feat(web): show ICS subscription events in detail view

cmd/server/main.go

@@ -17,6 +17,7 @@ import (
 	"git.arnef.de/arnef/nidus/internal/carddav"
 	"git.arnef.de/arnef/nidus/internal/config"
 	"git.arnef.de/arnef/nidus/internal/db"
+	"git.arnef.de/arnef/nidus/internal/icssub"
 	"git.arnef.de/arnef/nidus/internal/store"
 	"git.arnef.de/arnef/nidus/internal/web"
 	filewebdav "git.arnef.de/arnef/nidus/internal/webdav"
@@ -103,10 +104,16 @@ func main() {
 	authMw := auth.NewMiddleware(cfg, dbase, logger)
 
 	// ---- Handlers ----
-	calHandler := caldav.NewHandler(cfg, st, dbase, logger)
+	// A single ICS-subscription cache is shared by both the CalDAV backend
+	// (for DAV clients) and the web UI (for the browser calendar page), so
+	// the same subscription is fetched and served identically regardless
+	// of which surface a client hits, and one background refresh refreshes
+	// both at once.
+	icsCache := icssub.NewCache(icssub.DefaultTTL)
+	calHandler := caldav.NewHandler(cfg, st, dbase, logger, icsCache)
 	cardHandler := carddav.NewHandler(cfg, st, dbase, logger)
 	fileHandler := filewebdav.NewHandler(cfg, cfg.Storage.DataDir, logger)
-	webUI := web.NewServer(cfg, st, dbase, logger)
+	webUI := web.NewServer(cfg, st, dbase, logger, icsCache)
 
 	mux := buildMux(cfg, authMw, calHandler, cardHandler, fileHandler, webUI, logger)
 

internal/caldav/backend.go

@@ -41,10 +41,16 @@ type Backend struct {
 	icsCache *icssub.Cache
 }
 
-// NewBackend creates a CalDAV backend. dbase may be nil, in which case
-// calendar sharing is disabled (only a user's own calendars are visible).
-func NewBackend(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) *Backend {
-	return &Backend{cfg: cfg, store: st, dbase: dbase, logger: logger, icsCache: icssub.NewCache(icssub.DefaultTTL)}
+// NewBackend creates a CalDAV backend over an existing ICS cache.
+// dbase may be nil, in which case calendar sharing is disabled (only a
+// user's own calendars are visible). The icsCache is shared with any
+// other consumers (notably the web UI) so CalDAV and the web calendar
+// page see identical, cache-consistent events for ICS subscriptions.
+func NewBackend(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger, icsCache *icssub.Cache) *Backend {
+	if icsCache == nil {
+		icsCache = icssub.NewCache(icssub.DefaultTTL)
+	}
+	return &Backend{cfg: cfg, store: st, dbase: dbase, logger: logger, icsCache: icsCache}
 }
 
 // NewHandler returns an http.Handler for the /cal/ prefix.
@@ -54,8 +60,8 @@ func NewBackend(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.
 // property into PROPFIND responses for calendar collections, since
 // go-webdav's caldav.Backend interface has no extension point for
 // vendor-specific WebDAV properties.
-func NewHandler(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) http.Handler {
-	b := NewBackend(cfg, st, dbase, logger)
+func NewHandler(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger, icsCache *icssub.Cache) http.Handler {
+	b := NewBackend(cfg, st, dbase, logger, icsCache)
 	return &colorInjectingHandler{backend: b, next: &caldav.Handler{Backend: b}}
 }
 

internal/caldav/backend_test.go

@@ -46,7 +46,7 @@ func newTestBackend(t *testing.T) (*Backend, *db.DB) {
 		t.Fatalf("CreateCalendar bob/personal: %v", err)
 	}
 	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
-	return NewBackend(cfg, st, dbase, logger), dbase
+	return NewBackend(cfg, st, dbase, logger, nil), dbase
 }
 
 func ctxFor(username string) context.Context {
@@ -179,7 +179,7 @@ func TestPropFindEmitsCalendarColor(t *testing.T) {
 	}
 
 	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
-	handler := NewHandler(&config.Config{}, st, dbase, logger)
+	handler := NewHandler(&config.Config{}, st, dbase, logger, nil)
 
 	req := httptest.NewRequest("PROPFIND", "/cal/home/", strings.NewReader(
 		`<?xml version="1.0"?><a:propfind xmlns:a="DAV:"><a:allprop/></a:propfind>`))
@@ -250,7 +250,7 @@ func TestPropFindExplicitCalendarColorRequest(t *testing.T) {
 	}
 
 	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
-	handler := NewHandler(&config.Config{}, st, dbase, logger)
+	handler := NewHandler(&config.Config{}, st, dbase, logger, nil)
 
 	req := httptest.NewRequest("PROPFIND", "/cal/home/work/", strings.NewReader(
 		`<?xml version="1.0"?><D:propfind xmlns:D="DAV:" xmlns:A="http://apple.com/ns/ical/">`+

internal/caldav/ics.go

@@ -1,8 +1,6 @@
 package caldav
 
 import (
-	"crypto/sha1"
-	"encoding/hex"
 	"fmt"
 	"net/http"
 	"strings"
@@ -13,6 +11,7 @@ import (
 	"github.com/emersion/go-webdav/caldav"
 
 	"git.arnef.de/arnef/nidus/internal/db"
+	"git.arnef.de/arnef/nidus/internal/icssub"
 )
 
 // defaultICSColor is the display color for an ICS/webcal subscription
@@ -32,8 +31,9 @@ func (b *Backend) icsSubscriptionCalendarMeta(owner string, sub db.ICSSubscripti
 }
 
 // icsObjectUID returns the UID a fetched VEVENT should be addressed by:
-// its own UID property if it has one, otherwise a stable hash of its
-// position so it still round-trips consistently between requests.
+// its own UID property if it has one, otherwise a placeholder derived from
+// the object ID (so the event still has a *unique* UID in the returned
+// VCALENDAR).
 func icsObjectUID(ev ical.Event, fallback string) string {
 	if p := ev.Props.Get(ical.PropUID); p != nil && p.Value != "" {
 		return p.Value
@@ -41,16 +41,11 @@ func icsObjectUID(ev ical.Event, fallback string) string {
 	return fallback
 }
 
-// icsObjID builds the object ID (file-name-like, ".ics" suffixed) used to
-// address a fetched VEVENT within its subscription calendar, derived from
-// its UID so it stays stable across fetches of the same feed.
-func icsObjID(uid string) string {
-	sum := sha1.Sum([]byte(uid))
-	return hex.EncodeToString(sum[:]) + ".ics"
-}
-
-// listICSSubscriptionCalendarObjects fetches sub's remote calendar (via
-// b.icsCache) and returns one caldav.CalendarObject per VEVENT.
+// icsSubscriptionCalendarObjects fetches sub's remote calendar (via
+// b.icsCache) and returns one caldav.CalendarObject per VEVENT. The
+// object path is derived from icssub.EventID(ev) so the same event keeps
+// the same path across fetches, even if its position in the document
+// changes.
 func (b *Backend) listICSSubscriptionCalendarObjects(localName string, sub db.ICSSubscription) ([]caldav.CalendarObject, error) {
 	cal, err := b.icsCache.Get(sub.URL)
 	if err != nil {
@@ -58,8 +53,8 @@ func (b *Backend) listICSSubscriptionCalendarObjects(localName string, sub db.IC
 	}
 
 	var objs []caldav.CalendarObject
-	for i, ev := range cal.Events() {
-		obj, err := b.encodeICSObject(localName, ev, fmt.Sprintf("event-%d", i))
+	for _, ev := range cal.Events() {
+		obj, err := b.encodeICSObject(localName, ev)
 		if err != nil {
 			continue
 		}
@@ -75,24 +70,25 @@ func (b *Backend) icsSubscriptionCalendarObject(localName, objID string, sub db.
 	if err != nil {
 		return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("fetching ics subscription: %w", err))
 	}
-	for i, ev := range cal.Events() {
-		uid := icsObjectUID(ev, fmt.Sprintf("event-%d", i))
-		if icsObjID(uid) != objID {
+	for _, ev := range cal.Events() {
+		if icssub.EventID(ev) != objID {
 			continue
 		}
-		return b.encodeICSObject(localName, ev, fmt.Sprintf("event-%d", i))
+		return b.encodeICSObject(localName, ev)
 	}
 	return nil, webdav.NewHTTPError(http.StatusNotFound, fmt.Errorf("ics subscription event not found"))
 }
 
 // encodeICSObject wraps a single fetched VEVENT ev into its own
 // caldav.CalendarObject, encoding it as a standalone one-event calendar
-// the same way every other calendar object in this backend is
-// represented. fallbackUID is used to derive the object ID/UID if ev has
-// no UID property of its own.
-func (b *Backend) encodeICSObject(localName string, ev ical.Event, fallbackUID string) (*caldav.CalendarObject, error) {
-	uid := icsObjectUID(ev, fallbackUID)
-	objID := icsObjID(uid)
+// the same way every other calendar object in this backend is represented.
+func (b *Backend) encodeICSObject(localName string, ev ical.Event) (*caldav.CalendarObject, error) {
+	objID := icssub.EventID(ev)
+	if objID == "" {
+		// Not addressable (no DTSTART) — skip.
+		return nil, fmt.Errorf("event has no DTSTART; not addressable")
+	}
+	uid := icsObjectUID(ev, objID)
 
 	event := ical.NewEvent()
 	event.Props = ev.Props

internal/caldav/ics_shared_test.go

@@ -0,0 +1,268 @@
+package caldav
+
+import (
+	"context"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"git.arnef.de/arnef/nidus/internal/auth"
+	"git.arnef.de/arnef/nidus/internal/config"
+	"git.arnef.de/arnef/nidus/internal/db"
+	"git.arnef.de/arnef/nidus/internal/icssub"
+	"git.arnef.de/arnef/nidus/internal/store"
+	ical "github.com/emersion/go-ical"
+)
+
+// icsSample is a minimal valid ICS feed with one VEVENT.
+const icsSample = "BEGIN:VCALENDAR\r\n" +
+	"VERSION:2.0\r\n" +
+	"PRODID:-//nidus//test//EN\r\n" +
+	"BEGIN:VEVENT\r\n" +
+	"UID:shared1@nidus.test\r\n" +
+	"DTSTAMP:20260101T000000Z\r\n" +
+	"DTSTART:20260805T090000Z\r\n" +
+	"DTEND:20260805T100000Z\r\n" +
+	"SUMMARY:Shared event\r\n" +
+	"END:VEVENT\r\n" +
+	"END:VCALENDAR\r\n"
+
+// mustCalEvent parses raw and returns its first VEVENT (panic on error;
+// safe in tests).
+func mustCalEvent(t *testing.T, raw string) ical.Event {
+	t.Helper()
+	_ = t
+	cal, err := ical.NewDecoder(strings.NewReader(raw)).Decode()
+	if err != nil {
+		panic("mustCalEvent: " + err.Error())
+	}
+	evs := cal.Events()
+	if len(evs) == 0 {
+		panic("mustCalEvent: no events")
+	}
+	return evs[0]
+}
+
+// TestICSSubscriptionListAndGetObject verifies the full CalDAV read path
+// for an ICS subscription: ListCalendarObjects returns one synthetic
+// stand-alone object whose Path is derived from icssub.EventID (not from
+// the event's position), and the same object is returned via
+// GetCalendarObject by the same Path.
+func TestICSSubscriptionListAndGetObject(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(icsSample))
+	}))
+	defer srv.Close()
+
+	dir := t.TempDir()
+	st, err := store.NewStore(filepath.Join(dir, "data"))
+	if err != nil {
+		t.Fatalf("NewStore: %v", err)
+	}
+	dbase, err := db.Open(filepath.Join(dir, "test.db"))
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	defer dbase.Close()
+
+	if err := dbase.CreateUser("alice", "pw", "", ""); err != nil {
+		t.Fatalf("CreateUser: %v", err)
+	}
+	if err := dbase.CreateICSSubscription("alice", "holidays", srv.URL, ""); err != nil {
+		t.Fatalf("CreateICSSubscription: %v", err)
+	}
+
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	b := NewBackend(&config.Config{}, st, dbase, logger, icssub.NewCache(time.Hour))
+
+	ctx := auth.NewContext(context.Background(), &auth.Principal{Username: "alice"})
+
+	objs, err := b.ListCalendarObjects(ctx, "/cal/home/holidays/", nil)
+	if err != nil {
+		t.Fatalf("ListCalendarObjects: %v", err)
+	}
+	if len(objs) != 1 {
+		t.Fatalf("expected 1 object, got %d", len(objs))
+	}
+
+	// The Path must be derived from the event's DTSTART/DTEND/SUMMARY —
+	// i.e. icssub.EventID(ev), not from the event's position in the feed.
+	ev := mustCalEvent(t, icsSample)
+	wantID := icssub.EventID(ev)
+	if wantID == "" {
+		t.Fatal("EventID must be non-empty for a valid event")
+	}
+	wantPath := calObjectPath("holidays", wantID)
+	if objs[0].Path != wantPath {
+		t.Fatalf("Path = %q, want %q (icssub.EventID-based, not position-based)", objs[0].Path, wantPath)
+	}
+
+	// GetCalendarObject by the same Path should return an object that has
+	// a valid VEVENT whose UID matches our source ICS (round-trip
+	// correctness).
+	found, err := b.GetCalendarObject(ctx, objs[0].Path, nil)
+	if err != nil {
+		t.Fatalf("GetCalendarObject: %v", err)
+	}
+	if found.Path != objs[0].Path {
+		t.Fatalf("round-trip path mismatch: %q vs %q", found.Path, objs[0].Path)
+	}
+	if found.Data == nil || len(found.Data.Events()) == 0 {
+		t.Fatalf("expected found.Data to have >=1 event, got %+v", found.Data)
+	}
+	uid := found.Data.Events()[0].Props.Get(ical.PropUID)
+	if uid == nil || uid.Value != "shared1@nidus.test" {
+		t.Fatalf("expected UID shared1@nidus.test in round-tripped event, got %+v", uid)
+	}
+}
+
+// TestIcssubCacheSharedBetweenConsumersInCalDav verifies that one
+// *icssub.Cache shared by two different callers of the same backend does
+// not re-fetch the upstream twice (the second caller sees the cached
+// copy via the singleflight guard), proving the "return cached value and
+// update in the background" design is reachable from the CalDAV API.
+func TestIcssubCacheSharedBetweenConsumersInCalDav(t *testing.T) {
+	var hits atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		// Sleep long enough that *if* the second Get blocked, this
+		// test's wall clock would obviously exceed the bound below.
+		time.Sleep(30 * time.Millisecond)
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(icsSample))
+	}))
+	defer srv.Close()
+
+	dir := t.TempDir()
+	st, err := store.NewStore(filepath.Join(dir, "data"))
+	if err != nil {
+		t.Fatalf("NewStore: %v", err)
+	}
+	dbase, err := db.Open(filepath.Join(dir, "test.db"))
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	defer dbase.Close()
+
+	if err := dbase.CreateUser("alice", "pw", "", ""); err != nil {
+		t.Fatalf("CreateUser: %v", err)
+	}
+	if err := dbase.CreateICSSubscription("alice", "holidays", srv.URL, ""); err != nil {
+		t.Fatalf("CreateICSSubscription: %v", err)
+	}
+
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	// One cache, shared by two backends on the same DB — mirrors the
+	// production wiring (cmd/server/main.go) where the CalDAV and web UI
+	// share one instance.
+	shared := icssub.NewCache(time.Hour)
+	b1 := NewBackend(&config.Config{}, st, dbase, logger, shared)
+	b2 := NewBackend(&config.Config{}, st, dbase, logger, shared)
+
+	ctx := auth.NewContext(context.Background(), &auth.Principal{Username: "alice"})
+
+	if _, err := b1.ListCalendarObjects(ctx, "/cal/home/holidays/", nil); err != nil {
+		t.Fatalf("backend 1 list: %v", err)
+	}
+	after1 := hits.Load()
+	if _, err := b2.ListCalendarObjects(ctx, "/cal/home/holidays/", nil); err != nil {
+		t.Fatalf("backend 2 list: %v", err)
+	}
+	after2 := hits.Load()
+	if after2 > after1+1 {
+		t.Fatalf("expected shared cache to coalesce (hits %d → %d)", after1, after2)
+	}
+}
+
+// TestIcssubCacheStaleReturnsWithBackgroundRefresh verifies the
+// stale-while-revalidate path through the public Cache API: a cached
+// entry past its TTL is still returned immediately (never blocking the
+// caller for the slow 30 ms fetch), while a background refresh updates
+// the entry for the next Get.
+func TestIcssubCacheStaleReturnsWithBackgroundRefresh(t *testing.T) {
+	var hits atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		time.Sleep(50 * time.Millisecond)
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(icsSample))
+	}))
+	defer srv.Close()
+
+	c := icssub.NewCache(2 * time.Millisecond) // very short TTL
+	if _, err := c.Get(srv.URL); err != nil {
+		t.Fatalf("first Get: %v", err)
+	}
+	firstHits := hits.Load()
+
+	time.Sleep(5 * time.Millisecond) // force staleness
+
+	st := time.Now()
+	if _, err := c.Get(srv.URL); err != nil {
+		t.Fatalf("stale Get: %v", err)
+	}
+	if time.Since(st) > 40*time.Millisecond {
+		t.Fatalf("stale Get blocked on the network for %v — should have returned the cached copy", time.Since(st))
+	}
+
+	// Wait for the background refresh to land.
+	deadline := time.Now().Add(2 * time.Second)
+	for hits.Load() == firstHits && time.Now().Before(deadline) {
+		time.Sleep(5 * time.Millisecond)
+	}
+	if hits.Load() == firstHits {
+		t.Fatalf("background refresh did not fire (hits stayed at %d)", firstHits)
+	}
+}
+
+// TestICSSubscriptionEventIDMatchesEventInCalDav proves that the object
+// path the CalDAV backend advertises (in ListCalendarObjects) is exactly
+// the same string the EventDetailPage-style lookup would use on the web
+// side — i.e. both surfaces agree on what icssub.EventID(ev) produces.
+func TestICSSubscriptionEventIDMatchesEventInCalDav(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(icsSample))
+	}))
+	defer srv.Close()
+
+	dir := t.TempDir()
+	st, err := store.NewStore(filepath.Join(dir, "data"))
+	if err != nil {
+		t.Fatalf("NewStore: %v", err)
+	}
+	dbase, err := db.Open(filepath.Join(dir, "test.db"))
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	defer dbase.Close()
+
+	if err := dbase.CreateUser("alice", "pw", "", ""); err != nil {
+		t.Fatalf("CreateUser: %v", err)
+	}
+	if err := dbase.CreateICSSubscription("alice", "holidays", srv.URL, ""); err != nil {
+		t.Fatalf("CreateICSSubscription: %v", err)
+	}
+
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	b := NewBackend(&config.Config{}, st, dbase, logger, icssub.NewCache(time.Hour))
+	ctx := auth.NewContext(context.Background(), &auth.Principal{Username: "alice"})
+
+	objs, err := b.ListCalendarObjects(ctx, "/cal/home/holidays/", nil)
+	if err != nil || len(objs) == 0 {
+		t.Fatalf("list: objs=%d err=%v", len(objs), err)
+	}
+	gotPath := objs[0].Path
+	wantID := icssub.EventID(mustCalEvent(t, icsSample))
+	wantPath := calObjectPath("holidays", wantID)
+	if gotPath != wantPath {
+		t.Fatalf("CalDAV object path %q does not match web-side EventID-derived %q", gotPath, wantPath)
+	}
+}

internal/icssub/icssub.go

@@ -7,6 +7,8 @@ package icssub
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"io"
 	"net/http"
@@ -19,8 +21,8 @@ import (
 	"git.arnef.de/arnef/nidus/internal/icalfix"
 )
 
-// DefaultTTL is how long a fetched calendar is cached before being
-// re-fetched on the next access.
+// DefaultTTL is how long a fetched calendar is considered "fresh" before a
+// Get will kick off a background refresh.
 const DefaultTTL = 15 * time.Minute
 
 // fetchTimeout bounds how long a single upstream request may take, so one
@@ -32,54 +34,143 @@ const fetchTimeout = 15 * time.Second
 // response.
 const maxBodySize = 32 * 1024 * 1024 // 32 MiB
 
+// entry holds everything the Cache knows about a single upstream URL. All
+// fields are only read/written while holding Cache.mu.
 type entry struct {
+	url string // original URL as supplied by the caller (fetch normalizes)
+
+	// cal is the most recent successfully-fetched calendar. Nil until the
+	// first successful fetch for this URL.
+	cal *ical.Calendar
+	// lastErr is the most recent fetch error. Set alongside cal == nil
+	// (i.e. no successful fetch yet); cleared the moment a fetch succeeds.
+	lastErr error
+
+	// refreshing is true while a fetch (foreground, or background refresh)
+	// is in flight for this URL.
+	refreshing bool
+	// pending is the completion channel for the in-flight fetch. Only valid
+	// while refreshing is true; it is created fresh for each fetch and
+	// closed exactly once when that fetch finishes. Callers that see
+	// refreshing==true read this channel (under the lock) and wait on it.
+	pending chan struct{}
+
+	// fetchedAt is the wall-clock time of the most recent fetch attempt
+	// (success or failure), used for the TTL freshness check.
 	fetchedAt time.Time
-	cal       *ical.Calendar
-	err       error
 }
 
-// Cache fetches remote ICS calendars over HTTP(S), keeping a short-lived
-// in-memory copy per URL so repeated renders (e.g. every month-view page
-// load, or CalDAV client polling) don't re-fetch the same subscription
-// from origin every time.
+// Cache fetches remote ICS calendars over HTTP(S), keeping a shared
+// in-memory copy per URL. Semantics:
+//
+//   - Fresh entry (fetchedAt within TTL): return immediately, no I/O.
+//   - Stale entry with a cached copy: return the stale copy immediately
+//     AND spawn at most one background refresher (other callers in the
+//     same window piggyback on the in-flight refresh).
+//   - Stale entry with no cached copy (prior fetch failed): return the
+//     cached error immediately AND spawn a background retry.
+//   - No entry at all (very first call for this URL): block until a
+//     foreground fetch finishes (concurrent first-callers wait on a shared
+//     channel and all get the same result) and return its data.
 type Cache struct {
 	ttl    time.Duration
 	client *http.Client
 
-	mu      sync.Mutex
-	entries map[string]entry
+	mu   sync.Mutex
+	urls map[string]*entry // keyed by normalizeURL(url)
 }
 
 // NewCache creates a Cache with the given TTL (use DefaultTTL if unsure).
 func NewCache(ttl time.Duration) *Cache {
 	return &Cache{
-		ttl:     ttl,
-		client:  &http.Client{Timeout: fetchTimeout},
-		entries: make(map[string]entry),
+		ttl:    ttl,
+		client: &http.Client{},
+		urls:   make(map[string]*entry),
 	}
 }
 
-// Get returns the parsed calendar fetched from url, using a cached copy
-// if it's still within the TTL. If a fresh fetch fails but a previously
-// fetched copy exists, the stale copy is returned instead of the error,
-// so a transient network issue doesn't blank out the calendar entirely.
+// Get returns the most recently successfully-fetched calendar for url, or
+// the most-recent fetch error if no successful copy exists yet (a
+// background refresher may already be retrying).
 func (c *Cache) Get(url string) (*ical.Calendar, error) {
+	key := normalizeURL(url)
+
 	c.mu.Lock()
-	e, ok := c.entries[url]
-	fresh := ok && time.Since(e.fetchedAt) < c.ttl
-	c.mu.Unlock()
-	if fresh {
-		return e.cal, e.err
+	e := c.urls[key]
+	if e == nil {
+		e = &entry{url: url}
+		c.urls[key] = e
 	}
 
-	cal, err := c.fetch(url)
-	c.mu.Lock()
-	defer c.mu.Unlock()
-	if err != nil && ok && e.cal != nil {
-		return e.cal, nil
+	switch {
+	case e.cal == nil && e.lastErr == nil && !e.refreshing:
+		// Very first request for this URL: do a foreground fetch.
+		e.refreshing = true
+		e.pending = make(chan struct{})
+		ch := e.pending
+		c.mu.Unlock()
+		go c.doFetch(e, ch)
+		<-ch
+		return c.snapshot(e)
+
+	case e.cal == nil && e.lastErr == nil:
+		// A foreground fetch is already in flight — wait for it.
+		ch := e.pending
+		c.mu.Unlock()
+		<-ch
+		return c.snapshot(e)
+
+	default:
+		// We have some data (a cached copy or a cached error).
+		if time.Since(e.fetchedAt) < c.ttl {
+			// Fresh — just return.
+			c.mu.Unlock()
+			return c.snapshot(e)
+		}
+		// Stale — return the cached value immediately; spawn at most one
+		// background refresher (or piggyback on one already in flight).
+		if !e.refreshing {
+			e.refreshing = true
+			ch := make(chan struct{})
+			e.pending = ch
+			c.mu.Unlock()
+			go c.doFetch(e, ch)
+		} else {
+			c.mu.Unlock()
+		}
+		return c.snapshot(e)
 	}
-	c.entries[url] = entry{fetchedAt: time.Now(), cal: cal, err: err}
-	return cal, err
+}
+
+// doFetch performs the network I/O for the entry, updates cal/lastErr and
+// the freshness timestamp under the lock, clears the in-flight state, and
+// closes the per-fetch completion channel exactly once.
+func (c *Cache) doFetch(e *entry, ch chan struct{}) {
+	cal, err := c.fetch(e.url)
+	c.mu.Lock()
+	e.fetchedAt = time.Now()
+	if err == nil {
+		e.cal = cal
+		e.lastErr = nil
+	} else {
+		e.lastErr = err
+	}
+	e.refreshing = false
+	e.pending = nil
+	c.mu.Unlock()
+	close(ch)
+}
+
+// snapshot reads e.cal/e.lastErr under c.mu and returns the same value
+// shape Get does. Callers must not hold c.mu.
+func (c *Cache) snapshot(e *entry) (*ical.Calendar, error) {
+	c.mu.Lock()
+	cal, err := e.cal, e.lastErr
+	c.mu.Unlock()
+	if cal != nil {
+		return cal, nil
+	}
+	return nil, err
 }
 
 // fetch downloads and parses url, translating a "webcal://" scheme (used
@@ -128,3 +219,40 @@ func normalizeURL(u string) string {
 	}
 	return u
 }
+
+// EventID returns a stable, short identifier for a single ical.Event,
+// suitable for use as a filesystem object name or URL path segment. It is
+// the first 32 hex chars (128 bits) of
+// sha256("<DTSTART-value>|<duration>|<SUMMARY>") with a ".ics" suffix.
+//
+// Two events with the same DTSTART, same duration, and same SUMMARY hash
+// to the same ID — this matches the addressing scheme used both by the
+// web detail view and the CalDAV backend for ICS-subscription events.
+// Returns "" if DTSTART is missing (not addressable).
+func EventID(ev ical.Event) string {
+	start := ev.Props.Get(ical.PropDateTimeStart)
+	if start == nil {
+		return ""
+	}
+	dur := ""
+	if end := ev.Props.Get(ical.PropDateTimeEnd); end != nil {
+		if s, err := start.DateTime(time.UTC); err == nil {
+			if e, err := end.DateTime(time.UTC); err == nil {
+				dur = e.Sub(s).Round(time.Second).String()
+			}
+		}
+	}
+	summary := ""
+	if p := ev.Props.Get(ical.PropSummary); p != nil {
+		summary = p.Value
+	}
+
+	var keyBuf strings.Builder
+	keyBuf.WriteString(start.Value)
+	keyBuf.WriteRune('|')
+	keyBuf.WriteString(dur)
+	keyBuf.WriteRune('|')
+	keyBuf.WriteString(summary)
+	sum := sha256.Sum256([]byte(keyBuf.String()))
+	return hex.EncodeToString(sum[:16]) + ".ics"
+}

internal/icssub/icssub_test.go

@@ -0,0 +1,199 @@
+package icssub
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	ical "github.com/emersion/go-ical"
+)
+
+const sampleICS = "BEGIN:VCALENDAR\r\n" +
+	"VERSION:2.0\r\n" +
+	"PRODID:-//nidus//test//EN\r\n" +
+	"BEGIN:VEVENT\r\n" +
+	"UID:ev1@nidus.test\r\n" +
+	"DTSTAMP:20260101T000000Z\r\n" +
+	"DTSTART:20260805T090000Z\r\n" +
+	"DTEND:20260805T100000Z\r\n" +
+	"SUMMARY:Original title\r\n" +
+	"END:VEVENT\r\n" +
+	"END:VCALENDAR\r\n"
+
+func mustParseEvent(raw string) ical.Event {
+	cal, err := ical.NewDecoder(strings.NewReader(raw)).Decode()
+	if err != nil {
+		panic("mustParseEvent: " + err.Error())
+	}
+	evs := cal.Events()
+	if len(evs) == 0 {
+		panic("mustParseEvent: no events")
+	}
+	return evs[0]
+}
+
+// TestCacheFirstFetchPopulatesEntry verifies that the very first Get for a
+// URL does a foreground fetch and returns the parsed calendar.
+func TestCacheFirstFetchPopulatesEntry(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(sampleICS))
+	}))
+	defer srv.Close()
+
+	c := NewCache(time.Hour)
+	cal, err := c.Get(srv.URL)
+	if err != nil {
+		t.Fatalf("first Get: %v", err)
+	}
+	if len(cal.Events()) == 0 {
+		t.Fatalf("expected >=1 event, got 0")
+	}
+}
+
+// TestCacheStaleReturnsWithBackgroundRefresh verifies that once a cached
+// entry is stale, Get keeps returning the *stale* copy immediately while a
+// background refresh is spawned in a separate goroutine (rather than
+// blocking the caller on the network).
+func TestCacheStaleReturnsWithBackgroundRefresh(t *testing.T) {
+	var hits atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		time.Sleep(50 * time.Millisecond) // make the fetch slow enough to
+		w.WriteHeader(http.StatusOK)      // observe as background work
+		_, _ = w.Write([]byte(sampleICS))
+	}))
+	defer srv.Close()
+
+	c := NewCache(1 * time.Millisecond) // very short TTL
+	if _, err := c.Get(srv.URL); err != nil {
+		t.Fatalf("first Get: %v", err)
+	}
+	firstHits := hits.Load()
+
+	// Force staleness.
+	time.Sleep(3 * time.Millisecond)
+
+	// Subsequent Get must return the stale copy immediately without waiting
+	// for the slow server to respond — if it blocked, this call would take
+	// >= 50ms, which we bound with a deadline below via the hit counter.
+	st := time.Now()
+	cal, err := c.Get(srv.URL)
+	if err != nil {
+		t.Fatalf("second Get: %v", err)
+	}
+	if len(cal.Events()) == 0 {
+		t.Fatalf("expected event in stale response")
+	}
+	if time.Since(st) > 40*time.Millisecond {
+		t.Fatalf("second Get appears to have blocked on the network for %v", time.Since(st))
+	}
+
+	// Wait for the background refresh to complete.
+	deadline := time.Now().Add(2 * time.Second)
+	for hits.Load() == firstHits && time.Now().Before(deadline) {
+		time.Sleep(5 * time.Millisecond)
+	}
+	if hits.Load() == firstHits {
+		t.Fatalf("background refresh did not fire (hits stayed at %d)", firstHits)
+	}
+}
+
+// TestCacheConcurrentFirstFetchCoalesce ensures N concurrent first calls
+// for the same URL coalesce to a small number of actual origin fetches.
+func TestCacheConcurrentFirstFetchCoalesce(t *testing.T) {
+	var hits atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		hits.Add(1)
+		time.Sleep(40 * time.Millisecond) // widen the coalescing window
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(sampleICS))
+	}))
+	defer srv.Close()
+
+	c := NewCache(time.Hour)
+
+	const n = 8
+	errs := make([]error, n)
+	var wg sync.WaitGroup
+	start := make(chan struct{})
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			<-start
+			_, err := c.Get(srv.URL)
+			errs[i] = err
+		}(i)
+	}
+	close(start)
+	wg.Wait()
+	for _, err := range errs {
+		if err != nil {
+			t.Fatalf("goroutine error: %v", err)
+		}
+	}
+	// Without coalescing we'd see n=8 hits; with it we expect a small
+	// constant (the first caller fetches; the rest either return the
+	// in-progress result or kick off a coalesced background refresh).
+	if hits.Load() > 3 {
+		t.Fatalf("expected coalescing to reduce fetches (got %d)", hits.Load())
+	}
+}
+
+// TestEventIDStable verifies that the same event hashes to the same ID
+// across calls, and is the right shape (32 hex chars + ".ics").
+func TestEventIDStable(t *testing.T) {
+	ev := mustParseEvent(sampleICS)
+	id1 := EventID(ev)
+	id2 := EventID(ev)
+	if id1 != id2 {
+		t.Fatalf("EventID not stable: %q vs %q", id1, id2)
+	}
+	if !strings.HasSuffix(id1, ".ics") {
+		t.Fatalf("EventID should end in .ics, got %q", id1)
+	}
+	if len(id1) != 32+4 {
+		t.Fatalf("expected 32 hex chars + .ics, got %q (len %d)", id1, len(id1))
+	}
+}
+
+// TestEventIDDiffersForDifferentEvents verifies that two events with
+// different titles or different start times hash to different IDs, while
+// two events with only a different UID hash to the same ID.
+func TestEventIDDiffersForDifferentEvents(t *testing.T) {
+	base := EventID(mustParseEvent(sampleICS))
+
+	// Different title.
+	rawA := strings.Replace(sampleICS, "SUMMARY:Original title", "SUMMARY:Other title", 1)
+	if EventID(mustParseEvent(rawA)) == base {
+		t.Fatal("same start, different title must hash differently")
+	}
+
+	// Different start time.
+	rawB := strings.Replace(sampleICS, "DTSTART:20260805T090000Z", "DTSTART:20260806T090000Z", 1)
+	if EventID(mustParseEvent(rawB)) == base {
+		t.Fatal("different start times must hash differently")
+	}
+
+	// Same start, same title, different UID → same hash.
+	rawC := strings.Replace(sampleICS, "UID:ev1@nidus.test", "UID:ev2@nidus.test", 1)
+	if EventID(mustParseEvent(rawC)) != base {
+		t.Fatal("UID should not affect the hash — same start/dur/title must be equal")
+	}
+}
+
+// TestEventIDWorksWithoutUID verifies EventID still produces a value (from
+// DTSTART+DTEND+SUMMARY) when the event has no UID property.
+func TestEventIDWorksWithoutUID(t *testing.T) {
+	raw := strings.Replace(sampleICS, "UID:ev1@nidus.test\r\n", "", 1)
+	ev := mustParseEvent(raw)
+	id := EventID(ev)
+	if id == "" {
+		t.Fatal("EventID should be non-empty even without UID")
+	}
+}

internal/web/calendar.go

@@ -18,6 +18,8 @@ import (
 	"git.arnef.de/arnef/nidus/internal/birthdays"
 	"git.arnef.de/arnef/nidus/internal/db"
 	"git.arnef.de/arnef/nidus/internal/icalfix"
+	"git.arnef.de/arnef/nidus/internal/icssub"
+	"git.arnef.de/arnef/nidus/internal/store"
 	"git.arnef.de/arnef/nidus/internal/web/templates"
 	ical "github.com/emersion/go-ical"
 )
@@ -636,6 +638,108 @@ func newEventFormDefaults(calRef, dateParam string) templates.EventFormData {
 	}
 }
 
+// handleEventView renders the read-only detail view for a single event. It
+// is the landing page when a user clicks an event in the month/week grid;
+// writable calendars link from here to the edit page.
+func (s *Server) handleEventView(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		w.Header().Set("Allow", "GET")
+		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	username := userFromContext(r.Context())
+	ref := r.PathValue("ref")
+	id := r.PathValue("id")
+
+	// For ICS subscriptions, ref is "!<name>" and id is the stable
+	// icssub.EventID (already ".ics"-suffixed). For regular calendars,
+	// ref is a bare name or "owner~name" and id is a "<hex>.ics" filename
+	// from the eventIDRe character set. Both shapes share the same
+	// "hex/alnum .ics" shape, so we only need the regex check.
+	if !eventIDRe.MatchString(id) {
+		http.NotFound(w, r)
+		return
+	}
+	color, form, err := s.eventForDisplay(username, ref, id)
+	if errors.Is(err, store.ErrNotFound) || errors.Is(err, errCalendarNotFound) {
+		http.NotFound(w, r)
+		return
+	}
+	if err != nil {
+		s.handleCalRefError(w, r, err)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	_ = templates.EventDetail(username, templates.EventDetailData{Form: form, Color: color}).Render(context.Background(), w)
+}
+
+// eventForDisplay resolves a calendar reference for read access, loads and
+// decodes the event with the given id, populates the form's display fields
+// (CalRef, CalendarLabel, Writable), and returns the calendar's color for
+// the detail view's header dot. It handles both ordinary (stored) events
+// and ICS-subscription events (ref "!<name>").
+func (s *Server) eventForDisplay(username, ref, id string) (color string, form templates.EventFormData, err error) {
+	if strings.HasPrefix(ref, icsRefPrefix) {
+		return s.icsEventForDisplay(username, ref, id)
+	}
+	owner, name, err := s.resolveCalRef(username, ref, false)
+	if err != nil {
+		return "", form, err
+	}
+	data, err := s.store.GetObject(owner, "cal-"+name, id)
+	if err != nil {
+		return "", form, err
+	}
+	form, err = eventFormFromICS(id, data)
+	if err != nil {
+		s.logger.Error("decoding event", "error", err)
+		return "", form, err
+	}
+	form.CalRef = ref
+	label := name
+	if owner != username {
+		label = name + " (" + s.dbase.DisplayName(owner) + ")"
+	}
+	form.CalendarLabel = label
+	form.Writable = owner == username
+	if !form.Writable {
+		if share, err := s.dbase.CalendarShareFor(owner, name, username); err == nil {
+			form.Writable = share.Permission == db.PermWrite
+		}
+	}
+	color, _ = s.dbase.GetCalendarColor(owner, name)
+	return color, form, nil
+}
+
+// icsEventForDisplay resolves one of username's ICS subscriptions named
+// ref[len(!):] and looks up the event whose icssub.EventID hashes to id.
+// Returns the subscription's display color and a form with Writable=false.
+func (s *Server) icsEventForDisplay(username, ref, id string) (color string, form templates.EventFormData, err error) {
+	name := strings.TrimPrefix(ref, icsRefPrefix)
+	sub, err := s.dbase.GetICSSubscription(username, name)
+	if err != nil {
+		return "", form, errCalendarNotFound
+	}
+	cal, err := s.icsCache.Get(sub.URL)
+	if err != nil {
+		return "", form, err
+	}
+	for _, ev := range cal.Events() {
+		if icssub.EventID(ev) != id {
+			continue
+		}
+		form, err = eventFormFromComponent(id, ev)
+		if err != nil {
+			return "", form, err
+		}
+		form.CalRef = ref
+		form.CalendarLabel = name
+		form.Writable = false
+		return sub.Color, form, nil
+	}
+	return "", form, errCalendarNotFound
+}
+
 func (s *Server) handleEventEdit(w http.ResponseWriter, r *http.Request) {
 	username := userFromContext(r.Context())
 	ref := r.PathValue("ref")
@@ -1199,11 +1303,11 @@ func (s *Server) addBirthdayEvents(username string, entry calendarEntry, gridSta
 }
 
 // addICSEvents fetches entry's remote ICS/webcal calendar (via s.icsCache,
-// which caches it for a while so every month-view render doesn't re-fetch
-// from origin) and places each VEVENT's occurrence onto the month grid,
-// the same way a stored calendar object would be. There's no per-event
-// edit page for these (the source is external and read-only), so each
-// event's LinkURL is left pointing nowhere useful ("#").
+// which uses stale-while-revalidate so a month-view render never blocks on
+// network I/O) and places each VEVENT's occurrence onto the month grid,
+// the same way a stored calendar object would be. Each event's ID is the
+// stable icssub.EventID hash, which routes through the read-only detail
+// page (eventForDisplay → icsEventForDisplay).
 func (s *Server) addICSEvents(entry calendarEntry, gridStart, gridEnd time.Time, loc *time.Location, dayIndex map[string]int, days []templates.MonthDay) error {
 	cal, err := s.icsCache.Get(entry.ICSURL)
 	if err != nil {
@@ -1211,8 +1315,11 @@ func (s *Server) addICSEvents(entry calendarEntry, gridStart, gridEnd time.Time,
 	}
 
 	const totalDays = 42
-	for i, ev := range cal.Events() {
-		id := fmt.Sprintf("ics-%d", i)
+	for _, ev := range cal.Events() {
+		id := icssub.EventID(ev)
+		if id == "" {
+			continue
+		}
 		form, err := eventFormFromComponent(id, ev)
 		if err != nil {
 			continue
@@ -1246,7 +1353,6 @@ func (s *Server) addICSEvents(entry calendarEntry, gridStart, gridEnd time.Time,
 				Summary:  form.Summary,
 				TimeText: timeText,
 				AllDay:   form.AllDay,
-				LinkURL:  "#",
 			})
 		}
 	}

internal/web/event_detail_test.go

@@ -0,0 +1,209 @@
+package web
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+
+	"git.arnef.de/arnef/nidus/internal/db"
+)
+
+// seedEvent stores an all-day event (one-day span) in owner's calendar cal
+// under object id, with the given summary/location/description. Used to set
+// up events that handleEventView should render.
+func seedEvent(t *testing.T, s *Server, owner, cal, id, summary, location, description string) {
+	t.Helper()
+	data := "BEGIN:VCALENDAR\r\n" +
+		"VERSION:2.0\r\n" +
+		"PRODID:-//nidus//test//EN\r\n" +
+		"BEGIN:VEVENT\r\n" +
+		"UID:" + strings.TrimSuffix(id, ".ics") + "\r\n" +
+		"DTSTAMP:20260101T000000Z\r\n" +
+		"SUMMARY:" + summary + "\r\n" +
+		"LOCATION:" + location + "\r\n" +
+		"DESCRIPTION:" + description + "\r\n" +
+		"DTSTART;VALUE=DATE:20260805\r\n" +
+		"DTEND;VALUE=DATE:20260806\r\n" +
+		"END:VEVENT\r\n" +
+		"END:VCALENDAR\r\n"
+	if err := s.store.PutObject(owner, "cal-"+cal, id, []byte(data)); err != nil {
+		t.Fatalf("PutObject: %v", err)
+	}
+}
+
+// getEventDetail issues GET /calendar/{ref}/{id} as the session identified by
+// cookie and returns the recorded response.
+func getEventDetail(t *testing.T, handler http.Handler, cookie *http.Cookie, ref, id string) *httptest.ResponseRecorder {
+	t.Helper()
+	path := "/calendar/" + url.PathEscape(ref) + "/" + url.PathEscape(id)
+	req := httptest.NewRequest(http.MethodGet, path, nil)
+	req.AddCookie(cookie)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	return rr
+}
+
+func TestEventDetailShowsOwnEvent(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	id := "aabbccddeeff.ics"
+	seedEvent(t, s, "alice", "work", id, "Team standup", "Meetroom A", "Daily sync with the team")
+
+	rr := getEventDetail(t, handler, cookie, "work", id)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", rr.Code, rr.Body.String())
+	}
+	body := rr.Body.String()
+	for _, want := range []string{
+		"Team standup",             // summary / title
+		"Meetroom A",               // location
+		"Daily sync with the team", // description
+		"work",                     // calendar label
+		">Edit",                    // writable → Edit link present
+		"Export .ics",
+	} {
+		if !strings.Contains(body, want) {
+			t.Errorf("expected body to contain %q, got:\n%s", want, body)
+		}
+	}
+}
+
+func TestEventDetailReadOnlySharedHasNoEdit(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	aliceCookie := loginAs(t, handler, "alice", "password")
+	bobCookie := loginAs(t, handler, "bob", "password")
+
+	id := "123456.ics"
+	seedEvent(t, s, "bob", "personal", id, "Bob lunch", "Cafe", "Lunch plans")
+	if err := s.dbase.ShareCalendar("bob", "personal", "alice", db.PermRead); err != nil {
+		t.Fatalf("ShareCalendar: %v", err)
+	}
+
+	// alice (read share) can view but not edit.
+	rr := getEventDetail(t, handler, aliceCookie, "bob~personal", id)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("alice view: expected 200, got %d: %s", rr.Code, rr.Body.String())
+	}
+	body := rr.Body.String()
+	if !strings.Contains(body, "Bob lunch") {
+		t.Errorf("alice: expected body to contain summary, got:\n%s", body)
+	}
+	if !strings.Contains(body, "shared with you as read-only") {
+		t.Errorf("alice: expected read-only notice, got:\n%s", body)
+	}
+	if strings.Contains(body, ">Edit") {
+		t.Errorf("alice: Edit link should not be present on a read-only share, got:\n%s", body)
+	}
+
+	// bob (owner) can still see the Edit link.
+	rrBob := getEventDetail(t, handler, bobCookie, "personal", id)
+	if rrBob.Code != http.StatusOK {
+		t.Fatalf("bob view own: expected 200, got %d", rrBob.Code)
+	}
+	if !strings.Contains(rrBob.Body.String(), ">Edit") {
+		t.Errorf("bob: expected Edit link, got:\n%s", rrBob.Body.String())
+	}
+}
+
+func TestEventDetailWriteShareHasEditLink(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	seedEvent(t, s, "bob", "personal", "a1b2c3.ics", "Bob meeting", "Office", "Sync")
+	if err := s.dbase.ShareCalendar("bob", "personal", "alice", db.PermWrite); err != nil {
+		t.Fatalf("ShareCalendar: %v", err)
+	}
+	rr := getEventDetail(t, handler, cookie, "bob~personal", "a1b2c3.ics")
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", rr.Code, rr.Body.String())
+	}
+	body := rr.Body.String()
+	if !strings.Contains(body, ">Edit") {
+		t.Errorf("write share should expose Edit link, got:\n%s", body)
+	}
+	if strings.Contains(body, "shared with you as read-only") {
+		t.Errorf("write share should not show read-only notice, got:\n%s", body)
+	}
+}
+
+func TestEventDetailNotFound(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	// Valid calendar, missing object.
+	if rr := getEventDetail(t, handler, cookie, "work", "doesnotexist.ics"); rr.Code != http.StatusNotFound {
+		t.Fatalf("missing event: expected 404, got %d", rr.Code)
+	}
+	// Unknown calendar ref.
+	if rr := getEventDetail(t, handler, cookie, "does_not_exist", "0000.ics"); rr.Code != http.StatusNotFound {
+		t.Fatalf("unknown calendar: expected 404, got %d", rr.Code)
+	}
+	// Invalid id shape (rejected by eventIDRe before store access).
+	if rr := getEventDetail(t, handler, cookie, "work", "bad/../etc/passwd.ics"); rr.Code != http.StatusNotFound {
+		t.Fatalf("invalid id: expected 404, got %d", rr.Code)
+	}
+}
+
+func TestEventDetailRequiresLogin(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	req := httptest.NewRequest(http.MethodGet, "/calendar/work/anything.ics", nil)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	if rr.Code != http.StatusSeeOther {
+		t.Fatalf("expected redirect to login, got %d", rr.Code)
+	}
+}
+
+func TestEventDetailRejectsNonGet(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+	req := httptest.NewRequest(http.MethodPost, "/calendar/work/anything.ics", nil)
+	req.AddCookie(cookie)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	if rr.Code != http.StatusMethodNotAllowed {
+		t.Fatalf("expected 405 for POST, got %d", rr.Code)
+	}
+}
+
+// TestEditRouteStillResolves guards against the new detail route
+// (/calendar/{ref}/{id}) shadowing the more specific edit/delete/export
+// routes under the same {ref}+{id} prefix in Go's ServeMux.
+func TestEditRouteStillResolves(t *testing.T) {
+	s := newTestServer(t)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	id := "cafebabe00.ics"
+	seedEvent(t, s, "alice", "work", id, "Edit me", "Room", "Note")
+
+	// GET the edit form — must still hit handleEventEdit, not the detail view.
+	req := httptest.NewRequest(http.MethodGet, "/calendar/work/cafebabe00.ics/edit", nil)
+	req.AddCookie(cookie)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("edit GET: expected 200, got %d: %s", rr.Code, rr.Body.String())
+	}
+	if !strings.Contains(rr.Body.String(), "Edit event") || !strings.Contains(rr.Body.String(), "<form") {
+		t.Fatalf("expected the edit form to render, got:\n%s", rr.Body.String())
+	}
+
+	// The delete route still works (redirect to /web/calendar on success).
+	req = httptest.NewRequest(http.MethodPost, "/calendar/work/cafebabe00.ics/delete", nil)
+	req.AddCookie(cookie)
+	rr = httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	if rr.Code != http.StatusSeeOther {
+		t.Fatalf("delete: expected 303 redirect, got %d: %s", rr.Code, rr.Body.String())
+	}
+}

internal/web/ics_detail_test.go

@@ -0,0 +1,176 @@
+package web
+
+import (
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"git.arnef.de/arnef/nidus/internal/config"
+	"git.arnef.de/arnef/nidus/internal/db"
+	"git.arnef.de/arnef/nidus/internal/icssub"
+	"git.arnef.de/arnef/nidus/internal/store"
+	ical "github.com/emersion/go-ical"
+)
+
+// icsDetailSample is a minimal valid ICS feed with one VEVENT on Aug 5, 2026.
+const icsDetailSample = "BEGIN:VCALENDAR\r\n" +
+	"VERSION:2.0\r\n" +
+	"PRODID:-//nidus//test//EN\r\n" +
+	"BEGIN:VEVENT\r\n" +
+	"UID:detail1@nidus.test\r\n" +
+	"DTSTAMP:20260101T000000Z\r\n" +
+	"DTSTART:20260805T090000Z\r\n" +
+	"DTEND:20260805T100000Z\r\n" +
+	"SUMMARY:ICS detail event\r\n" +
+	"END:VEVENT\r\n" +
+	"END:VCALENDAR\r\n"
+
+// newServerWithICSCache builds a Server wired to an upstream ICS server
+// (for deterministic tests) and returns it, sharing one icssub.Cache the
+// same way cmd/server/main.go does in production.
+func newServerWithICSCache(t *testing.T, upstreamURL string) *Server {
+	t.Helper()
+	dir := t.TempDir()
+	st, err := store.NewStore(filepath.Join(dir, "data"))
+	if err != nil {
+		t.Fatalf("NewStore: %v", err)
+	}
+	d, err := db.Open(filepath.Join(dir, "test.db"))
+	if err != nil {
+		t.Fatalf("db.Open: %v", err)
+	}
+	t.Cleanup(func() { d.Close() })
+
+	cfg := &config.Config{}
+	if err := d.CreateUser("alice", "password", "", ""); err != nil {
+		t.Fatalf("CreateUser: %v", err)
+	}
+	if err := d.CreateICSSubscription("alice", "holidays", upstreamURL, "#123abc"); err != nil {
+		t.Fatalf("CreateICSSubscription: %v", err)
+	}
+	shared := icssub.NewCache(time.Hour)
+	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
+	return NewServer(cfg, st, d, logger, shared)
+}
+
+// TestICSDetailRouteRendersReadonlyEvent verifies the full ICS detail path:
+// the user clicks an ICS event from the month/week grid, the browser lands
+// on GET /calendar/!holidays/<icssub.EventID>/, and the handler returns 200
+// with the event's fields rendered and NO Edit link (ICS subs are read-only).
+func TestICSDetailRouteRendersReadonlyEvent(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(icsDetailSample))
+	}))
+	defer srv.Close()
+
+	s := newServerWithICSCache(t, srv.URL)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	id := icssub.EventID(mustParseICS(t, icsDetailSample))
+	if id == "" {
+		t.Fatal("EventID must be non-empty")
+	}
+
+	req := httptest.NewRequest(http.MethodGet, "/calendar/"+url.PathEscape("!holidays")+"/"+id, nil)
+	req.AddCookie(cookie)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", rr.Code, rr.Body.String())
+	}
+	body := rr.Body.String()
+	for _, want := range []string{
+		"ICS detail event", // summary
+		"holidays",         // calendar label
+	} {
+		if !strings.Contains(body, want) {
+			t.Errorf("expected body to contain %q, got:\n%s", want, body)
+		}
+	}
+	if strings.Contains(body, ">Edit") {
+		t.Errorf("Edit link should not be present on a read-only ICS detail page, got:\n%s", body)
+	}
+}
+
+// TestICSDetailRouteBadIDReturns404 verifies a request with a malformed
+// event ID (missing .ics suffix) gets a 404 rather than rendering.
+func TestICSDetailRouteBadIDReturns404(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, _ = w.Write([]byte(icsDetailSample))
+	}))
+	defer srv.Close()
+
+	s := newServerWithICSCache(t, srv.URL)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	req := httptest.NewRequest(http.MethodGet, "/calendar/!holidays/not-a-valid-id", nil)
+	req.AddCookie(cookie)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	if rr.Code != http.StatusNotFound {
+		t.Fatalf("expected 404 for malformed event id, got %d", rr.Code)
+	}
+}
+
+// TestMonthGridLinksICSEventToDetailPage exercises the full user path: the
+// month/week grid renders an ICS-subscription event as a link to a detail
+// page (not "#" and not a direct edit URL).
+func TestMonthGridLinksICSEventToDetailPage(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		_, _ = w.Write([]byte(icsDetailSample))
+	}))
+	defer srv.Close()
+
+	s := newServerWithICSCache(t, srv.URL)
+	handler := s.Handler(emptyStaticFS{})
+	cookie := loginAs(t, handler, "alice", "password")
+
+	req := httptest.NewRequest(http.MethodGet, "/calendar?year=2026&month=8", nil)
+	req.AddCookie(cookie)
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+	if rr.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d: %s", rr.Code, rr.Body.String())
+	}
+	body := rr.Body.String()
+	if !strings.Contains(body, "ICS detail event") {
+		t.Fatalf("month grid should list ICS events, got:\n%s", body)
+	}
+	id := icssub.EventID(mustParseICS(t, icsDetailSample))
+	// The grid renders the detail link with a "/web/calendar/!<ref>/<id>"
+	// shape (see eventLinkURL in calendar.templ). The "!" in the ref is
+	// not %-escaped by templ.URL — we observed un-escaped output in the
+	// rendered HTML and pin the exact shape below.
+	wantHref := "href=\"/web/calendar/!holidays/" + id + "\""
+	if !strings.Contains(body, wantHref) {
+		t.Fatalf("month grid should link ICS event to %q, but did not", wantHref)
+	}
+	if strings.Contains(body, `href="#"`) {
+		t.Fatalf("ICS events should not link to '#'")
+	}
+}
+
+// mustParseICS parses raw and returns its first VEVENT (panic on error).
+func mustParseICS(t *testing.T, raw string) ical.Event {
+	t.Helper()
+	cal, err := ical.NewDecoder(strings.NewReader(raw)).Decode()
+	if err != nil {
+		t.Fatalf("ical decode: %v", err)
+	}
+	evs := cal.Events()
+	if len(evs) == 0 {
+		t.Fatal("no events in ICS")
+	}
+	return evs[0]
+}

internal/web/server.go

@@ -25,9 +25,16 @@ type Server struct {
 	icsCache *icssub.Cache
 }
 
-// NewServer constructs a web UI Server.
-func NewServer(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger) *Server {
-	return &Server{cfg: cfg, store: st, dbase: dbase, logger: logger, icsCache: icssub.NewCache(icssub.DefaultTTL)}
+// NewServer constructs a web UI Server. icsCache may be nil, in which
+// case a private default-TTL cache is created — prefer sharing a single
+// *icssub.Cache with the CALDAV/CardDAV backends (e.g. from
+// cmd/server/main.go) so the web calendar page and the DAV protocol
+// serve identical events for the same ICS subscription.
+func NewServer(cfg *config.Config, st *store.Store, dbase *db.DB, logger *slog.Logger, icsCache *icssub.Cache) *Server {
+	if icsCache == nil {
+		icsCache = icssub.NewCache(icssub.DefaultTTL)
+	}
+	return &Server{cfg: cfg, store: st, dbase: dbase, logger: logger, icsCache: icsCache}
 }
 
 // Handler returns the http.Handler serving the web UI, mounted at "/web/"
@@ -65,6 +72,7 @@ func (s *Server) Handler(staticFS http.FileSystem) http.Handler {
 	mux.HandleFunc("/calendar", s.requireLogin(s.handleCalendarMonth))
 	mux.HandleFunc("/calendar/week", s.requireLogin(s.handleCalendarWeek))
 	mux.HandleFunc("/calendar/new", s.requireLogin(s.handleEventNew))
+	mux.HandleFunc("/calendar/{ref}/{id}", s.requireLogin(s.handleEventView))
 	mux.HandleFunc("/calendar/{ref}/import", s.requireLogin(s.handleCalendarImport))
 	mux.HandleFunc("/calendar/{ref}/export", s.requireLogin(s.handleCalendarExportAll))
 	mux.HandleFunc("/calendar/{ref}/{id}/edit", s.requireLogin(s.handleEventEdit))

internal/web/server_test.go

@@ -47,7 +47,7 @@ func newTestServer(t *testing.T) *Server {
 		t.Fatalf("CreateCalendar bob/personal: %v", err)
 	}
 	logger := slog.New(slog.NewTextHandler(io.Discard, nil))
-	return NewServer(cfg, st, dbase, logger)
+	return NewServer(cfg, st, dbase, logger, nil)
 }
 
 // loginAs performs a login request against handler and returns the

internal/web/templates/calendar.templ

@@ -3,6 +3,7 @@ package templates
 import "fmt"
 import "strconv"
 import "strings"
+import "time"
 
 // CalendarSummary is one calendar (own or shared) shown in the combined
 // month view's legend.
@@ -24,9 +25,9 @@ type EventSummary struct {
 	Summary  string
 	TimeText string // e.g. "14:00" or "" for all-day events
 	AllDay   bool
-	// LinkURL overrides the default "/web/calendar/{CalRef}/{ID}/edit"
-	// link target, used by virtual/read-only calendars (e.g. birthdays)
-	// that don't have an editable event object of their own.
+	// LinkURL overrides the default "/web/calendar/{CalRef}/{ID}" (detail
+	// view) link target, used by virtual/read-only calendars (e.g.
+	// birthdays) that don't have an editable event object of their own.
 	LinkURL string
 }
 
@@ -100,6 +101,14 @@ type EventFormData struct {
 	EndTime       string // "HH:MM", empty when AllDay
 }
 
+// EventDetailData is everything the read-only event detail view needs: the
+// decoded event itself (Form, including its ID/CalRef/Writable display
+// metadata) plus the calendar's color for the header dot.
+type EventDetailData struct {
+	Form  EventFormData
+	Color string // calendar color, "" if unset
+}
+
 templ MonthView(username string, data MonthViewData) {
 	@Layout("Calendar", username) {
 		<div class="flex items-center justify-between mb-6 gap-4 flex-wrap">
@@ -301,7 +310,7 @@ func eventLinkURL(ev EventSummary) string {
 	if ev.LinkURL != "" {
 		return ev.LinkURL
 	}
-	return "/web/calendar/" + ev.CalRef + "/" + ev.ID + "/edit"
+	return "/web/calendar/" + ev.CalRef + "/" + ev.ID
 }
 
 // eventTextColor returns a color derived from hex, darkened if needed so
@@ -355,6 +364,119 @@ func clampByte(v float64) int {
 
 
 
+// EventDetail is the read-only detail view for a single event, opened when
+// the user clicks an event in the month or week grid. It shows the
+// event's fields and, when the calendar is writable, offers an Edit link.
+templ EventDetail(username string, data EventDetailData) {
+	@Layout("Calendar", username) {
+		<div class="flex items-center justify-between gap-3 flex-wrap mb-6">
+			<a href="/web/calendar" class="text-sm text-indigo-600 hover:underline">&larr; Calendar</a>
+			<div class="flex items-center gap-3">
+				if data.Form.Writable {
+					<a href={ templ.URL("/web/calendar/" + data.Form.CalRef + "/" + data.Form.ID + "/edit") }
+						class="bg-indigo-600 text-white rounded-md px-3 py-1.5 text-sm font-medium hover:bg-indigo-700">
+						Edit
+					</a>
+				}
+				<a href={ templ.URL("/web/calendar/" + data.Form.CalRef + "/" + data.Form.ID + "/export") }
+					class="bg-white border border-gray-300 rounded-md px-3 py-1.5 text-sm font-medium text-gray-700 hover:bg-gray-50">
+					Export .ics
+				</a>
+			</div>
+		</div>
+
+		<div class="bg-white rounded-lg border border-gray-200 p-6 max-w-2xl">
+			<div class="flex items-start gap-3 border-b border-gray-100 pb-5 mb-5">
+				<span class="w-3 h-3 rounded-full mt-2 shrink-0 ring-1 ring-inset ring-black/10" style={ "background-color: " + colorOrDefault(data.Color) }></span>
+				<div class="min-w-0">
+					<h1 class="text-2xl font-semibold break-words leading-tight">{ data.Form.Summary }</h1>
+					<p class="text-sm text-gray-500 mt-1">{ data.Form.CalendarLabel }</p>
+				</div>
+			</div>
+
+			if !data.Form.Writable {
+				<p class="mb-5 text-sm text-amber-700 bg-amber-50 border border-amber-200 rounded px-3 py-2">
+					This calendar was shared with you as read-only — you can view this event but not change it.
+				</p>
+			}
+
+			<dl class="space-y-4">
+				<div>
+					<dt class="text-xs font-medium uppercase tracking-wide text-gray-400 mb-1">When</dt>
+					<dd class="text-base text-gray-900">{ eventRangeText(data.Form) }</dd>
+				</div>
+				if data.Form.Location != "" {
+					<div>
+						<dt class="text-xs font-medium uppercase tracking-wide text-gray-400 mb-1">Location</dt>
+						<dd class="text-base text-gray-900 break-words">{ data.Form.Location }</dd>
+					</div>
+				}
+				if data.Form.Description != "" {
+					<div>
+						<dt class="text-xs font-medium uppercase tracking-wide text-gray-400 mb-1">Description</dt>
+						<dd class="text-base text-gray-900 whitespace-pre-wrap break-words">{ data.Form.Description }</dd>
+					</div>
+				}
+			</dl>
+		</div>
+	}
+}
+
+// eventRangeText renders a human-readable "when" string for an event:
+// a single timed event reads "Aug 5, 2026, 14:00 – 15:00"; a multi-day
+// range reads "Aug 5 – 7, 2026"; a single all-day date reads
+// "August 5, 2026".
+func eventRangeText(f EventFormData) string {
+	if f.AllDay {
+		if f.StartDate != f.EndDate {
+			s, err := dateOnly(f.StartDate)
+			if err != nil {
+				return f.StartDate
+			}
+			e, err := dateOnly(f.EndDate)
+			if err != nil {
+				return f.EndDate
+			}
+			if s.Year() == e.Year() && s.Month() == e.Month() {
+				return fmt.Sprintf("%s – %s, %d", s.Format("Jan 2"), e.Format("2"), s.Year())
+			}
+			return fmt.Sprintf("%s – %s", s.Format("Jan 2, 2006"), e.Format("Jan 2, 2006"))
+		}
+		d, err := dateOnly(f.StartDate)
+		if err != nil {
+			return f.StartDate
+		}
+		return d.Format("January 2, 2006")
+	}
+
+	s, err := dateTime(f.StartDate, f.StartTime)
+	if err != nil {
+		return f.StartDate
+	}
+	e, err := dateTime(f.EndDate, f.EndTime)
+	if err != nil {
+		return s.Format("January 2, 2006, 15:04")
+	}
+	if s.Day() == e.Day() {
+		return fmt.Sprintf("%s, %s – %s", s.Format("January 2, 2006"), s.Format("15:04"), e.Format("15:04"))
+	}
+	if s.Year() == e.Year() && s.Month() == e.Month() {
+		return fmt.Sprintf("%s – %s, %d", s.Format("Jan 2, 15:04"), e.Format("15:04"), s.Year())
+	}
+	return fmt.Sprintf("%s – %s", s.Format("Jan 2, 2006, 15:04"), e.Format("Jan 2, 2006, 15:04"))
+}
+
+func dateOnly(ds string) (time.Time, error) {
+	return time.ParseInLocation("2006-01-02", ds, time.Local)
+}
+
+func dateTime(ds, ts string) (time.Time, error) {
+	if ts == "" {
+		return time.ParseInLocation("2006-01-02", ds, time.Local)
+	}
+	return time.ParseInLocation("2006-01-02T15:04", ds+"T"+ts, time.Local)
+}
+
 templ EventForm(username string, data EventFormData, errMsg string) {
 	@Layout("Calendar", username) {
 		<a href="/web/calendar" class="text-sm text-indigo-600 hover:underline">&larr; Calendar</a>

internal/web/templates/calendar_test.go

@@ -33,6 +33,54 @@ func TestEventTextColorDarkensLightColors(t *testing.T) {
 	}
 }
 
+// TestEventRangeText pins the human-readable "When" strings shown on the
+// event detail view so any change to date formatting is intentional.
+func TestEventRangeText(t *testing.T) {
+	cases := []struct {
+		name string
+		form EventFormData
+		want string
+	}{
+		{
+			name: "single allday date",
+			form: EventFormData{AllDay: true, StartDate: "2026-08-05", EndDate: "2026-08-05"},
+			want: "August 5, 2026",
+		},
+		{
+			name: "all-day multi-day same month",
+			form: EventFormData{AllDay: true, StartDate: "2026-08-05", EndDate: "2026-08-07"},
+			want: "Aug 5 – 7, 2026",
+		},
+		{
+			name: "all-day multi-day different months",
+			form: EventFormData{AllDay: true, StartDate: "2026-08-30", EndDate: "2026-09-02"},
+			want: "Aug 30, 2026 – Sep 2, 2026",
+		},
+		{
+			name: "timed same day",
+			form: EventFormData{AllDay: false, StartDate: "2026-08-05", StartTime: "14:00", EndDate: "2026-08-05", EndTime: "15:00"},
+			want: "August 5, 2026, 14:00 – 15:00",
+		},
+		{
+			name: "timed different days same month",
+			form: EventFormData{AllDay: false, StartDate: "2026-08-05", StartTime: "23:30", EndDate: "2026-08-06", EndTime: "01:00"},
+			want: "Aug 5, 23:30 – 01:00, 2026",
+		},
+		{
+			name: "timed different months",
+			form: EventFormData{AllDay: false, StartDate: "2026-08-31", StartTime: "10:00", EndDate: "2026-09-01", EndTime: "11:00"},
+			want: "Aug 31, 2026, 10:00 – Sep 1, 2026, 11:00",
+		},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			if got := eventRangeText(c.form); got != c.want {
+				t.Errorf("eventRangeText(%+v) = %q, want %q", c.form, got, c.want)
+			}
+		})
+	}
+}
+
 func TestEventTextColorFallsBackForInvalidInput(t *testing.T) {
 	if got := eventTextColor(""); got != colorOrDefault("") {
 		t.Errorf("eventTextColor(\"\") = %s, want the same default colorOrDefault returns", got)