ledger refactoring: fix catchpoint tracker (#3214)

## Summary

When implementing the catchpoint tracker, I missed the re-initilization location for some of the local variables.
This would generate incorrect catchpoint labels after a node completes a fast-catchup.

#3085

## Test Plan

- [x] Add unit tests
- [x] Perform manual testing

ledger/catchpointtracker.go

@@ -159,6 +159,12 @@ func (ct *catchpointTracker) loadFromDisk(l ledgerForTracker, lastBalancesRound
 	ct.log = l.trackerLog()
 	ct.dbs = l.trackerDB()
 
+	ct.roundDigest = nil
+	ct.catchpointWriting = 0
+	// keep these channel closed if we're not generating catchpoint
+	ct.catchpointSlowWriting = make(chan struct{}, 1)
+	close(ct.catchpointSlowWriting)
+
 	err = ct.dbs.Wdb.Atomic(func(ctx context.Context, tx *sql.Tx) error {
 		err0 := ct.accountsInitializeHashes(ctx, tx, lastBalancesRound)
 		if err0 != nil {
@@ -207,12 +213,6 @@ func (ct *catchpointTracker) loadFromDisk(l ledgerForTracker, lastBalancesRound
 	}
 	blockHeaderDigest := blk.Digest()
 
-	ct.catchpointWriting = 0
-	// keep these channel closed if we're not generating catchpoint
-	ct.catchpointSlowWriting = make(chan struct{}, 1)
-	close(ct.catchpointSlowWriting)
-	ct.roundDigest = nil
-
 	ct.generateCatchpoint(context.Background(), basics.Round(writingCatchpointRound), ct.lastCatchpointLabel, blockHeaderDigest, time.Duration(0))
 	return nil
 }

ledger/catchpointtracker_test.go

@@ -372,4 +372,44 @@ func TestReproducibleCatchpointLabels(t *testing.T) {
 			}
 		}
 	}
+
+	// test to see that after loadFromDisk, all the tracker content is lost ( as expected )
+	require.NotZero(t, len(ct.roundDigest))
+	require.NoError(t, ct.loadFromDisk(ml, ml.Latest()))
+	require.Zero(t, len(ct.roundDigest))
+	require.Zero(t, ct.catchpointWriting)
+	select {
+	case _, closed := <-ct.catchpointSlowWriting:
+		require.False(t, closed)
+	default:
+		require.FailNow(t, "The catchpointSlowWriting should have been a closed channel; it seems to be a nil ?!")
+	}
+}
+
+func TestCatchpointTrackerPrepareCommit(t *testing.T) {
+	partitiontest.PartitionTest(t)
+
+	ct := &catchpointTracker{}
+	const maxOffset = 40
+	const maxLookback = 320
+	ct.roundDigest = make([]crypto.Digest, maxOffset+maxLookback)
+	for i := 0; i < len(ct.roundDigest); i++ {
+		ct.roundDigest[i] = crypto.Hash([]byte{byte(i), byte(i / 256)})
+	}
+	dcc := &deferredCommitContext{}
+	for offset := uint64(1); offset < maxOffset; offset++ {
+		dcc.offset = offset
+		for lookback := basics.Round(0); lookback < maxLookback; lookback += 20 {
+			dcc.lookback = lookback
+			for _, isCatchpointRound := range []bool{false, true} {
+				dcc.isCatchpointRound = isCatchpointRound
+				require.NoError(t, ct.prepareCommit(dcc))
+				if isCatchpointRound {
+					expectedRound := offset + uint64(lookback) - 1
+					expectedHash := crypto.Hash([]byte{byte(expectedRound), byte(expectedRound / 256)})
+					require.Equal(t, expectedHash[:], dcc.committedRoundDigest[:])
+				}
+			}
+		}
+	}
 }