1 files changed, 145 insertions, 0 deletions
diff --git a/internal/peer.go b/internal/peer.go
new file mode 100644
index 0000000..bc0d12e
--- /dev/null
+++ b/internal/peer.go
@@ -0,0 +1,145 @@
+package internal
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"os"
+	"time"
+)
+
+type peerReport struct {
+	LastUpdated string `json:"lastUpdated"`
+}
+
+func peerActive(ctx context.Context, conf config) (bool, string) {
+	if conf.PeerURL == "" {
+		return true, "Peer failover: disabled (PeerURL not set)"
+	}
+
+	hostname, err := os.Hostname()
+	if err != nil {
+		return true, fmt.Sprintf("Peer failover: hostname lookup failed (%v); staying active", err)
+	}
+
+	return peerActiveAt(ctx, conf, time.Now(), hostname, fetchPeerLastUpdated)
+}
+
+func peerActiveAt(
+	ctx context.Context,
+	conf config,
+	now time.Time,
+	hostname string,
+	fetch func(context.Context, string) (time.Time, error),
+) (bool, string) {
+	if conf.PeerURL == "" {
+		return true, "Peer failover: disabled (PeerURL not set)"
+	}
+
+	primary := conf.PeerPrimaryName
+	if primary == "" {
+		primary = hostname
+	}
+
+	secondary := conf.PeerSecondaryName
+	if secondary == "" {
+		if parsedURL, err := url.Parse(conf.PeerURL); err == nil && parsedURL.Hostname() != "" {
+			secondary = parsedURL.Hostname()
+		}
+	}
+
+	if primary == "" || secondary == "" {
+		return true, "Peer failover: missing peer names; staying active"
+	}
+
+	if hostname != primary && hostname != secondary {
+		return true, fmt.Sprintf("Peer failover: local hostname %s not in [%s, %s]; staying active",
+			hostname, primary, secondary)
+	}
+
+	staleThresholdS := conf.PeerStaleThresholdS
+	if staleThresholdS == 0 {
+		staleThresholdS = 600
+	}
+
+	lastUpdated, err := fetch(ctx, conf.PeerURL)
+	if err != nil {
+		return true, fmt.Sprintf("Peer failover: peer check failed (%v); staying active", err)
+	}
+
+	age := now.Sub(lastUpdated)
+	if age > time.Duration(staleThresholdS)*time.Second {
+		return true, fmt.Sprintf("Peer failover: peer stale (%v > %ds); staying active",
+			age, staleThresholdS)
+	}
+
+	master := scheduledMaster(primary, secondary, now)
+	if hostname == master {
+		return true, fmt.Sprintf("Peer failover: peer healthy; scheduled master is %s", master)
+	}
+
+	return false, fmt.Sprintf("Peer failover: peer healthy; scheduled master is %s", master)
+}
+
+func scheduledMaster(primary, secondary string, now time.Time) string {
+	week := weekNumberSunday(now)
+	if week%2 == 0 {
+		return secondary
+	}
+	return primary
+}
+
+// weekNumberSunday matches strftime %U (Sunday-based week number, 00-53).
+func weekNumberSunday(t time.Time) int {
+	tUTC := t.In(time.UTC)
+	yearStart := time.Date(tUTC.Year(), 1, 1, 0, 0, 0, 0, time.UTC)
+
+	// Find the first Sunday on or after Jan 1.
+	daysUntilSunday := (7 - int(yearStart.Weekday())) % 7
+	firstSunday := yearStart.AddDate(0, 0, daysUntilSunday)
+
+	if tUTC.Before(firstSunday) {
+		return 0
+	}
+
+	daysSinceFirstSunday := int(tUTC.Sub(firstSunday).Hours() / 24)
+	return 1 + (daysSinceFirstSunday / 7)
+}
+
+func fetchPeerLastUpdated(ctx context.Context, peerURL string) (time.Time, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, peerURL, nil)
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	client := http.Client{
+		Timeout: 5 * time.Second,
+	}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return time.Time{}, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode < http.StatusOK || resp.StatusCode >= http.StatusMultipleChoices {
+		return time.Time{}, fmt.Errorf("unexpected status %d", resp.StatusCode)
+	}
+
+	var report peerReport
+	if err := json.NewDecoder(resp.Body).Decode(&report); err != nil {
+		return time.Time{}, err
+	}
+	if report.LastUpdated == "" {
+		return time.Time{}, fmt.Errorf("missing lastUpdated")
+	}
+
+	lastUpdated, err := time.Parse(time.RFC3339, report.LastUpdated)
+	if err != nil {
+		return time.Time{}, err
+	}
+
+	return lastUpdated, nil
+}