region_cache: Add comments for LocateBucket (#497)

* update

Signed-off-by: SpadeA-Tang <u6748471@anu.edu.au>

* update

Signed-off-by: SpadeA-Tang <u6748471@anu.edu.au>

* update

Signed-off-by: SpadeA-Tang <u6748471@anu.edu.au>

internal/locate/region_cache.go

@@ -37,6 +37,7 @@ package locate
 import (
 	"bytes"
 	"context"
+	"encoding/hex"
 	"fmt"
 	"math/rand"
 	"sort"
@@ -755,23 +756,33 @@ func (l *KeyLocation) GetBucketVersion() uint64 {
 	return l.Buckets.GetVersion()
 }
 
-// LocateBucket calls locateBucket and check the result.
-// When the key is in [KeyLocation.StartKey, first Bucket key), the result returned by locateBucket will be nil
+// LocateBucket handles with a type of edge case of locateBucket that returns nil.
+// There are two cases where locateBucket returns nil:
+// Case one is that the key neither does not belong to any bucket nor does not belong to the region.
+// Case two is that the key belongs to the region but not any bucket.
+// LocateBucket will not return nil in the case two.
+// Specifically, when the key is in [KeyLocation.StartKey, first Bucket key), the result returned by locateBucket will be nil
 // as there's no bucket containing this key. LocateBucket will return Bucket{KeyLocation.StartKey, first Bucket key}
-//  --- it's reasonable to assume that Bucket{KeyLocation.StartKey, first Bucket key} is a bucket belonging to the region.
+// as it's reasonable to assume that Bucket{KeyLocation.StartKey, first Bucket key} is a bucket belonging to the region.
 // Key in [last Bucket key, KeyLocation.EndKey) is handled similarly.
 func (l *KeyLocation) LocateBucket(key []byte) *Bucket {
 	bucket := l.locateBucket(key)
-	if bucket != nil || !l.Contains(key) {
+	// Return the bucket when locateBucket can locate the key
+	if bucket != nil {
 		return bucket
 	}
+	// Case one returns nil too.
+	if !l.Contains(key) {
+		return nil
+	}
 	counts := len(l.Buckets.Keys)
 	if counts == 0 {
 		return &Bucket{
 			l.StartKey,
 			l.EndKey,
 		}
 	}
+	// Handle case two
 	firstBucketKey := l.Buckets.Keys[0]
 	if bytes.Compare(key, firstBucketKey) < 0 {
 		return &Bucket{
@@ -786,10 +797,13 @@ func (l *KeyLocation) LocateBucket(key []byte) *Bucket {
 			l.EndKey,
 		}
 	}
-	return bucket
+	// unreachable
+	logutil.Logger(context.Background()).Info(
+		"Unreachable place", zap.String("KeyLocation", l.String()), zap.String("Key", hex.EncodeToString(key)))
+	panic("Unreachable")
 }
 
-// locateBucket returns the bucket the key is located.
+// locateBucket returns the bucket the key is located. It returns nil if the key is outside the bucket.
 func (l *KeyLocation) locateBucket(key []byte) *Bucket {
 	keys := l.Buckets.GetKeys()
 	searchLen := len(keys) - 1