add context cancellation checks on merging GetLabel slices (#5837)

* add context cancellation checks on merging GetLabel slices

Signed-off-by: Erlan Zholdubai uulu <erlanz@amazon.com>

* add context cancellation checks on merging GetLabel slices, update changelog

Signed-off-by: Erlan Zholdubai uulu <erlanz@amazon.com>

---------

Signed-off-by: Erlan Zholdubai uulu <erlanz@amazon.com>

Author: erlan-z

CHANGELOG.md

@@ -33,6 +33,7 @@
 * [ENHANCEMENT] Query: Added additional max query length check at Query Frontend and Ruler. Added `-querier.ignore-max-query-length` flag to disable max query length check at Querier. #5808
 * [ENHANCEMENT] Querier: Add context error check when converting Metrics to SeriesSet for GetSeries on distributorQuerier. #5827
 * [ENHANCEMENT] Ruler: Improve GetRules response time by refactoring mutexes and introducing a temporary rules cache in `ruler/manager.go`. #5805
+* [ENHANCEMENT] Querier: Add context error check when merging slices from ingesters for GetLabel operations. #5837
 * [BUGFIX] Distributor: Do not use label with empty values for sharding #5717
 * [BUGFIX] Query Frontend: queries with negative offset should check whether it is cacheable or not. #5719
 * [BUGFIX] Redis Cache: pass `cache_size` config correctly. #5734

pkg/distributor/distributor.go

@@ -973,7 +973,10 @@ func (d *Distributor) LabelValuesForLabelNameCommon(ctx context.Context, from, t
 	for i, resp := range resps {
 		values[i] = resp.([]string)
 	}
-	r := util.MergeSlicesParallel(mergeSlicesParallelism, values...)
+	r, err := util.MergeSlicesParallel(ctx, mergeSlicesParallelism, values...)
+	if err != nil {
+		return nil, err
+	}
 	span.SetTag("result_length", len(r))
 	return r, nil
 }
@@ -1043,7 +1046,10 @@ func (d *Distributor) LabelNamesCommon(ctx context.Context, from, to model.Time,
 	for i, resp := range resps {
 		values[i] = resp.([]string)
 	}
-	r := util.MergeSlicesParallel(mergeSlicesParallelism, values...)
+	r, err := util.MergeSlicesParallel(ctx, mergeSlicesParallelism, values...)
+	if err != nil {
+		return nil, err
+	}
 	span.SetTag("result_length", len(r))
 
 	return r, nil

pkg/util/strings.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"context"
 	"sync"
 	"unsafe"
 
@@ -37,17 +38,18 @@ func StringsClone(s string) string {
 
 // MergeSlicesParallel merge sorted slices in parallel
 // using the MergeSortedSlices function
-func MergeSlicesParallel(parallelism int, a ...[]string) []string {
+func MergeSlicesParallel(ctx context.Context, parallelism int, a ...[]string) ([]string, error) {
 	if parallelism <= 1 {
-		return MergeSortedSlices(a...)
+		return MergeSortedSlices(ctx, a...)
 	}
 	if len(a) == 0 {
-		return nil
+		return nil, nil
 	}
 	if len(a) == 1 {
-		return a[0]
+		return a[0], nil
 	}
 	c := make(chan []string, len(a))
+	errCh := make(chan error, 1)
 	wg := sync.WaitGroup{}
 	var r [][]string
 	p := min(parallelism, len(a)/2)
@@ -57,21 +59,31 @@ func MergeSlicesParallel(parallelism int, a ...[]string) []string {
 		wg.Add(1)
 		go func(i int) {
 			m := min(len(a), i+batchSize)
-			c <- MergeSortedSlices(a[i:m]...)
+			r, e := MergeSortedSlices(ctx, a[i:m]...)
+			if e != nil {
+				errCh <- e
+				wg.Done()
+				return
+			}
+			c <- r
 			wg.Done()
 		}(i)
 	}
 
 	go func() {
 		wg.Wait()
 		close(c)
+		close(errCh)
 	}()
 
+	if err := <-errCh; err != nil {
+		return nil, err
+	}
 	for s := range c {
 		r = append(r, s)
 	}
 
-	return MergeSortedSlices(r...)
+	return MergeSortedSlices(ctx, r...)
 }
 
 func NewStringListIter(s []string) *StringListIter {
@@ -98,9 +110,9 @@ var MAX_STRING = string([]byte{0xff})
 
 // MergeSortedSlices merges a set of sorted string slices into a single ones
 // while removing all duplicates.
-func MergeSortedSlices(a ...[]string) []string {
+func MergeSortedSlices(ctx context.Context, a ...[]string) ([]string, error) {
 	if len(a) == 1 {
-		return a[0]
+		return a[0], nil
 	}
 	its := make([]*StringListIter, 0, len(a))
 	sumLengh := 0
@@ -111,16 +123,19 @@ func MergeSortedSlices(a ...[]string) []string {
 	lt := loser.New(its, MAX_STRING)
 
 	if sumLengh == 0 {
-		return []string{}
+		return []string{}, nil
 	}
 
 	r := make([]string, 0, sumLengh*2/10)
 	var current string
 	for lt.Next() {
+		if ctx.Err() != nil {
+			return nil, ctx.Err()
+		}
 		if lt.At() != current {
 			current = lt.At()
 			r = append(r, current)
 		}
 	}
-	return r
+	return r, nil
 }

pkg/util/strings_test.go

@@ -1,6 +1,7 @@
 package util
 
 import (
+	"context"
 	"fmt"
 	"math/rand"
 	"sort"
@@ -96,12 +97,14 @@ func BenchmarkMergeSlicesParallel(b *testing.B) {
 				b.ReportAllocs()
 				b.ResetTimer()
 				var r []string
+				var err error
 				for i := 0; i < b.N; i++ {
 					if p == usingMap {
 						r = sortUsingMap(input...)
 						require.NotEmpty(b, r)
 					} else {
-						r = MergeSlicesParallel(int(p), input...)
+						r, err = MergeSlicesParallel(context.Background(), int(p), input...)
+						require.NoError(b, err)
 						require.NotEmpty(b, r)
 					}
 				}