Bound the undated-item set per feed (V-641)

rss.Poller.seen held every undated item ever seen, one entry per id, for as
long as mavend ran. fresh() added and nothing removed. A feed that ships items
with no <pubDate> grew it forever.

seenIDs is the same set with a bound: the map answers the lookup, a slice
remembers insertion order, and the oldest id falls out past 512. The cap has
to stay above any one feed's front page or an item still listed there would be
written a second time, and a few hundred covers the largest page anyone
publishes. The set only ever had to span one poll window plus the resync
guard, not all of history.

Dedupe behaviour is unchanged. The comment at fresh() explains why the set
does not survive a restart; it never bounded it within one run.

internal/rss/poller.go

@@ -89,8 +89,8 @@ type Poller struct {
 	ranker  Ranker
 	cfg     Config
 	nextDue map[string]time.Time
-	seen    map[string]map[string]bool // feed → item ID, for items with no date
-	polled  map[string]bool            // feed → polled at least once in THIS process
+	seen    map[string]*seenIDs // feed → item IDs, for items with no date
+	polled  map[string]bool     // feed → polled at least once in THIS process
 }
 
 // NewPoller wires a poller. Returns nil when there is nothing to poll — a
@@ -122,7 +122,7 @@ func NewPoller(feeds []FeedConfig, fetch Fetcher, notes Notes, marks Marks, embe
 		feeds: valid, fetch: fetch, notes: notes, marks: marks,
 		embed: embed, ranker: ranker, cfg: cfg,
 		nextDue: map[string]time.Time{},
-		seen:    map[string]map[string]bool{},
+		seen:    map[string]*seenIDs{},
 		polled:  map[string]bool{},
 	}
 }
@@ -283,6 +283,38 @@ func (p *Poller) mark(ctx context.Context, feed string, now time.Time) (time.Tim
 	return at, true
 }
 
+// maxSeenPerFeed bounds the undated-item set. It has to stay comfortably above
+// any one feed's front page, or an item still listed there would fall out of the
+// set and be written a second time. A few hundred entries covers the largest
+// page anyone publishes, and the set only has to span one poll window plus the
+// resync guard, not all of history.
+const maxSeenPerFeed = 512
+
+// seenIDs is a bounded insertion-ordered set. The map answers the lookup, the
+// slice remembers what to drop first, so an undated feed cannot grow the poller
+// for as long as mavend runs.
+type seenIDs struct {
+	ids   map[string]bool
+	order []string
+}
+
+// add records id and reports whether it was new.
+func (s *seenIDs) add(id string) bool {
+	if s.ids == nil {
+		s.ids = make(map[string]bool, maxSeenPerFeed)
+	}
+	if s.ids[id] {
+		return false
+	}
+	s.ids[id] = true
+	s.order = append(s.order, id)
+	if len(s.order) > maxSeenPerFeed {
+		delete(s.ids, s.order[0])
+		s.order = s.order[1:]
+	}
+	return true
+}
+
 // fresh — two dedup rules, because feeds are inconsistent about dates. A dated
 // item must be newer than the mark; an undated one is kept once per process by
 // ID.
@@ -308,12 +340,11 @@ func (p *Poller) fresh(f FeedConfig, it Item, mark, now time.Time, resync bool)
 		id = it.Title
 	}
 	if p.seen[f.Name] == nil {
-		p.seen[f.Name] = map[string]bool{}
+		p.seen[f.Name] = &seenIDs{}
 	}
-	if p.seen[f.Name][id] {
+	if !p.seen[f.Name].add(id) {
 		return false
 	}
-	p.seen[f.Name][id] = true
 	return !resync
 }
 

internal/rss/poller_test.go

@@ -3,6 +3,7 @@ package rss
 import (
 	"context"
 	"errors"
+	"fmt"
 	"strings"
 	"testing"
 	"time"
@@ -208,3 +209,27 @@ func TestNoFeedsMeansNoPoller(t *testing.T) {
 		t.Fatal("a feed with no name or url is not a configuration")
 	}
 }
+
+// An undated feed used to grow p.seen for as long as mavend ran. The set is
+// bounded now, and the bound must not cost the dedupe an item still on the
+// front page — only ids far older than any page fall out.
+func TestSeenIDsBounded(t *testing.T) {
+	var s seenIDs
+	for i := 0; i < maxSeenPerFeed*3; i++ {
+		if !s.add(fmt.Sprintf("item-%d", i)) {
+			t.Fatalf("item-%d read as already seen", i)
+		}
+		if len(s.ids) > maxSeenPerFeed || len(s.order) > maxSeenPerFeed {
+			t.Fatalf("after %d inserts: ids=%d order=%d, cap is %d",
+				i+1, len(s.ids), len(s.order), maxSeenPerFeed)
+		}
+	}
+	// The newest insert is still deduped; the oldest was evicted.
+	last := fmt.Sprintf("item-%d", maxSeenPerFeed*3-1)
+	if s.add(last) {
+		t.Fatalf("%s read as new, so the most recent id was dropped", last)
+	}
+	if !s.add("item-0") {
+		t.Fatal("item-0 survived, so nothing was evicted")
+	}
+}