fix: Changes the backend.Walk implementation to return non-truncated result if both delimiter and max-keys are provided

Fixes #816

`ListObjects(V2)` used to return truncated response, if delimiter is provided and the result is limited by max-keys and the number of common prefixes is the same as `max-keys`.
e.g
PUT -> `foo/bar`
PUT -> `foo/quxx`
LIST: `max-keys=1;delim=/` -> foo/

`ListObjects(V2)` should return `foo/` as common prefix and `truncated` should be `false`.

The PR makes this fix to return `non-truncated` response for the above described case.

backend/walk.go

@@ -44,12 +44,19 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 	cpmap := make(map[string]struct{})
 	var objects []s3response.Object
 
+	// if max is 0, it should return empty non-truncated result
+	if max == 0 {
+		return WalkResults{
+			Truncated: false,
+		}, nil
+	}
+
 	var pastMarker bool
 	if marker == "" {
 		pastMarker = true
 	}
 
-	pastMax := max == 0
+	var pastMax bool
 	var newMarker string
 	var truncated bool
 
@@ -76,14 +83,6 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 			return fs.SkipDir
 		}
 
-		if pastMax {
-			if len(objects) != 0 {
-				newMarker = *objects[len(objects)-1].Key
-				truncated = true
-			}
-			return fs.SkipAll
-		}
-
 		// After this point, return skipflag instead of nil
 		// so we can skip a directory without an early return
 		var skipflag error
@@ -116,7 +115,15 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 					if err != nil {
 						return fmt.Errorf("directory to object %q: %w", path, err)
 					}
+					if pastMax {
+						truncated = true
+						return fs.SkipAll
+					}
 					objects = append(objects, dirobj)
+					if (len(objects) + len(cpmap)) == int(max) {
+						newMarker = path
+						pastMax = true
+					}
 
 					return skipflag
 				}
@@ -160,9 +167,15 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 			if err != nil {
 				return fmt.Errorf("file to object %q: %w", path, err)
 			}
+			if pastMax {
+				truncated = true
+				return fs.SkipAll
+			}
+
 			objects = append(objects, obj)
 
-			if max > 0 && (len(objects)+len(cpmap)) == int(max) {
+			if (len(objects) + len(cpmap)) == int(max) {
+				newMarker = path
 				pastMax = true
 			}
 
@@ -200,8 +213,13 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 			if err != nil {
 				return fmt.Errorf("file to object %q: %w", path, err)
 			}
+			if pastMax {
+				truncated = true
+				return fs.SkipAll
+			}
 			objects = append(objects, obj)
 			if (len(objects) + len(cpmap)) == int(max) {
+				newMarker = path
 				pastMax = true
 			}
 			return skipflag
@@ -222,11 +240,14 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 			return skipflag
 		}
 
+		if pastMax {
+			truncated = true
+			return fs.SkipAll
+		}
 		cpmap[cpref] = struct{}{}
 		if (len(objects) + len(cpmap)) == int(max) {
 			newMarker = cpref
-			truncated = true
-			return fs.SkipAll
+			pastMax = true
 		}
 
 		return skipflag
@@ -252,6 +273,10 @@ func Walk(ctx context.Context, fileSystem fs.FS, prefix, delimiter, marker strin
 		})
 	}
 
+	if !truncated {
+		newMarker = ""
+	}
+
 	return WalkResults{
 		CommonPrefixes: commonPrefixes,
 		Objects:        objects,