// InternalRangeLookup is used to look up RangeDescriptors - a RangeDescriptor
// is a metadata structure which describes the key range and replica locations
// of a distinct range in the cluster.
//
// RangeDescriptors are stored as values in the cockroach cluster's key-value
// store. However, they are always stored using special "Range Metadata keys",
// which are "ordinary" keys with a special prefix prepended. The Range Metadata
// Key for an ordinary key can be generated with the `keys.RangeMetaKey(key)`
// function. The RangeDescriptor for the range which contains a given key can be
// retrieved by generating its Range Metadata Key and dispatching it to
// InternalRangeLookup.
//
// Note that the Range Metadata Key sent to InternalRangeLookup is NOT the key
// at which the desired RangeDescriptor is stored. Instead, this method returns
// the RangeDescriptor stored at the _lowest_ existing key which is _greater_
// than the given key. The returned RangeDescriptor will thus contain the
// ordinary key which was originally used to generate the Range Metadata Key
// sent to InternalRangeLookup.
//
// The "Range Metadata Key" for a range is built by appending the end key of
// the range to the meta[12] prefix because the RocksDB iterator only supports
// a Seek() interface which acts as a Ceil(). Using the start key of the range
// would cause Seek() to find the key after the meta indexing record we're
// looking for, which would result in having to back the iterator up, an option
// which is both less efficient and not available in all cases.
//
// This method has an important optimization: instead of just returning the
// request RangeDescriptor, it also returns a slice of additional range
// descriptors immediately consecutive to the desired RangeDescriptor. This is
// intended to serve as a sort of caching pre-fetch, so that the requesting
// nodes can aggressively cache RangeDescriptors which are likely to be desired
// by their current workload.
func (r *Range) InternalRangeLookup(batch engine.Engine, args *proto.InternalRangeLookupRequest, reply *proto.InternalRangeLookupResponse) []proto.Intent {
	if err := keys.ValidateRangeMetaKey(args.Key); err != nil {
		reply.SetGoError(err)
		return nil
	}

	rangeCount := int64(args.MaxRanges)
	if rangeCount < 1 {
		reply.SetGoError(util.Errorf(
			"Range lookup specified invalid maximum range count %d: must be > 0", rangeCount))
		return nil
	}
	if args.IgnoreIntents {
		rangeCount = 1 // simplify lookup because we may have to retry to read new
	}

	// We want to search for the metadata key just greater than args.Key. Scan
	// for both the requested key and the keys immediately afterwards, up to
	// MaxRanges.
	startKey, endKey := keys.MetaScanBounds(args.Key)
	// Scan inconsistently. Any intents encountered are bundled up, but other-
	// wise ignored.
	kvs, intents, err := engine.MVCCScan(batch, startKey, endKey, rangeCount,
		args.Timestamp, false /* !consistent */, args.Txn)
	if err != nil {
		// An error here would likely amount to something seriously going
		// wrong.
		reply.SetGoError(err)
		return nil
	}
	if args.IgnoreIntents && len(intents) > 0 {
		// NOTE (subtle): in general, we want to try to clean up dangling
		// intents on meta records. However, if we're in the process of
		// cleaning up a dangling intent on a meta record by pushing the
		// transaction, we don't want to create an infinite loop:
		//
		// intent! -> push-txn -> range-lookup -> intent! -> etc...
		//
		// Instead we want:
		//
		// intent! -> push-txn -> range-lookup -> ignore intent, return old/new ranges
		//
		// On the range-lookup from a push transaction, we therefore
		// want to suppress WriteIntentErrors and return a value
		// anyway. But which value? We don't know whether the range
		// update succeeded or failed, but if we don't return the
		// correct range descriptor we may not be able to find the
		// transaction to push. Since we cannot know the correct answer,
		// we choose randomly between the pre- and post- transaction
		// values. If we guess wrong, the client will try again and get
		// the other value (within a few tries).
		if rand.Intn(2) == 0 {
			key, txn := intents[0].Key, &intents[0].Txn
			val, _, err := engine.MVCCGet(batch, key, txn.Timestamp, true, txn)
			if err != nil {
				reply.SetGoError(err)
				return nil
			}
			kvs = []proto.KeyValue{{Key: key, Value: *val}}
		}
	}

	if len(kvs) == 0 {
		// No matching results were returned from the scan. This could
		// indicate a very bad system error, but for now we will just
		// treat it as a retryable Key Mismatch error.
		err := proto.NewRangeKeyMismatchError(args.Key, args.EndKey, r.Desc())
		reply.SetGoError(err)
//.........这里部分代码省略.........

// InternalRangeLookup is used to look up RangeDescriptors - a RangeDescriptor
// is a metadata structure which describes the key range and replica locations
// of a distinct range in the cluster.
//
// RangeDescriptors are stored as values in the cockroach cluster's key-value
// store. However, they are always stored using special "Range Metadata keys",
// which are "ordinary" keys with a special prefix appended. The Range Metadata
// Key for an ordinary key can be generated with the `engine.RangeMetaKey(key)`
// function. The RangeDescriptor for the range which contains a given key can be
// retrieved by generating its Range Metadata Key and dispatching it to
// InternalRangeLookup.
//
// Note that the Range Metadata Key sent to InternalRangeLookup is NOT the key
// at which the desired RangeDescriptor is stored. Instead, this method returns
// the RangeDescriptor stored at the _lowest_ existing key which is _greater_
// than the given key. The returned RangeDescriptor will thus contain the
// ordinary key which was originally used to generate the Range Metadata Key
// sent to InternalRangeLookup.
//
// This method has an important optimization: instead of just returning the
// request RangeDescriptor, it also returns a slice of additional range
// descriptors immediately consecutive to the desired RangeDescriptor. This is
// intended to serve as a sort of caching pre-fetch, so that the requesting
// nodes can aggressively cache RangeDescriptors which are likely to be desired
// by their current workload.
func (r *Range) InternalRangeLookup(args *proto.InternalRangeLookupRequest, reply *proto.InternalRangeLookupResponse) {
	if err := engine.ValidateRangeMetaKey(args.Key); err != nil {
		reply.SetGoError(err)
		return
	}

	rangeCount := int64(args.MaxRanges)
	if rangeCount < 1 {
		reply.SetGoError(util.Errorf(
			"Range lookup specified invalid maximum range count %d: must be > 0", rangeCount))
		return
	}

	// We want to search for the metadata key just greater than args.Key.  Scan
	// for both the requested key and the keys immediately afterwards, up to
	// MaxRanges.
	metaPrefix := args.Key[:len(engine.KeyMeta1Prefix)]
	nextKey := engine.NextKey(args.Key)
	kvs, err := r.engine.Scan(nextKey, engine.PrefixEndKey(metaPrefix), rangeCount)
	if err != nil {
		reply.SetGoError(err)
		return
	}

	// The initial key must have the same metadata level prefix as we queried.
	if len(kvs) == 0 {
		// At this point the range has been verified to contain the requested
		// key, but no matching results were returned from the scan. This could
		// indicate a very bad system error, but for now we will just treat it
		// as a retryable Key Mismatch error.
		err := proto.NewRangeKeyMismatchError(args.Key, args.Key, r.Meta)
		reply.SetGoError(err)
		log.Errorf("InternalRangeLookup dispatched to correct range, but no matching RangeDescriptor was found. %s", err)
		return
	}

	// Decode all scanned range descriptors, stopping if a range is encountered
	// which does not have the same metadata prefix as the queried key.
	rds := make([]proto.RangeDescriptor, len(kvs))
	for i := range kvs {
		if err = gogoproto.Unmarshal(kvs[i].Value, &rds[i]); err != nil {
			reply.SetGoError(err)
			return
		}
	}

	reply.Ranges = rds
	return
}