func testRangeCacheHandleDoubleSplit(t *testing.T, useReverseScan bool) {
	db := initTestDescriptorDB(t)
	db.disablePrefetch = true

	// A request initially looks up the range descriptor ["a"-"b").
	doLookup(t, db.cache, "aa")
	db.assertLookupCountEq(t, 2, "aa")

	// A split breaks up the range into ["a"-"an"), ["an"-"at"), ["at"-"b").
	db.splitRange(t, roachpb.RKey("an"))
	db.splitRange(t, roachpb.RKey("at"))

	// A request is sent to the stale descriptor on the right half
	// such that a RangeKeyMismatchError is returned.
	_, evictToken := doLookup(t, db.cache, "az")
	// mismatchErrRange mocks out a RangeKeyMismatchError.Range response.
	ranges, _, pErr := db.getDescriptors(roachpb.RKey("aa"), false, false)
	if pErr != nil {
		t.Fatal(pErr)
	}
	mismatchErrRange := ranges[0]
	// The stale descriptor is evicted, the new descriptor from the error is
	// replaced, and a new lookup is initialized.
	if err := evictToken.EvictAndReplace(context.Background(), mismatchErrRange); err != nil {
		t.Fatal(err)
	}

	// Requests to all parts of the split are sent:

	// [reverse case]
	// - "aa" and "an" will hit the cache
	// - all others will join a coalesced request to "az"
	//   + will lookup the meta2 desc
	//   + will lookup the ["at"-"b") desc
	// - "az" will get the right range back
	// - "at" will make a second lookup
	//   + will lookup the ["an"-"at") desc
	//
	// [non-reverse case]
	// - "aa" will hit the cache
	// - all others will join a coalesced request to "an"
	//   + will lookup the meta2 desc
	//   + will lookup the ["an"-"at") desc
	// - "an" and "ao" will get the right range back
	// - "at" and "az" will make a second lookup
	//   + will lookup the ["at"-"b") desc
	var wg, waitJoin sync.WaitGroup
	db.pauseRangeLookups()
	numRetries := int64(0)
	for _, k := range []string{"aa", "an", "ao", "at", "az"} {
		wg.Add(1)
		waitJoin.Add(1)
		go func(key roachpb.RKey) {
			reqEvictToken := evictToken
			waitJoinCopied := &waitJoin
			var desc *roachpb.RangeDescriptor
			for {
				// Each request goes to a different key.
				var err error
				if desc, reqEvictToken, err = db.cache.lookupRangeDescriptorInternal(
					context.Background(), key, reqEvictToken, false, /* considerIntents */
					useReverseScan, waitJoinCopied); err != nil {
					waitJoinCopied = nil
					atomic.AddInt64(&numRetries, 1)
					continue
				}
				break
			}
			if useReverseScan {
				if !desc.ContainsExclusiveEndKey(key) {
					t.Errorf("desc %s does not contain exclusive end key %s", desc, key)
				}
			} else {
				if !desc.ContainsKey(key) {
					t.Errorf("desc %s does not contain key %s", desc, key)
				}
			}

			wg.Done()
		}(roachpb.RKey(k))
	}
	// Wait until all lookup requests hit the cache or join into a coalesced request.
	waitJoin.Wait()
	db.resumeRangeLookups()

	wg.Wait()
	db.assertLookupCountEq(t, 3, "an and az")
	if numRetries == 0 {
		t.Error("expected retry on desc lookup")
	}

	// All three descriptors are now correctly cached.
	doLookup(t, db.cache, "aa")
	db.assertLookupCountEq(t, 0, "aa")
	doLookup(t, db.cache, "ao")
	db.assertLookupCountEq(t, 0, "ao")
	doLookup(t, db.cache, "az")
	db.assertLookupCountEq(t, 0, "az")
}
Exemple #2
0
// sendChunk is in charge of sending an "admissible" piece of batch, i.e. one
// which doesn't need to be subdivided further before going to a range (so no
// mixing of forward and reverse scans, etc). The parameters and return values
// correspond to client.Sender with the exception of the returned boolean,
// which is true when indicating that the caller should retry but needs to send
// EndTransaction in a separate request.
func (ds *DistSender) sendChunk(ctx context.Context, ba roachpb.BatchRequest) (*roachpb.BatchResponse, *roachpb.Error, bool) {
	isReverse := ba.IsReverse()

	// TODO(radu): when contexts are properly plumbed, we should be able to get
	// the tracer from ctx, not from the DistSender.
	ctx, cleanup := tracing.EnsureContext(ctx, tracing.TracerFromCtx(ds.Ctx))
	defer cleanup()

	// The minimal key range encompassing all requests contained within.
	// Local addressing has already been resolved.
	// TODO(tschottdorf): consider rudimentary validation of the batch here
	// (for example, non-range requests with EndKey, or empty key ranges).
	rs, err := keys.Range(ba)
	if err != nil {
		return nil, roachpb.NewError(err), false
	}
	var br *roachpb.BatchResponse

	// Send the request to one range per iteration.
	for {
		// Increase the sequence counter only once before sending RPCs to
		// the ranges involved in this chunk of the batch (as opposed to for
		// each RPC individually). On RPC errors, there's no guarantee that
		// the request hasn't made its way to the target regardless of the
		// error; we'd like the second execution to be caught by the sequence
		// cache if that happens. There is a small chance that that we address
		// a range twice in this chunk (stale/suboptimal descriptors due to
		// splits/merges) which leads to a transaction retry.
		// TODO(tschottdorf): it's possible that if we don't evict from the
		//   cache we could be in for a busy loop.
		ba.SetNewRequest()

		var curReply *roachpb.BatchResponse
		var desc *roachpb.RangeDescriptor
		var evictToken *evictionToken
		var needAnother bool
		var pErr *roachpb.Error
		var finished bool
		var numAttempts int
		for r := retry.StartWithCtx(ctx, ds.rpcRetryOptions); r.Next(); {
			numAttempts++
			{
				const magicLogCurAttempt = 20

				var seq int32
				if ba.Txn != nil {
					seq = ba.Txn.Sequence
				}

				if numAttempts%magicLogCurAttempt == 0 || seq%magicLogCurAttempt == 0 {
					// Log a message if a request appears to get stuck for a long
					// time or, potentially, forever. See #8975.
					// The local counter captures this loop here; the Sequence number
					// should capture anything higher up (as it needs to be
					// incremented every time this method is called).
					log.Warningf(
						ctx,
						"%d retries for an RPC at sequence %d, last error was: %s, remaining key ranges %s: %s",
						numAttempts, seq, pErr, rs, ba,
					)
				}
			}
			// Get range descriptor (or, when spanning range, descriptors). Our
			// error handling below may clear them on certain errors, so we
			// refresh (likely from the cache) on every retry.
			log.Trace(ctx, "meta descriptor lookup")
			var err error
			desc, needAnother, evictToken, err = ds.getDescriptors(ctx, rs, evictToken, isReverse)

			// getDescriptors may fail retryably if, for example, the first
			// range isn't available via Gossip. Assume that all errors at
			// this level are retryable. Non-retryable errors would be for
			// things like malformed requests which we should have checked
			// for before reaching this point.
			if err != nil {
				log.Trace(ctx, "range descriptor lookup failed: "+err.Error())
				if log.V(1) {
					log.Warning(ctx, err)
				}
				pErr = roachpb.NewError(err)
				continue
			}

			if needAnother && br == nil {
				// TODO(tschottdorf): we should have a mechanism for discovering
				// range merges (descriptor staleness will mostly go unnoticed),
				// or we'll be turning single-range queries into multi-range
				// queries for no good reason.

				// If there's no transaction and op spans ranges, possibly
				// re-run as part of a transaction for consistency. The
				// case where we don't need to re-run is if the read
				// consistency is not required.
				if ba.Txn == nil && ba.IsPossibleTransaction() &&
					ba.ReadConsistency != roachpb.INCONSISTENT {
					return nil, roachpb.NewError(&roachpb.OpRequiresTxnError{}), false
				}
				// If the request is more than but ends with EndTransaction, we
				// want the caller to come again with the EndTransaction in an
				// extra call.
				if l := len(ba.Requests) - 1; l > 0 && ba.Requests[l].GetInner().Method() == roachpb.EndTransaction {
					return nil, roachpb.NewError(errors.New("cannot send 1PC txn to multiple ranges")), true /* shouldSplitET */
				}
			}

			// It's possible that the returned descriptor misses parts of the
			// keys it's supposed to scan after it's truncated to match the
			// descriptor. Example revscan [a,g), first desc lookup for "g"
			// returns descriptor [c,d) -> [d,g) is never scanned.
			// We evict and retry in such a case.
			includesFrontOfCurSpan := func(rd *roachpb.RangeDescriptor) bool {
				if isReverse {
					return desc.ContainsExclusiveEndKey(rs.EndKey)
				}
				return desc.ContainsKey(rs.Key)
			}
			if !includesFrontOfCurSpan(desc) {
				if err := evictToken.Evict(ctx); err != nil {
					return nil, roachpb.NewError(err), false
				}
				// On addressing errors, don't backoff; retry immediately.
				r.Reset()
				continue
			}

			curReply, pErr = func() (*roachpb.BatchResponse, *roachpb.Error) {
				// Truncate the request to our current key range.
				intersected, iErr := rs.Intersect(desc)
				if iErr != nil {
					return nil, roachpb.NewError(iErr)
				}
				truncBA, numActive, trErr := truncate(ba, intersected)
				if numActive == 0 && trErr == nil {
					// This shouldn't happen in the wild, but some tests
					// exercise it.
					return nil, roachpb.NewErrorf("truncation resulted in empty batch on [%s,%s): %s",
						rs.Key, rs.EndKey, ba)
				}
				if trErr != nil {
					return nil, roachpb.NewError(trErr)
				}
				return ds.sendSingleRange(ctx, truncBA, desc)
			}()
			// If sending succeeded, break this loop.
			if pErr == nil {
				finished = true
				break
			}

			log.VTracef(1, ctx, "reply error %s: %s", ba, pErr)

			// Error handling: If the error indicates that our range
			// descriptor is out of date, evict it from the cache and try
			// again. Errors that apply only to a single replica were
			// handled in send().
			//
			// TODO(bdarnell): Don't retry endlessly. If we fail twice in a
			// row and the range descriptor hasn't changed, return the error
			// to our caller.
			switch tErr := pErr.GetDetail().(type) {
			case *roachpb.SendError:
				// We've tried all the replicas without success. Either
				// they're all down, or we're using an out-of-date range
				// descriptor. Invalidate the cache and try again with the new
				// metadata.
				if err := evictToken.Evict(ctx); err != nil {
					return nil, roachpb.NewError(err), false
				}
				continue
			case *roachpb.RangeKeyMismatchError:
				// Range descriptor might be out of date - evict it. This is
				// likely the result of a range split. If we have new range
				// descriptors, insert them instead as long as they are different
				// from the last descriptor to avoid endless loops.
				var replacements []roachpb.RangeDescriptor
				different := func(rd *roachpb.RangeDescriptor) bool {
					return !desc.RSpan().Equal(rd.RSpan())
				}
				if tErr.MismatchedRange != nil && different(tErr.MismatchedRange) {
					replacements = append(replacements, *tErr.MismatchedRange)
				}
				if tErr.SuggestedRange != nil && different(tErr.SuggestedRange) {
					if includesFrontOfCurSpan(tErr.SuggestedRange) {
						replacements = append(replacements, *tErr.SuggestedRange)

					}
				}
				// Same as Evict() if replacements is empty.
				if err := evictToken.EvictAndReplace(ctx, replacements...); err != nil {
					return nil, roachpb.NewError(err), false
				}
				// On addressing errors, don't backoff; retry immediately.
				r.Reset()
				if log.V(1) {
					log.Warning(ctx, tErr)
				}
				continue
			}
			break
		}

		// Immediately return if querying a range failed non-retryably.
		if pErr != nil {
			return nil, pErr, false
		} else if !finished {
			select {
			case <-ds.rpcRetryOptions.Closer:
				return nil, roachpb.NewError(&roachpb.NodeUnavailableError{}), false
			case <-ctx.Done():
				return nil, roachpb.NewError(ctx.Err()), false
			default:
				log.Fatal(ctx, "exited retry loop with nil error but finished=false")
			}
		}

		ba.UpdateTxn(curReply.Txn)

		if br == nil {
			// First response from a Range.
			br = curReply
		} else {
			// This was the second or later call in a cross-Range request.
			// Combine the new response with the existing one.
			if err := br.Combine(curReply); err != nil {
				return nil, roachpb.NewError(err), false
			}
		}

		if isReverse {
			// In next iteration, query previous range.
			// We use the StartKey of the current descriptor as opposed to the
			// EndKey of the previous one since that doesn't have bugs when
			// stale descriptors come into play.
			rs.EndKey, err = prev(ba, desc.StartKey)
		} else {
			// In next iteration, query next range.
			// It's important that we use the EndKey of the current descriptor
			// as opposed to the StartKey of the next one: if the former is stale,
			// it's possible that the next range has since merged the subsequent
			// one, and unless both descriptors are stale, the next descriptor's
			// StartKey would move us to the beginning of the current range,
			// resulting in a duplicate scan.
			rs.Key, err = next(ba, desc.EndKey)
		}
		if err != nil {
			return nil, roachpb.NewError(err), false
		}

		if ba.MaxSpanRequestKeys > 0 {
			// Count how many results we received.
			var numResults int64
			for _, resp := range curReply.Responses {
				numResults += resp.GetInner().Header().NumKeys
			}
			if numResults > ba.MaxSpanRequestKeys {
				panic(fmt.Sprintf("received %d results, limit was %d", numResults, ba.MaxSpanRequestKeys))
			}
			ba.MaxSpanRequestKeys -= numResults
			if ba.MaxSpanRequestKeys == 0 {
				// prepare the batch response after meeting the max key limit.
				fillSkippedResponses(ba, br, rs)
				// done, exit loop.
				return br, nil, false
			}
		}

		// If this was the last range accessed by this call, exit loop.
		if !needAnother {
			return br, nil, false
		}

		// key cannot be less that the end key.
		if !rs.Key.Less(rs.EndKey) {
			panic(fmt.Sprintf("start key %s is less than %s", rs.Key, rs.EndKey))
		}

		log.Trace(ctx, "querying next range")
	}
}
Exemple #3
0
// sendChunk is in charge of sending an "admissible" piece of batch, i.e. one
// which doesn't need to be subdivided further before going to a range (so no
// mixing of forward and reverse scans, etc). The parameters and return values
// correspond to client.Sender with the exception of the returned boolean,
// which is true when indicating that the caller should retry but needs to send
// EndTransaction in a separate request.
func (ds *DistSender) sendChunk(ctx context.Context, ba roachpb.BatchRequest) (*roachpb.BatchResponse, *roachpb.Error, bool) {
	isReverse := ba.IsReverse()

	ctx, cleanup := tracing.EnsureContext(ctx, ds.Tracer)
	defer cleanup()

	// The minimal key range encompassing all requests contained within.
	// Local addressing has already been resolved.
	// TODO(tschottdorf): consider rudimentary validation of the batch here
	// (for example, non-range requests with EndKey, or empty key ranges).
	rs, err := keys.Range(ba)
	if err != nil {
		return nil, roachpb.NewError(err), false
	}
	var br *roachpb.BatchResponse

	// Send the request to one range per iteration.
	for {
		// Increase the sequence counter only once before sending RPCs to
		// the ranges involved in this chunk of the batch (as opposed to for
		// each RPC individually). On RPC errors, there's no guarantee that
		// the request hasn't made its way to the target regardless of the
		// error; we'd like the second execution to be caught by the sequence
		// cache if that happens. There is a small chance that that we address
		// a range twice in this chunk (stale/suboptimal descriptors due to
		// splits/merges) which leads to a transaction retry.
		// TODO(tschottdorf): it's possible that if we don't evict from the
		//   cache we could be in for a busy loop.
		ba.SetNewRequest()

		var curReply *roachpb.BatchResponse
		var desc *roachpb.RangeDescriptor
		var evictToken *evictionToken
		var needAnother bool
		var pErr *roachpb.Error
		var finished bool
		for r := retry.Start(ds.rpcRetryOptions); r.Next(); {
			// Get range descriptor (or, when spanning range, descriptors). Our
			// error handling below may clear them on certain errors, so we
			// refresh (likely from the cache) on every retry.
			log.Trace(ctx, "meta descriptor lookup")
			var err error
			desc, needAnother, evictToken, err = ds.getDescriptors(ctx, rs, evictToken, isReverse)

			// getDescriptors may fail retryably if, for example, the first
			// range isn't available via Gossip. Assume that all errors at
			// this level are retryable. Non-retryable errors would be for
			// things like malformed requests which we should have checked
			// for before reaching this point.
			if err != nil {
				log.Trace(ctx, "range descriptor lookup failed: "+err.Error())
				if log.V(1) {
					log.Warning(err)
				}
				pErr = roachpb.NewError(err)
				continue
			}

			if needAnother && br == nil {
				// TODO(tschottdorf): we should have a mechanism for discovering
				// range merges (descriptor staleness will mostly go unnoticed),
				// or we'll be turning single-range queries into multi-range
				// queries for no good reason.

				// If there's no transaction and op spans ranges, possibly
				// re-run as part of a transaction for consistency. The
				// case where we don't need to re-run is if the read
				// consistency is not required.
				if ba.Txn == nil && ba.IsPossibleTransaction() &&
					ba.ReadConsistency != roachpb.INCONSISTENT {
					return nil, roachpb.NewError(&roachpb.OpRequiresTxnError{}), false
				}
				// If the request is more than but ends with EndTransaction, we
				// want the caller to come again with the EndTransaction in an
				// extra call.
				if l := len(ba.Requests) - 1; l > 0 && ba.Requests[l].GetInner().Method() == roachpb.EndTransaction {
					return nil, roachpb.NewError(errors.New("cannot send 1PC txn to multiple ranges")), true /* shouldSplitET */
				}
			}

			// It's possible that the returned descriptor misses parts of the
			// keys it's supposed to scan after it's truncated to match the
			// descriptor. Example revscan [a,g), first desc lookup for "g"
			// returns descriptor [c,d) -> [d,g) is never scanned.
			// We evict and retry in such a case.
			includesFrontOfCurSpan := func(rd *roachpb.RangeDescriptor) bool {
				if isReverse {
					return desc.ContainsExclusiveEndKey(rs.EndKey)
				}
				return desc.ContainsKey(rs.Key)
			}
			if !includesFrontOfCurSpan(desc) {
				if err := evictToken.Evict(ctx); err != nil {
					return nil, roachpb.NewError(err), false
				}
				// On addressing errors, don't backoff; retry immediately.
				r.Reset()
				continue
			}

			curReply, pErr = func() (*roachpb.BatchResponse, *roachpb.Error) {
				// Truncate the request to our current key range.
				intersected, iErr := rs.Intersect(desc)
				if iErr != nil {
					return nil, roachpb.NewError(iErr)
				}
				truncBA, numActive, trErr := truncate(ba, intersected)
				if numActive == 0 && trErr == nil {
					// This shouldn't happen in the wild, but some tests
					// exercise it.
					return nil, roachpb.NewErrorf("truncation resulted in empty batch on [%s,%s): %s",
						rs.Key, rs.EndKey, ba)
				}
				if trErr != nil {
					return nil, roachpb.NewError(trErr)
				}
				return ds.sendSingleRange(ctx, truncBA, desc)
			}()
			// If sending succeeded, break this loop.
			if pErr == nil {
				finished = true
				break
			}

			if log.V(1) {
				log.Warningf("failed to invoke %s: %s", ba, pErr)
			}
			log.Trace(ctx, fmt.Sprintf("reply error: %s", pErr))

			// Error handling: If the error indicates that our range
			// descriptor is out of date, evict it from the cache and try
			// again. Errors that apply only to a single replica were
			// handled in send().
			//
			// TODO(bdarnell): Don't retry endlessly. If we fail twice in a
			// row and the range descriptor hasn't changed, return the error
			// to our caller.
			switch tErr := pErr.GetDetail().(type) {
			case *roachpb.SendError:
				// We've tried all the replicas without success. Either
				// they're all down, or we're using an out-of-date range
				// descriptor. Invalidate the cache and try again with the new
				// metadata.
				if err := evictToken.Evict(ctx); err != nil {
					return nil, roachpb.NewError(err), false
				}
				continue
			case *roachpb.RangeKeyMismatchError:
				// Range descriptor might be out of date - evict it. This is
				// likely the result of a range split. If we have new range
				// descriptors, insert them instead as long as they are different
				// from the last descriptor to avoid endless loops.
				var replacements []roachpb.RangeDescriptor
				different := func(rd *roachpb.RangeDescriptor) bool {
					return !desc.RSpan().Equal(rd.RSpan())
				}
				if tErr.MismatchedRange != nil && different(tErr.MismatchedRange) {
					replacements = append(replacements, *tErr.MismatchedRange)
				}
				if tErr.SuggestedRange != nil && different(tErr.SuggestedRange) {
					if includesFrontOfCurSpan(tErr.SuggestedRange) {
						replacements = append(replacements, *tErr.SuggestedRange)

					}
				}
				// Same as Evict() if replacements is empty.
				if err := evictToken.EvictAndReplace(ctx, replacements...); err != nil {
					return nil, roachpb.NewError(err), false
				}
				// On addressing errors, don't backoff; retry immediately.
				r.Reset()
				if log.V(1) {
					log.Warning(tErr)
				}
				continue
			}
			break
		}

		// Immediately return if querying a range failed non-retryably.
		if pErr != nil {
			return nil, pErr, false
		} else if !finished {
			select {
			case <-ds.rpcRetryOptions.Closer:
				return nil, roachpb.NewError(&roachpb.NodeUnavailableError{}), false
			default:
				log.Fatal("exited retry loop with nil error but finished=false")
			}
		}

		ba.Txn.Update(curReply.Txn)

		if br == nil {
			// First response from a Range.
			br = curReply
		} else {
			// This was the second or later call in a cross-Range request.
			// Combine the new response with the existing one.
			if err := br.Combine(curReply); err != nil {
				return nil, roachpb.NewError(err), false
			}
		}

		if ba.MaxScanResults > 0 {
			// Count how many results we received.
			var numResults int64
			for _, resp := range curReply.Responses {
				if cResp, ok := resp.GetInner().(roachpb.Countable); ok {
					numResults += cResp.Count()
				}
			}
			if numResults > ba.MaxScanResults {
				panic(fmt.Sprintf("received %d results, limit was %d", numResults, ba.MaxScanResults))
			}
			ba.MaxScanResults -= numResults
			if ba.MaxScanResults == 0 {
				// We are done with this batch. Some requests might have NoopResponses; we must
				// replace them with empty responses of the proper type.
				for i, req := range ba.Requests {
					if _, ok := br.Responses[i].GetInner().(*roachpb.NoopResponse); !ok {
						continue
					}
					union := roachpb.ResponseUnion{}
					var reply roachpb.Response
					if _, ok := req.GetInner().(*roachpb.ScanRequest); ok {
						reply = &roachpb.ScanResponse{}
					} else {
						_ = req.GetInner().(*roachpb.ReverseScanRequest)
						reply = &roachpb.ReverseScanResponse{}
					}
					union.MustSetInner(reply)
					br.Responses[i] = union
				}
				return br, nil, false
			}
		}

		// If this request has a bound (such as MaxResults in
		// ScanRequest) and we are going to query at least one more range,
		// check whether enough rows have been retrieved.
		// TODO(tschottdorf): need tests for executing a multi-range batch
		// with various bounded requests which saturate at different times.
		if needAnother {
			// Start with the assumption that all requests are saturated.
			// Below, we look at each and decide whether that's true.
			// Everything that is indeed saturated is "masked out" from the
			// batch request; only if that's all requests does needAnother
			// remain false.
			needAnother = false
			if br == nil {
				// Clone ba.Requests. This is because we're multi-range, and
				// some requests may be bounded, which could lead to them being
				// masked out once they're saturated. We don't want to risk
				// removing requests that way in the "master copy" since that
				// could lead to omitting requests in certain retry scenarios.
				ba.Requests = append([]roachpb.RequestUnion(nil), ba.Requests...)
			}
			for i, union := range ba.Requests {
				args := union.GetInner()
				if _, ok := args.(*roachpb.NoopRequest); ok {
					// NoopRequests are skipped.
					continue
				}
				boundedArg, ok := args.(roachpb.Bounded)
				if !ok {
					// Non-bounded request. We will have to query all ranges.
					needAnother = true
					continue
				}
				prevBound := boundedArg.GetBound()
				cReply, ok := curReply.Responses[i].GetInner().(roachpb.Countable)
				if !ok || prevBound <= 0 {
					// Request bounded, but without max results. Again, will
					// need to query everything we can. The case in which the reply
					// isn't countable occurs when the request wasn't active for
					// that range (since it didn't apply to it), so the response
					// is a NoopResponse.
					needAnother = true
					continue
				}
				nextBound := prevBound - cReply.Count()
				if nextBound <= 0 {
					// We've hit max results for this piece of the batch. Mask
					// it out (we've copied the requests slice above, so this
					// is kosher).
					union := &ba.Requests[i] // avoid working on copy
					union.MustSetInner(&noopRequest)
					continue
				}
				// The request isn't saturated yet.
				needAnother = true
				boundedArg.SetBound(nextBound)
			}
		}

		// If this was the last range accessed by this call, exit loop.
		if !needAnother {
			return br, nil, false
		}

		if isReverse {
			// In next iteration, query previous range.
			// We use the StartKey of the current descriptor as opposed to the
			// EndKey of the previous one since that doesn't have bugs when
			// stale descriptors come into play.
			rs.EndKey, err = prev(ba, desc.StartKey)
		} else {
			// In next iteration, query next range.
			// It's important that we use the EndKey of the current descriptor
			// as opposed to the StartKey of the next one: if the former is stale,
			// it's possible that the next range has since merged the subsequent
			// one, and unless both descriptors are stale, the next descriptor's
			// StartKey would move us to the beginning of the current range,
			// resulting in a duplicate scan.
			rs.Key, err = next(ba, desc.EndKey)
		}
		if err != nil {
			return nil, roachpb.NewError(err), false
		}
		log.Trace(ctx, "querying next range")
	}
}