fix: feed repitition

internal/repository/feed_repo.go

@@ -0,0 +1,76 @@
+package repository
+
+import (
+	"context"
+	"time"
+
+	"go.uber.org/zap"
+)
+
+// feedSeenTTL bounds a home-feed "session": after this idle window the
+// per-user seen-set expires and the feed starts fresh from the top.
+const feedSeenTTL = 30 * time.Minute
+
+// FeedRepo tracks, per user, which post ids have already been served in the
+// current home-feed session (a Redis SET) so paginated requests never repeat
+// a post — and so the viewer-state-dependent feed ordering can't cause an
+// OFFSET to drift over already-served rows.
+//
+// Nil-safe: with no Redis configured every method degrades to a no-op /
+// empty, and the feed falls back to plain offset paging.
+type FeedRepo struct {
+	redis  *Redis
+	logger *zap.Logger
+}
+
+func NewFeedRepo(redis *Redis, logger *zap.Logger) *FeedRepo {
+	return &FeedRepo{redis: redis, logger: logger}
+}
+
+// Enabled reports whether the Redis-backed seen-set is available.
+func (r *FeedRepo) Enabled() bool { return r != nil && r.redis != nil }
+
+func (r *FeedRepo) key(userID string) string { return "feed:seen:" + userID }
+
+// Seen returns the post ids already served to the user this session (empty
+// when Redis is unavailable or the session is fresh).
+func (r *FeedRepo) Seen(ctx context.Context, userID string) []string {
+	if !r.Enabled() {
+		return nil
+	}
+	ids, err := r.redis.Client.SMembers(ctx, r.key(userID)).Result()
+	if err != nil {
+		// Degrade rather than fail the feed — worst case is a possible repeat.
+		r.logger.Warn("feed seen: read failed", zap.String("user", userID), zap.Error(err))
+		return nil
+	}
+	return ids
+}
+
+// MarkSeen records the ids just served and refreshes the session TTL.
+func (r *FeedRepo) MarkSeen(ctx context.Context, userID string, ids []string) {
+	if !r.Enabled() || len(ids) == 0 {
+		return
+	}
+	members := make([]any, len(ids))
+	for i, id := range ids {
+		members[i] = id
+	}
+	pipe := r.redis.Client.Pipeline()
+	pipe.SAdd(ctx, r.key(userID), members...)
+	pipe.Expire(ctx, r.key(userID), feedSeenTTL)
+	if _, err := pipe.Exec(ctx); err != nil {
+		r.logger.Warn("feed seen: write failed", zap.String("user", userID), zap.Error(err))
+	}
+}
+
+// Reset clears the user's seen-set — called when a fresh feed session begins
+// (the first page request) so reopening the feed serves from the top again.
+func (r *FeedRepo) Reset(ctx context.Context, userID string) {
+	if !r.Enabled() {
+		return
+	}
+	if err := r.redis.Client.Del(ctx, r.key(userID)).Err(); err != nil {
+		r.logger.Warn("feed seen: reset failed", zap.String("user", userID), zap.Error(err))
+	}
+}

internal/repository/post_repo.go

@@ -219,14 +219,27 @@ func (r *PostRepo) ListByUser(ctx context.Context, userID string, limit, offset
 //	tier 0 — posts by people the viewer follows that they haven't liked yet
 //	tier 1 — everything else (followed-and-liked, followers', strangers')
 //
-// The viewer's own posts and anything involving a block (either direction)
-// are excluded. The tier is computed in ORDER BY via correlated EXISTS, so no
-// extra columns leak into scanPost. No total is computed (infinite scroll
-// infers "more" from a full page); offset paging keeps it simple. At larger
-// scale this would move to a precomputed fan-out / keyset cursor.
-func (r *PostRepo) FeedForViewer(ctx context.Context, viewerID string, limit, offset int) ([]model.Post, error) {
+// The viewer's own posts, anything involving a block (either direction), and
+// any id in `exclude` (the already-served set) are filtered out. The tier is
+// computed in ORDER BY via correlated EXISTS, so no extra columns leak into
+// scanPost. No total is computed (infinite scroll infers "more" from a full
+// page).
+//
+// With a non-empty `exclude` the caller drives pagination purely by exclusion
+// (pass offset 0): since the tier depends on viewer state — liking a post
+// moves it between tiers — an OFFSET would drift and repeat/skip rows, so the
+// seen-set is the correct cursor. Without Redis, `exclude` is empty and OFFSET
+// paginates (the frontend de-dups the rare drift). At larger scale this moves
+// to a precomputed fan-out / keyset cursor.
+func (r *PostRepo) FeedForViewer(ctx context.Context, viewerID string, limit, offset int, exclude []string) ([]model.Post, error) {
+	// nil would encode as SQL NULL, and `id <> ALL(NULL)` is NULL (excludes
+	// every row); a non-nil empty array means "exclude nothing".
+	if exclude == nil {
+		exclude = []string{}
+	}
 	q := `SELECT ` + postSelectColumns + postSelectFrom + `
 		WHERE p.user_id <> $1
+		  AND p.id <> ALL($4)
 		  AND NOT EXISTS (
 		      SELECT 1 FROM user_blocks b
 		      WHERE (b.blocker_id = $1 AND b.blocked_id = p.user_id)
@@ -247,7 +260,7 @@ func (r *PostRepo) FeedForViewer(ctx context.Context, viewerID string, limit, of
 		   END) ASC,
 		  p.created_at DESC, p.id DESC
 		LIMIT $2 OFFSET $3`
-	posts, _, err := r.collectPosts(ctx, 0, q, viewerID, limit, offset)
+	posts, _, err := r.collectPosts(ctx, 0, q, viewerID, limit, offset, exclude)
 	return posts, err
 }