From 393fc78a79614bb3ceeeeacf691d90fa3479a782 Mon Sep 17 00:00:00 2001
From: AE <98699388+A-EDev@users.noreply.github.com>
Date: Thu, 29 Oct 2026 21:34:59 +0200
Subject: [PATCH] fix(sync): keep a superseded snapshot until every device has
 read past it

---
 .../2026-10-29-fix-sync2-gc-manifest-ack.md           |  2 ++
 .../aedev/flow/sync2/engine/GarbageCollection.kt      |  8 +++++---
 .../github/aedev/flow/sync2/GarbageCollectionTest.kt  | 11 +++++++++++
 3 files changed, 18 insertions(+), 3 deletions(-)
 create mode 100644 .github/nightly-notes/2026-10-29-fix-sync2-gc-manifest-ack.md

diff --git a/.github/nightly-notes/2026-10-29-fix-sync2-gc-manifest-ack.md b/.github/nightly-notes/2026-10-29-fix-sync2-gc-manifest-ack.md
new file mode 100644
index 000000000..38a6c21f2
--- /dev/null
+++ b/.github/nightly-notes/2026-10-29-fix-sync2-gc-manifest-ack.md
@@ -0,0 +1,2 @@
+## Fixed
+- Linked devices: a device no longer gets stuck behind another one that cleaned up a snapshot it hadn't read yet (#1075)
diff --git a/app/src/main/java/io/github/aedev/flow/sync2/engine/GarbageCollection.kt b/app/src/main/java/io/github/aedev/flow/sync2/engine/GarbageCollection.kt
index 5c1f0d7c0..faf01e007 100644
--- a/app/src/main/java/io/github/aedev/flow/sync2/engine/GarbageCollection.kt
+++ b/app/src/main/java/io/github/aedev/flow/sync2/engine/GarbageCollection.kt
@@ -77,10 +77,12 @@ class Facts(
 
     private fun acknowledged(link: String?): List<Logged> = manifests.filter { it.manifest.link == link && acked(it.did) >= it.seq }
 
+    /** A newer acknowledged snapshot covers [old], and every device read past [old] itself, so its removal leaves no gap. */
     private fun superseded(old: Logged): Boolean =
-        acknowledged(old.manifest.link).any { newer ->
-            newer != old && old.manifest.vv.all { (did, tip) -> (newer.manifest.vv[did]?.seq ?: -1) >= tip.seq }
-        }
+        acked(old.did) >= old.seq &&
+            acknowledged(old.manifest.link).any { newer ->
+                newer != old && old.manifest.vv.all { (did, tip) -> (newer.manifest.vv[did]?.seq ?: -1) >= tip.seq }
+            }
 
     /** How far every link's newest acknowledged snapshot covers [did]'s log. */
     private fun covered(did: String): Long =
diff --git a/app/src/test/java/io/github/aedev/flow/sync2/GarbageCollectionTest.kt b/app/src/test/java/io/github/aedev/flow/sync2/GarbageCollectionTest.kt
index 7398336c7..977b2d1b6 100644
--- a/app/src/test/java/io/github/aedev/flow/sync2/GarbageCollectionTest.kt
+++ b/app/src/test/java/io/github/aedev/flow/sync2/GarbageCollectionTest.kt
@@ -71,6 +71,17 @@ class GarbageCollectionTest {
         assertTrue(plan(setOf("a", "b"), setOf("a"), mapOf("b" to mapOf("a" to 4L)), listOf(old, new)).manifests.isEmpty())
     }
 
+    @Test
+    fun a_snapshot_stays_until_every_device_read_past_it_even_when_another_covers_it() {
+        // The Mac's manifest 573 is covered by the phone's acknowledged snapshot, but the phone has read the Mac's log only to 572.
+        val mac = snapshot("mac", 573, "mac" to 572, "phone" to 40)
+        val phone = snapshot("phone", 41, "mac" to 572, "phone" to 40)
+        val acks = mapOf("mac" to mapOf("phone" to 41L))
+        val devices = setOf("mac", "phone")
+        assertTrue(plan(devices, setOf("mac"), acks + ("phone" to mapOf("mac" to 572L)), listOf(mac, phone)).manifests.isEmpty())
+        assertEquals(listOf(mac), plan(devices, setOf("mac"), acks + ("phone" to mapOf("mac" to 573L)), listOf(mac, phone)).manifests)
+    }
+
     @Test
     fun every_link_batches_name_needs_an_acknowledged_snapshot() {
         val acks = mapOf("b" to mapOf("a" to 9L))