mavcaldav: reconcile the render collection on startup

Withdrawal read published, which is in-memory, so the second loop only
ever withdrew reminders this process had published. Fire a reminder,
restart mavcaldav, and its event stayed in the collection forever with
nothing left to revisit it. "Losing it costs nothing, the next tick
rebuilds it" holds for events that should be there and not for the ones
that should not.

The first tick now PROPFINDs the collection and reconciles what it finds
against what is pending. Only hrefs carrying ReminderUIDPrefix are read
back, so the pass can never propose deleting a file maven did not create.
A failed read is retried on the next tick rather than skipped for the
life of the process.

Two smaller things from the same review. checkRenderTarget takes the
whole read set, so a second calendar to read cannot quietly fall outside
the guarantee the package comment makes. writeIfChanged loses its
confidence parameter, which every caller passed 1.0 and nothing read.
Found in review of #56.

cmd/mavcaldav/render.go

@@ -2,15 +2,18 @@ package main
 
 import (
 	"context"
+	"encoding/xml"
 	"fmt"
 	"io"
 	"log"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 
 	"github.com/kami/maven/internal/calendar"
 	"github.com/kami/maven/internal/ipc"
+	"github.com/kami/maven/internal/store"
 )
 
 // renderer is the write half of maven's own local calendar (Vikunja #127).
@@ -38,6 +41,13 @@ type renderer struct {
 	// unchanged reminder costs nothing. Purely an optimisation: a restart
 	// re-publishes every reminder once, which is idempotent.
 	published map[int64]string
+
+	// reconciled — whether the collection has been read once since start. It
+	// has to be, because published is in-memory: withdrawal used to cover only
+	// the reminders THIS process published, so a reminder that fired while the
+	// daemon was down kept its event in the calendar forever, and nothing ever
+	// revisited it.
+	reconciled bool
 }
 
 func newRenderer(core ipc.CoreAPI, hc *http.Client, url, user, pass string, dur time.Duration) *renderer {
@@ -64,7 +74,7 @@ func (r *renderer) renderOnce(ctx context.Context) {
 
 	live := make(map[int64]bool, len(reminders))
 	for _, rem := range reminders {
-		if rem.Status != "pending" {
+		if rem.Status != store.ReminderPending {
 			continue
 		}
 		live[rem.ID] = true
@@ -81,10 +91,28 @@ func (r *renderer) renderOnce(ctx context.Context) {
 		log.Printf("mavcaldav: rendered reminder %d (%s)", rem.ID, e.Summary)
 	}
 
+	stale := make(map[int64]bool)
 	for id := range r.published {
-		if live[id] {
-			continue
+		if !live[id] {
+			stale[id] = true
 		}
+	}
+	if !r.reconciled {
+		remote, err := r.listPublished(ctx)
+		if err != nil {
+			// Try again next tick. A collection maven cannot read is not a
+			// reason to stop publishing to it.
+			log.Printf("mavcaldav: reconcile: %v", err)
+		} else {
+			r.reconciled = true
+			for _, id := range remote {
+				if !live[id] {
+					stale[id] = true
+				}
+			}
+		}
+	}
+	for id := range stale {
 		if err := r.delete(ctx, calendar.ReminderPath(id)); err != nil {
 			log.Printf("mavcaldav: withdraw reminder %d: %v", id, err)
 			continue
@@ -94,6 +122,55 @@ func (r *renderer) renderOnce(ctx context.Context) {
 	}
 }
 
+// listPublished PROPFINDs the collection and returns the reminder ids maven has
+// events for in it. Only resources carrying calendar.ReminderUIDPrefix are
+// reported, so a reconciliation pass can never propose deleting a file maven
+// did not create — the same bound every other path in this file has.
+func (r *renderer) listPublished(ctx context.Context) ([]int64, error) {
+	const body = `<?xml version="1.0" encoding="utf-8"?>` +
+		`<D:propfind xmlns:D="DAV:"><D:prop><D:resourcetype/></D:prop></D:propfind>`
+	req, err := http.NewRequestWithContext(ctx, "PROPFIND", r.url+"/", strings.NewReader(body))
+	if err != nil {
+		return nil, err
+	}
+	req.SetBasicAuth(r.user, r.pass)
+	req.Header.Set("Content-Type", "application/xml; charset=utf-8")
+	req.Header.Set("Depth", "1")
+
+	resp, err := r.http.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	raw, err := io.ReadAll(io.LimitReader(resp.Body, 4<<20))
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != http.StatusMultiStatus && (resp.StatusCode < 200 || resp.StatusCode >= 300) {
+		return nil, fmt.Errorf("PROPFIND %s: %s", r.url, resp.Status)
+	}
+
+	var ms struct {
+		Responses []struct {
+			Href string `xml:"href"`
+		} `xml:"response"`
+	}
+	if err := xml.Unmarshal(raw, &ms); err != nil {
+		return nil, fmt.Errorf("PROPFIND %s: %w", r.url, err)
+	}
+	var ids []int64
+	for _, resp := range ms.Responses {
+		href, err := url.PathUnescape(strings.TrimSpace(resp.Href))
+		if err != nil {
+			continue
+		}
+		if id, ok := calendar.ReminderIDFromPath(href); ok {
+			ids = append(ids, id)
+		}
+	}
+	return ids, nil
+}
+
 // renderMaxReminders bounds the read. Reminders past this count are older than
 // anything a calendar view is useful for.
 const renderMaxReminders = 200