refac: atomic merge, lost-update race, RunPurge cost, duplicated walks

test/media_purge_http_test.go

@@ -743,16 +743,22 @@ func TestPurgeRun(t *testing.T) {
 		env.useImages(22, 3, 4)
 		env.useImages(23, 3)
 		env.images.failDeleteFor = map[uint32]error{2: errRepositoryFailure}
+		originalA := env.bodies()[20]
+		copyURL := env.urlOf(2)
 
 		expectRaw(t, env.run(t, `{}`), 200, counters(2, 1, 1, 1, 1))
-		if strings.Contains(env.bodies()[20], env.urlOf(2)) {
-			t.Errorf("expected article 20 to stay switched to the kept copy")
+		if env.bodies()[20] != originalA {
+			t.Errorf("expected group A's bodies back to their original text")
 		}
 		if got := numberOf(env.preview(t)["groups"]); got != 1 {
 			t.Errorf("expected group A to be planned again, got %d groups", got)
 		}
 		env.images.failDeleteFor = nil
-		expectRaw(t, env.run(t, `{}`), 200, counters(1, 1, 1, 0, 0))
+		expectRaw(t, env.run(t, `{}`), 200, counters(1, 1, 1, 1, 0))
+		if strings.Contains(env.bodies()[20], copyURL) {
+			t.Errorf("expected article 20 to use the kept copy after the next run")
+		}
+		expectIds(t, env.imageIds(), []uint32{1, 3})
 	})
 
 	t.Run("Every group failing is still a 200 with the counters", func(t *testing.T) {