From 466c8e042b140078d9c800ef420d99bf9f7cb7f8 Mon Sep 17 00:00:00 2001
From: Norbert Hartl <norbert@hartl.name>
Date: Wed, 22 Jul 2026 12:27:38 +0200
Subject: [PATCH] Add tombstone GC for GossipListValue
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Tombstones (removed items) were never physically dropped from the
entries dict, only ever marked present: false — fine for correctness
(each GossipListValue is always replaced as a whole atomic unit via
the enclosing GossipApplicationState's version, so there's no
per-item merge/resurrection risk the way Cassandra's cell-level
tombstones have), but unbounded over a long-running process with many
distinct items added then removed.

gcTombstonesOlderThan: drops any tombstoned entry whose itemVersion is
more than the given gap behind the list's own versionCounter. Uses
the list's own logical version clock rather than wall-clock time,
since nothing in Gossip carries real timestamps. Deliberately left as
an explicit method the caller invokes with its own threshold, not
auto-wired into Gossiper's periodic gossip loop — picking a concrete
grace window is a policy call that depends on gossip round frequency,
not something to default silently.

New GossipListValueTest (5 tests) covers add/remove/tombstone
semantics and the GC boundary (kept when recent, removed when past
the gap, present items never touched).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 .../Gossip-Tests/GossipListValueTest.class.st | 61 +++++++++++++++++++
 source/Gossip/GossipListValue.class.st        |  6 ++
 2 files changed, 67 insertions(+)
 create mode 100644 source/Gossip-Tests/GossipListValueTest.class.st

diff --git a/source/Gossip-Tests/GossipListValueTest.class.st b/source/Gossip-Tests/GossipListValueTest.class.st
new file mode 100644
index 0000000..4a47012
--- /dev/null
+++ b/source/Gossip-Tests/GossipListValueTest.class.st
@@ -0,0 +1,61 @@
+Class {
+	#name : 'GossipListValueTest',
+	#superclass : 'TestCase',
+	#category : 'Gossip-Tests',
+	#package : 'Gossip-Tests'
+}
+
+{ #category : 'as yet unclassified' }
+GossipListValueTest >> testAddMakesItemPresent [
+	| lv |
+	lv := GossipListValue new.
+	lv add: #a.
+	self assert: (lv includes: #a).
+	self assert: lv presentItems asArray equals: #(#a)
+]
+
+{ #category : 'as yet unclassified' }
+GossipListValueTest >> testGcTombstonesOlderThanKeepsRecentTombstones [
+	| lv |
+	lv := GossipListValue new.
+	lv add: #a.
+	lv remove: #a.
+	lv add: #b.
+	lv gcTombstonesOlderThan: 5.
+	self assert: (lv entries includesKey: #a)
+]
+
+{ #category : 'as yet unclassified' }
+GossipListValueTest >> testGcTombstonesOlderThanNeverRemovesPresentItems [
+	| lv |
+	lv := GossipListValue new.
+	lv add: #a.
+	lv add: #b.
+	lv add: #c.
+	lv gcTombstonesOlderThan: 0.
+	self assert: (lv entries includesKey: #a).
+	self assert: lv presentItems asSortedCollection asArray equals: #(#a #b #c)
+]
+
+{ #category : 'as yet unclassified' }
+GossipListValueTest >> testGcTombstonesOlderThanRemovesOldTombstones [
+	| lv |
+	lv := GossipListValue new.
+	lv add: #a.
+	lv remove: #a.
+	lv add: #b.
+	lv add: #c.
+	lv gcTombstonesOlderThan: 1.
+	self deny: (lv entries includesKey: #a)
+]
+
+{ #category : 'as yet unclassified' }
+GossipListValueTest >> testRemoveTombstonesRatherThanDeletes [
+	| lv |
+	lv := GossipListValue new.
+	lv add: #a.
+	lv remove: #a.
+	self deny: (lv includes: #a).
+	self assert: lv presentItems isEmpty.
+	self assert: (lv entries includesKey: #a)
+]
diff --git a/source/Gossip/GossipListValue.class.st b/source/Gossip/GossipListValue.class.st
index 8ac0017..f17ca05 100644
--- a/source/Gossip/GossipListValue.class.st
+++ b/source/Gossip/GossipListValue.class.st
@@ -19,6 +19,12 @@ GossipListValue >> entries [
 	^ entries
 ]
 
+{ #category : 'as yet unclassified' }
+GossipListValue >> gcTombstonesOlderThan: aVersionGap [
+	entries := entries reject: [ :pair | 
+		pair first not and: [ (versionCounter - pair second) > aVersionGap ] ]
+]
+
 { #category : 'not defined protocol' }
 GossipListValue >> includes: anItem [
 	^ (entries at: anItem ifAbsent: [ ^ false ]) first