From ca8cbefb24cc005bba5913993f6320f9580e83cf Mon Sep 17 00:00:00 2001
From: Sandeep Sukhani <sandeep.d.sukhani@gmail.com>
Date: Mon, 15 Feb 2021 12:24:43 +0530
Subject: [PATCH] convert sync.Mutex to sync.RWMutex in deduper to allow
 concurrent checks for isSeen

---
 pkg/storage/stores/shipper/util/queries.go | 23 +++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)

diff --git a/pkg/storage/stores/shipper/util/queries.go b/pkg/storage/stores/shipper/util/queries.go
index 3085497f76e6..1150e2086f14 100644
--- a/pkg/storage/stores/shipper/util/queries.go
+++ b/pkg/storage/stores/shipper/util/queries.go
@@ -58,7 +58,7 @@ func DoParallelQueries(ctx context.Context, tableQuerier TableQuerier, queries [
 type IndexDeduper struct {
 	callback        chunk_util.Callback
 	seenRangeValues map[string]map[string]struct{}
-	mtx             sync.Mutex
+	mtx             sync.RWMutex
 }
 
 func NewIndexDeduper(callback chunk_util.Callback) *IndexDeduper {
@@ -77,19 +77,32 @@ func (i *IndexDeduper) Callback(query chunk.IndexQuery, batch chunk.ReadBatch) b
 }
 
 func (i *IndexDeduper) isSeen(hashValue string, rangeValue []byte) bool {
-	i.mtx.Lock()
-	defer i.mtx.Unlock()
+	i.mtx.RLock()
 
 	// index entries are never modified during query processing so it should be safe to reference a byte slice as a string.
 	rangeValueStr := yoloString(rangeValue)
-	if _, ok := i.seenRangeValues[hashValue]; !ok {
-		i.seenRangeValues[hashValue] = map[string]struct{}{}
+
+	if _, ok := i.seenRangeValues[hashValue][rangeValueStr]; ok {
+		i.mtx.RUnlock()
+		return true
 	}
 
+	i.mtx.RUnlock()
+
+	i.mtx.Lock()
+	defer i.mtx.Unlock()
+
+	// re-check if another concurrent call added the values already, if so do not add it again and return true
 	if _, ok := i.seenRangeValues[hashValue][rangeValueStr]; ok {
 		return true
 	}
 
+	// add the hashValue first if missing
+	if _, ok := i.seenRangeValues[hashValue]; !ok {
+		i.seenRangeValues[hashValue] = map[string]struct{}{}
+	}
+
+	// add the rangeValue
 	i.seenRangeValues[hashValue][rangeValueStr] = struct{}{}
 	return false
 }