Render maven's own reminders to a calendar she owns (#127)

Radicale becomes a write-only render target, not a store. sqlite stays
canonical: every poll mavcaldav reads the pending reminders out of core and
publishes each one as a single-event iCal resource, withdrawing the ones that
have fired or been cancelled. Losing the collection costs nothing — the next
tick rebuilds it, and nothing is ever read back from it.

It structurally cannot write to a calendar maven only reads. The render URL and
credential are their own flags, and -render-url is refused at startup when it
names the collection -url reads; the only paths it addresses carry the
maven-reminder- prefix, so even aimed at the wrong collection it can only touch
resources it created. Rendering is off unless -render-url is given.

The calendar data model now lives in one place, internal/calendar: the Event,
the iCal parse it comes from and the render it goes to, the fact key/value
encoding, and the source constants that say which calendars may be written to.
It was a parse inlined in cmd/mavcaldav and a Sprintf in two files; #126 and
#128 both need to agree with it.

Fixes a latent day-boundary bug moved out of that inline parse: it took the day
number off a local clock reading but built the window boundaries in UTC, so on
a box east of Greenwich part of the evening fell outside "today" and the poller
saw an empty calendar after 20:00 UTC. Today is now the owner's day in the
owner's location, which is what the busy gate and the day plan mean.

cmd/mavcaldav/render.go

@@ -0,0 +1,145 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/kami/maven/internal/calendar"
+	"github.com/kami/maven/internal/ipc"
+)
+
+// renderer is the write half of maven's own local calendar (Vikunja #127).
+//
+// It is a RENDER TARGET, not a store. sqlite stays canonical: every tick the
+// renderer reads the pending reminders out of core and publishes each one as a
+// single-event iCal resource in a CalDAV collection maven owns. Nothing is ever
+// read back from that collection, and losing it costs nothing — the next tick
+// rebuilds it.
+//
+// It structurally cannot write to a calendar maven only reads. The URL comes
+// from its own flag, checked at startup against every read URL (see
+// run in main.go), and the only paths it ever addresses carry
+// calendar.ReminderUIDPrefix — so even pointed at the wrong collection it can
+// only touch resources it created.
+type renderer struct {
+	core ipc.CoreAPI
+	http *http.Client
+	url  string
+	user string
+	pass string
+	dur  time.Duration
+
+	// published maps reminder id → the body last successfully PUT, so an
+	// unchanged reminder costs nothing. Purely an optimisation: a restart
+	// re-publishes every reminder once, which is idempotent.
+	published map[int64]string
+}
+
+func newRenderer(core ipc.CoreAPI, hc *http.Client, url, user, pass string, dur time.Duration) *renderer {
+	return &renderer{
+		core:      core,
+		http:      hc,
+		url:       strings.TrimRight(url, "/"),
+		user:      user,
+		pass:      pass,
+		dur:       dur,
+		published: make(map[int64]string),
+	}
+}
+
+// renderOnce publishes every pending reminder and withdraws the ones that are
+// no longer pending. Errors are logged and skipped: a calendar maven cannot
+// reach must never break the reminder itself, which lives in sqlite.
+func (r *renderer) renderOnce(ctx context.Context) {
+	reminders, err := r.core.ListReminders(ctx, renderMaxReminders)
+	if err != nil {
+		log.Printf("mavcaldav: list reminders: %v", err)
+		return
+	}
+
+	live := make(map[int64]bool, len(reminders))
+	for _, rem := range reminders {
+		if rem.Status != "pending" {
+			continue
+		}
+		live[rem.ID] = true
+		e := calendar.ReminderEvent(rem.ID, fireTime(rem), rem.Payload, r.dur)
+		body := calendar.RenderICal([]calendar.Event{e})
+		if r.published[rem.ID] == body {
+			continue
+		}
+		if err := r.put(ctx, calendar.ReminderPath(rem.ID), body); err != nil {
+			log.Printf("mavcaldav: render reminder %d: %v", rem.ID, err)
+			continue
+		}
+		r.published[rem.ID] = body
+		log.Printf("mavcaldav: rendered reminder %d (%s)", rem.ID, e.Summary)
+	}
+
+	for id := range r.published {
+		if live[id] {
+			continue
+		}
+		if err := r.delete(ctx, calendar.ReminderPath(id)); err != nil {
+			log.Printf("mavcaldav: withdraw reminder %d: %v", id, err)
+			continue
+		}
+		delete(r.published, id)
+		log.Printf("mavcaldav: withdrew reminder %d", id)
+	}
+}
+
+// renderMaxReminders bounds the read. Reminders past this count are older than
+// anything a calendar view is useful for.
+const renderMaxReminders = 200
+
+// fireTime prefers NextFireTs — for a recurring reminder that is the occurrence
+// worth showing; FireTs is the original statement.
+func fireTime(rem ipc.Reminder) time.Time {
+	if !rem.NextFireTs.IsZero() {
+		return rem.NextFireTs
+	}
+	return rem.FireTs
+}
+
+func (r *renderer) put(ctx context.Context, name, body string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPut, r.url+"/"+name, strings.NewReader(body))
+	if err != nil {
+		return err
+	}
+	req.SetBasicAuth(r.user, r.pass)
+	req.Header.Set("Content-Type", "text/calendar; charset=utf-8")
+	return r.do(req, name)
+}
+
+func (r *renderer) delete(ctx context.Context, name string) error {
+	req, err := http.NewRequestWithContext(ctx, http.MethodDelete, r.url+"/"+name, nil)
+	if err != nil {
+		return err
+	}
+	req.SetBasicAuth(r.user, r.pass)
+	return r.do(req, name)
+}
+
+// do runs the request and treats any 2xx, plus 404 on a DELETE, as success —
+// a resource that is already gone is the state the caller wanted.
+func (r *renderer) do(req *http.Request, name string) error {
+	resp, err := r.http.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	io.Copy(io.Discard, io.LimitReader(resp.Body, 1<<16))
+	switch {
+	case resp.StatusCode >= 200 && resp.StatusCode < 300:
+		return nil
+	case req.Method == http.MethodDelete && resp.StatusCode == http.StatusNotFound:
+		return nil
+	}
+	return fmt.Errorf("%s %s: %s", req.Method, name, resp.Status)
+}