Add per-remote sync_interval and spread GitHub syncs across it

internal/database/github_sync.go

@@ -3,6 +3,7 @@ package database
 import (
 	"context"
 	"errors"
+	"hash/fnv"
 	"time"
 
 	"github.com/jackc/pgx/v5"
@@ -47,9 +48,11 @@ func (db *DB) ReleaseGitHubSyncLease(ctx context.Context, remoteName, owner stri
 // claimSyncLease atomically claims a per-remote sync lease in table. It succeeds
 // (claimed=true) only when the remote is due and no live lease is held by
 // another replica. Due means: a pending retry after a failed scan has reached
-// next_retry_at, or, with no retry pending, the remote was never synced or was
-// synced longer than freshness ago. A zero freshness (prime scans) ignores the
-// recency gate but still honours a pending retry's backoff. The returned etag is
+// next_retry_at, or, with no retry pending, the remote was never synced or not
+// since its latest slot boundary. Boundaries recur every freshness, offset by a
+// stable per-remote phase so remotes sharing a TTL are spread across it rather
+// than due together. A zero freshness (prime scans) ignores the recency gate
+// but still honours a pending retry's backoff. The returned etag is
 // the stored releases-list ETag, shared across replicas so a conditional request
 // can short-circuit an unchanged repo.
 func (db *DB) claimSyncLease(ctx context.Context, table, remoteName, owner string, freshness, lease time.Duration) (bool, string, error) {
@@ -60,10 +63,11 @@ func (db *DB) claimSyncLease(ctx context.Context, table, remoteName, owner strin
 			SET sync_lease_owner   = $2,
 			    sync_lease_expires = now() + make_interval(secs => $4)
 			WHERE (CASE WHEN s.next_retry_at IS NOT NULL THEN s.next_retry_at <= now()
-			            ELSE s.last_synced_at IS NULL OR s.last_synced_at < now() - make_interval(secs => $3) END)
+			            ELSE s.last_synced_at IS NULL OR s.last_synced_at < CASE WHEN $3 <= 0 THEN now()
+			                 ELSE to_timestamp(floor((extract(epoch from now())::float8 - $5) / NULLIF($3, 0)) * $3 + $5) END END)
 			  AND (s.sync_lease_expires IS NULL OR s.sync_lease_expires < now())
 		RETURNING s.etag
-	`, remoteName, owner, freshness.Seconds(), lease.Seconds())
+	`, remoteName, owner, freshness.Seconds(), lease.Seconds(), syncPhase(remoteName, freshness).Seconds())
 
 	var etag string
 	if err := row.Scan(&etag); err != nil {
@@ -75,6 +79,17 @@ func (db *DB) claimSyncLease(ctx context.Context, table, remoteName, owner strin
 	return true, etag, nil
 }
 
+// syncPhase is remoteName's stable offset within period, so every replica
+// agrees on when each remote is due.
+func syncPhase(remoteName string, period time.Duration) time.Duration {
+	if period <= 0 {
+		return 0
+	}
+	h := fnv.New64a()
+	_, _ = h.Write([]byte(remoteName))
+	return time.Duration(h.Sum64() % uint64(period))
+}
+
 // releaseSyncLease records a scan outcome and frees the lease; only the owning
 // replica may release. Success advances last_synced_at, persists the etag and
 // clears any retry. Failure leaves last_synced_at and etag untouched (the last