Initial commit

CalDAV and CardDAV checkers sharing a single Go module. Discovery follows
RFC 6764 (/.well-known + SRV/TXT), authenticated probes cover principal,
home-set, collections and a minimal REPORT query on top of go-webdav.
Common shape in internal/dav/; CalDAV adds a scheduling rule.

Surfaces its context URL (and each secure-SRV target) as TLS endpoints via
the EndpointDiscoverer interface, so the dedicated TLS checker can pick
them up without re-parsing observations.

HTML report foregrounds common misconfigs (well-known returning 200,
missing SRV, plaintext-only SRV, missing DAV capability, skipped auth
phase) as action-item callouts before the full phase breakdown.

internal/dav/endpoints.go

@@ -0,0 +1,89 @@
+package dav
+
+import (
+	"net/url"
+	"strconv"
+
+	sdk "git.happydns.org/checker-sdk-go/checker"
+)
+
+// DiscoverEndpoints derives TLS endpoints worth handing off to downstream
+// checkers (notably the dedicated TLS checker) from a completed Observation.
+//
+// A CalDAV/CardDAV context URL always implies a direct-TLS HTTPS endpoint, so
+// we emit a single `tls` entry for the resolved context URL's host:port. If
+// the endpoint was reached via SRV, we also surface each SRV target as its
+// own endpoint — those are the names operators actually need certificates on,
+// and they may differ from the queried domain.
+func DiscoverEndpoints(obs *Observation) []sdk.DiscoveredEndpoint {
+	if obs == nil || obs.Discovery.ContextURL == "" {
+		return nil
+	}
+	var out []sdk.DiscoveredEndpoint
+	seen := map[string]struct{}{}
+
+	add := func(host string, port uint16, sni string) {
+		if host == "" || port == 0 {
+			return
+		}
+		key := host + ":" + strconv.Itoa(int(port))
+		if _, dup := seen[key]; dup {
+			return
+		}
+		seen[key] = struct{}{}
+		ep := sdk.DiscoveredEndpoint{
+			Type: "tls",
+			Host: host,
+			Port: port,
+		}
+		if sni != "" && sni != host {
+			ep.SNI = sni
+		}
+		out = append(out, ep)
+	}
+
+	// Primary endpoint: the resolved context URL.
+	if host, port, ok := hostPortFromURL(obs.Discovery.ContextURL); ok {
+		add(host, port, "")
+	}
+
+	// Secondary endpoints: every TLS SRV target. Clients may connect to any
+	// of them per weight/priority, and all of them need a valid certificate.
+	for _, r := range obs.Discovery.SecureSRV {
+		port := r.Port
+		if port == 0 {
+			port = 443
+		}
+		add(r.Target, port, "")
+	}
+
+	return out
+}
+
+// hostPortFromURL extracts the (host, port) pair from an absolute URL. The
+// port defaults to 443 for https and 80 for http. Returns ok=false for
+// malformed URLs so callers can silently skip them.
+func hostPortFromURL(raw string) (host string, port uint16, ok bool) {
+	u, err := url.Parse(raw)
+	if err != nil {
+		return "", 0, false
+	}
+	host = u.Hostname()
+	if host == "" {
+		return "", 0, false
+	}
+	if p := u.Port(); p != "" {
+		n, convErr := strconv.ParseUint(p, 10, 16)
+		if convErr != nil {
+			return "", 0, false
+		}
+		return host, uint16(n), true
+	}
+	switch u.Scheme {
+	case "https":
+		return host, 443, true
+	case "http":
+		return host, 80, true
+	}
+	return "", 0, false
+}