publish: lock pool on non MultiDist publish

* revert mutex on LinkFromPool
* add tests

deb/publish.go

@@ -612,6 +612,15 @@ func (p *PublishedRepo) Key() []byte {
 	return []byte("U" + p.StoragePrefix() + ">>" + p.Distribution)
 }
 
+// PrefixPoolLockKey returns the task-queue resource key that serialises all
+// publish operations sharing the same pool directory under storagePrefix.
+// It must be held whenever a non-MultiDist publish may read or clean the
+// shared pool, to prevent concurrent cleanup runs from deleting each other's
+// files.  See docs/Resource-Locking.md for the full key-namespace table.
+func PrefixPoolLockKey(storagePrefix string) string {
+	return "P" + storagePrefix
+}
+
 // RefKey is a unique id for package reference list
 func (p *PublishedRepo) RefKey(component string) []byte {
 	return []byte("E" + p.UUID + component)

deb/publish_test.go

@@ -873,7 +873,10 @@ func (s *PublishedRepoCollectionSuite) TestListReferencedFiles(c *C) {
 	snap3 := NewSnapshotFromRefList("snap3", []*Snapshot{}, s.snap2.RefList(), "desc3")
 	_ = s.snapshotCollection.Add(snap3)
 
-	// Ensure that adding a second publish point with matching files doesn't give duplicate results.
+	// When a second publish point references the same package (snap3 is a clone of snap2,
+	// both containing p3/lonely-strangers), listReferencedFilesByComponent deduplicates by
+	// package ref so the file appears only once.  StrSlicesSubstract handles a single entry
+	// correctly, so no duplicate is needed for cleanup safety.
 	repo3, err := NewPublishedRepo("", "", "anaconda-2", []string{}, []string{"main"}, []interface{}{snap3}, s.factory, false)
 	c.Check(err, IsNil)
 	c.Check(s.collection.Add(repo3), IsNil)
@@ -888,7 +891,9 @@ func (s *PublishedRepoCollectionSuite) TestListReferencedFiles(c *C) {
 			"a/alien-arena/alien-arena-common_7.40-2_i386.deb",
 			"a/alien-arena/mars-invaders_7.40-2_i386.deb",
 		},
-		"main": {"a/alien-arena/lonely-strangers_7.40-2_i386.deb"},
+		"main": {
+			"a/alien-arena/lonely-strangers_7.40-2_i386.deb",
+		},
 	})
 }