Delay static manifest until our head is within the target period

Unfortunately, due to
filecoin-project/lotus#12557, time in our
lotus tests is basically broken. So we start by relying on time but also
check the head so we don't switch too early.

This doesn't matter in practice, but I'd really like to be able to test
this in CI, fully integrated.

manifest/fusing_provider.go

@@ -90,10 +90,30 @@ func (m *FusingManifestProvider) Start(ctx context.Context) error {
 		for m.runningCtx.Err() == nil {
 			select {
 			case <-timer.C:
+				// Make sure we're actually at the target epoch. This shouldn't be
+				// an issue unless our clocks are really funky, the network is
+				// behind, or we're in a lotus integration test
+				// (https://github.com/filecoin-project/lotus/issues/12557).
+				head, err := m.ec.GetHead(m.runningCtx)
+				switch {
+				case err != nil:
+					log.Errorw("failed to get head in fusing manifest provider", "error", err)
+					fallthrough
+				case head.Epoch() < switchEpoch:
+					log.Infow("delaying fusing manifest switch-over because head is behind the target epoch",
+						"head", head.Epoch(),
+						"target epoch", switchEpoch,
+						"bootstrap epoch", m.static.BootstrapEpoch,
+					)
+					timer.Reset(m.static.EC.Period)
+					continue
+				}
+
 				log.Infow(
 					"fusing to the static manifest, stopping the dynamic manifest provider",
 					"network", m.static.NetworkName,
 					"bootstrap epoch", m.static.BootstrapEpoch,
+					"current epoch", head.Epoch(),
 				)
 				m.updateManifest(m.static)
 				// Log any errors and move on. We don't bubble it because we don't