)
 	if err != nil {
 		h.serverError(w, err)
 		return