func tryTxn(kv engine.MVCCKeyValue) (string, error) {
	var txn roachpb.Transaction
	if err := maybeUnmarshalInline(kv.Value, &txn); err != nil {
		return "", err
	}
	return txn.String() + "\n", nil
}

// TestGCQueueProcess creates test data in the range over various time
// scales and verifies that scan queue process properly GCs test data.
func TestGCQueueProcess(t *testing.T) {
	defer leaktest.AfterTest(t)()
	tc := testContext{}
	tc.Start(t)
	defer tc.Stop()

	const now int64 = 48 * 60 * 60 * 1E9 // 2d past the epoch
	tc.manualClock.Set(now)

	ts1 := makeTS(now-2*24*60*60*1E9+1, 0)                     // 2d old (add one nanosecond so we're not using zero timestamp)
	ts2 := makeTS(now-25*60*60*1E9, 0)                         // GC will occur at time=25 hours
	ts2m1 := ts2.Prev()                                        // ts2 - 1 so we have something not right at the GC time
	ts3 := makeTS(now-intentAgeThreshold.Nanoseconds(), 0)     // 2h old
	ts4 := makeTS(now-(intentAgeThreshold.Nanoseconds()-1), 0) // 2h-1ns old
	ts5 := makeTS(now-1E9, 0)                                  // 1s old
	key1 := roachpb.Key("a")
	key2 := roachpb.Key("b")
	key3 := roachpb.Key("c")
	key4 := roachpb.Key("d")
	key5 := roachpb.Key("e")
	key6 := roachpb.Key("f")
	key7 := roachpb.Key("g")
	key8 := roachpb.Key("h")
	key9 := roachpb.Key("i")
	key10 := roachpb.Key("j")
	key11 := roachpb.Key("k")

	data := []struct {
		key roachpb.Key
		ts  hlc.Timestamp
		del bool
		txn bool
	}{
		// For key1, we expect first value to GC.
		{key1, ts1, false, false},
		{key1, ts2, false, false},
		{key1, ts5, false, false},
		// For key2, we expect values to GC, even though most recent is deletion.
		{key2, ts1, false, false},
		{key2, ts2m1, false, false}, // use a value < the GC time to verify it's kept
		{key2, ts5, true, false},
		// For key3, we expect just ts1 to GC, because most recent deletion is intent.
		{key3, ts1, false, false},
		{key3, ts2, false, false},
		{key3, ts5, true, true},
		// For key4, expect oldest value to GC.
		{key4, ts1, false, false},
		{key4, ts2, false, false},
		// For key5, expect all values to GC (most recent value deleted).
		{key5, ts1, false, false},
		{key5, ts2, true, false}, // deleted, so GC
		// For key6, expect no values to GC because most recent value is intent.
		{key6, ts1, false, false},
		{key6, ts5, false, true},
		// For key7, expect no values to GC because intent is exactly 2h old.
		{key7, ts2, false, false},
		{key7, ts4, false, true},
		// For key8, expect most recent value to resolve by aborting, which will clean it up.
		{key8, ts2, false, false},
		{key8, ts3, true, true},
		// For key9, resolve naked intent with no remaining values.
		{key9, ts3, false, true},
		// For key10, GC ts1 because it's a delete but not ts3 because it's above the threshold.
		{key10, ts1, true, false},
		{key10, ts3, true, false},
		{key10, ts4, false, false},
		{key10, ts5, false, false},
		// For key11, we can't GC anything because ts1 isn't a delete.
		{key11, ts1, false, false},
		{key11, ts3, true, false},
		{key11, ts4, true, false},
		{key11, ts5, true, false},
	}

	for i, datum := range data {
		if datum.del {
			dArgs := deleteArgs(datum.key)
			var txn *roachpb.Transaction
			if datum.txn {
				txn = newTransaction("test", datum.key, 1, enginepb.SERIALIZABLE, tc.clock)
				txn.OrigTimestamp = datum.ts
				txn.Timestamp = datum.ts
			}
			if _, err := tc.SendWrappedWith(roachpb.Header{
				Timestamp: datum.ts,
				Txn:       txn,
			}, &dArgs); err != nil {
				t.Fatalf("%d: could not delete data: %s", i, err)
			}
		} else {
			pArgs := putArgs(datum.key, []byte("value"))
			var txn *roachpb.Transaction
			if datum.txn {
				txn = newTransaction("test", datum.key, 1, enginepb.SERIALIZABLE, tc.clock)
				txn.OrigTimestamp = datum.ts
				txn.Timestamp = datum.ts
			}
			if _, err := tc.SendWrappedWith(roachpb.Header{
//.........这里部分代码省略.........

// updateState updates the transaction state in both the success and
// error cases, applying those updates to the corresponding txnMeta
// object when adequate. It also updates certain errors with the
// updated transaction for use by client restarts.
func (tc *TxnCoordSender) updateState(
	startNS int64,
	ctx context.Context,
	ba roachpb.BatchRequest,
	br *roachpb.BatchResponse,
	pErr *roachpb.Error,
) *roachpb.Error {

	tc.Lock()
	defer tc.Unlock()

	if ba.Txn == nil {
		// Not a transactional request.
		return pErr
	}

	var newTxn roachpb.Transaction
	newTxn.Update(ba.Txn)
	if pErr == nil {
		newTxn.Update(br.Txn)
	} else if errTxn := pErr.GetTxn(); errTxn != nil {
		newTxn.Update(errTxn)
	}

	switch t := pErr.GetDetail().(type) {
	case *roachpb.TransactionStatusError:
		// Likely already committed or more obscure errors such as epoch or
		// timestamp regressions; consider txn dead.
		if txn := pErr.GetTxn(); txn != nil {
			defer tc.cleanupTxnLocked(ctx, *txn)
		}
	case *roachpb.OpRequiresTxnError:
		panic("OpRequiresTxnError must not happen at this level")
	case *roachpb.ReadWithinUncertaintyIntervalError:
		// If the reader encountered a newer write within the uncertainty
		// interval, we advance the txn's timestamp just past the last observed
		// timestamp from the node.
		restartTS, ok := newTxn.GetObservedTimestamp(pErr.OriginNode)
		if !ok {
			pErr = roachpb.NewError(errors.Errorf("no observed timestamp for node %d found on uncertainty restart", pErr.OriginNode))
		} else {
			newTxn.Timestamp.Forward(restartTS)
			newTxn.Restart(ba.UserPriority, newTxn.Priority, newTxn.Timestamp)
		}
	case *roachpb.TransactionAbortedError:
		// Increase timestamp if applicable.
		newTxn.Timestamp.Forward(pErr.GetTxn().Timestamp)
		newTxn.Priority = pErr.GetTxn().Priority
		// Clean up the freshly aborted transaction in defer(), avoiding a
		// race with the state update below.
		defer tc.cleanupTxnLocked(ctx, newTxn)
	case *roachpb.TransactionPushError:
		// Increase timestamp if applicable, ensuring that we're
		// just ahead of the pushee.
		newTxn.Timestamp.Forward(t.PusheeTxn.Timestamp)
		newTxn.Restart(ba.UserPriority, t.PusheeTxn.Priority-1, newTxn.Timestamp)
	case *roachpb.TransactionRetryError:
		// Increase timestamp so on restart, we're ahead of any timestamp
		// cache entries or newer versions which caused the restart.
		newTxn.Restart(ba.UserPriority, pErr.GetTxn().Priority, newTxn.Timestamp)
	case *roachpb.WriteTooOldError:
		newTxn.Restart(ba.UserPriority, newTxn.Priority, t.ActualTimestamp)
	case nil:
		// Nothing to do here, avoid the default case.
	default:
		// Do not clean up the transaction here since the client might still
		// want to continue the transaction. For example, a client might
		// continue its transaction after receiving ConditionFailedError, which
		// can come from a unique index violation.
		//
		// TODO(bdarnell): Is this valid? Unless there is a single CPut in
		// the batch, it is difficult to be able to continue after a
		// ConditionFailedError because it is unclear which parts of the
		// batch had succeeded on other ranges before one range hit the
		// failed condition. It may be better to clean up the transaction here.
	}

	txnID := *newTxn.ID

	txnMeta := tc.txns[txnID]
	// For successful transactional requests, keep the written intents and
	// the updated transaction record to be sent along with the reply.
	// The transaction metadata is created with the first writing operation.
	// A tricky edge case is that of a transaction which "fails" on the
	// first writing request, but actually manages to write some intents
	// (for example, due to being multi-range). In this case, there will
	// be an error, but the transaction will be marked as Writing and the
	// coordinator must track the state, for the client's retry will be
	// performed with a Writing transaction which the coordinator rejects
	// unless it is tracking it (on top of it making sense to track it;
	// after all, it **has** laid down intents and only the coordinator
	// can augment a potential EndTransaction call). See #3303.
	if txnMeta != nil || pErr == nil || newTxn.Writing {
		// Adding the intents even on error reduces the likelihood of dangling
		// intents blocking concurrent writers for extended periods of time.
		// See #3346.
//.........这里部分代码省略.........

// TestGCQueueProcess creates test data in the range over various time
// scales and verifies that scan queue process properly GCs test data.
func TestGCQueueProcess(t *testing.T) {
	defer leaktest.AfterTest(t)
	tc := testContext{}
	tc.Start(t)
	defer tc.Stop()

	const now int64 = 48 * 60 * 60 * 1E9 // 2d past the epoch
	tc.manualClock.Set(now)

	ts1 := makeTS(now-2*24*60*60*1E9+1, 0)                     // 2d old (add one nanosecond so we're not using zero timestamp)
	ts2 := makeTS(now-25*60*60*1E9, 0)                         // GC will occur at time=25 hours
	ts3 := makeTS(now-intentAgeThreshold.Nanoseconds(), 0)     // 2h old
	ts4 := makeTS(now-(intentAgeThreshold.Nanoseconds()-1), 0) // 2h-1ns old
	ts5 := makeTS(now-1E9, 0)                                  // 1s old
	key1 := roachpb.Key("a")
	key2 := roachpb.Key("b")
	key3 := roachpb.Key("c")
	key4 := roachpb.Key("d")
	key5 := roachpb.Key("e")
	key6 := roachpb.Key("f")
	key7 := roachpb.Key("g")
	key8 := roachpb.Key("h")
	key9 := roachpb.Key("i")

	data := []struct {
		key roachpb.Key
		ts  roachpb.Timestamp
		del bool
		txn bool
	}{
		// For key1, we expect first two values to GC.
		{key1, ts1, false, false},
		{key1, ts2, false, false},
		{key1, ts5, false, false},
		// For key2, we expect all values to GC, because most recent is deletion.
		{key2, ts1, false, false},
		{key2, ts2, false, false},
		{key2, ts5, true, false},
		// For key3, we expect just ts1 to GC, because most recent deletion is intent.
		{key3, ts1, false, false},
		{key3, ts2, false, false},
		{key3, ts5, true, true},
		// For key4, expect oldest value to GC.
		{key4, ts1, false, false},
		{key4, ts2, false, false},
		// For key5, expect all values to GC (most recent value deleted).
		{key5, ts1, false, false},
		{key5, ts2, true, false},
		// For key6, expect no values to GC because most recent value is intent.
		{key6, ts1, false, false},
		{key6, ts5, false, true},
		// For key7, expect no values to GC because intent is exactly 2h old.
		{key7, ts2, false, false},
		{key7, ts4, false, true},
		// For key8, expect most recent value to resolve by aborting, which will clean it up.
		{key8, ts2, false, false},
		{key8, ts3, true, true},
		// For key9, resolve naked intent with no remaining values.
		{key9, ts3, true, false},
	}

	for i, datum := range data {
		if datum.del {
			dArgs := deleteArgs(datum.key)
			var txn *roachpb.Transaction
			if datum.txn {
				txn = newTransaction("test", datum.key, 1, roachpb.SERIALIZABLE, tc.clock)
				txn.OrigTimestamp = datum.ts
				txn.Timestamp = datum.ts
			}
			if _, err := client.SendWrappedWith(tc.Sender(), tc.rng.context(), roachpb.BatchRequest_Header{
				Timestamp: datum.ts,
				Txn:       txn,
			}, &dArgs); err != nil {
				t.Fatalf("%d: could not delete data: %s", i, err)
			}
		} else {
			pArgs := putArgs(datum.key, []byte("value"))
			var txn *roachpb.Transaction
			if datum.txn {
				txn = newTransaction("test", datum.key, 1, roachpb.SERIALIZABLE, tc.clock)
				txn.OrigTimestamp = datum.ts
				txn.Timestamp = datum.ts
			}
			if _, err := client.SendWrappedWith(tc.Sender(), tc.rng.context(), roachpb.BatchRequest_Header{
				Timestamp: datum.ts,
				Txn:       txn,
			}, &pArgs); err != nil {
				t.Fatalf("%d: could not put data: %s", i, err)
			}
		}
	}

	cfg := tc.gossip.GetSystemConfig()
	if cfg == nil {
		t.Fatal("nil config")
	}

//.........这里部分代码省略.........

// processTransactionTable scans the transaction table and updates txnMap with
// those transactions which are old and either PENDING or with intents
// registered. In the first case we want to push the transaction so that it is
// aborted, and in the second case we may have to resolve the intents success-
// fully before GCing the entry. The transaction records which can be gc'ed are
// returned separately and are not added to txnMap nor intentSpanMap.
func processTransactionTable(
	ctx context.Context,
	snap engine.Engine,
	desc *roachpb.RangeDescriptor,
	txnMap map[uuid.UUID]*roachpb.Transaction,
	cutoff roachpb.Timestamp,
	infoMu *lockableGCInfo,
	resolveIntents resolveFunc,
) ([]roachpb.GCRequest_GCKey, error) {
	infoMu.Lock()
	defer infoMu.Unlock()

	var gcKeys []roachpb.GCRequest_GCKey
	handleOne := func(kv roachpb.KeyValue) error {
		var txn roachpb.Transaction
		if err := kv.Value.GetProto(&txn); err != nil {
			return err
		}
		infoMu.TransactionSpanTotal++
		if !txn.LastActive().Less(cutoff) {
			return nil
		}

		txnID := *txn.ID

		// The transaction record should be considered for removal.
		switch txn.Status {
		case roachpb.PENDING:
			// Marked as running, so we need to push it to abort it but won't
			// try to GC it in this cycle (for convenience).
			// TODO(tschottdorf): refactor so that we can GC PENDING entries
			// in the same cycle, but keeping the calls to pushTxn in a central
			// location (keeping it easy to batch them up in the future).
			infoMu.TransactionSpanGCPending++
			txnMap[txnID] = &txn
			return nil
		case roachpb.ABORTED:
			// If we remove this transaction, it effectively still counts as
			// ABORTED (by design). So this can be GC'ed even if we can't
			// resolve the intents.
			// Note: Most aborted transaction weren't aborted by their client,
			// but instead by the coordinator - those will not have any intents
			// persisted, though they still might exist in the system.
			infoMu.TransactionSpanGCAborted++
			func() {
				infoMu.Unlock() // intentional
				defer infoMu.Lock()
				if err := resolveIntents(roachpb.AsIntents(txn.Intents, &txn),
					true /* wait */, false /* !poison */); err != nil {
					log.Warningf("failed to resolve intents of aborted txn on gc: %s", err)
				}
			}()
		case roachpb.COMMITTED:
			// It's committed, so it doesn't need a push but we can only
			// GC it after its intents are resolved.
			if err := func() error {
				infoMu.Unlock() // intentional
				defer infoMu.Lock()
				return resolveIntents(roachpb.AsIntents(txn.Intents, &txn), true /* wait */, false /* !poison */)
			}(); err != nil {
				log.Warningf("unable to resolve intents of committed txn on gc: %s", err)
				// Returning the error here would abort the whole GC run, and
				// we don't want that. Instead, we simply don't GC this entry.
				return nil
			}
			infoMu.TransactionSpanGCCommitted++
		default:
			panic(fmt.Sprintf("invalid transaction state: %s", txn))
		}
		gcKeys = append(gcKeys, roachpb.GCRequest_GCKey{Key: kv.Key}) // zero timestamp
		return nil
	}

	startKey := keys.TransactionKey(desc.StartKey.AsRawKey(), uuid.EmptyUUID)
	endKey := keys.TransactionKey(desc.EndKey.AsRawKey(), uuid.EmptyUUID)

	_, err := engine.MVCCIterate(ctx, snap, startKey, endKey,
		roachpb.ZeroTimestamp, true /* consistent */, nil, /* txn */
		false /* !reverse */, func(kv roachpb.KeyValue) (bool, error) {
			return false, handleOne(kv)
		})
	return gcKeys, err
}