[#210] Fix multipart object reader

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/layer/multi_object_reader.go

@@ -93,15 +93,22 @@ func newMultiObjectReader(ctx context.Context, cfg multiObjectReaderConfig) (*mu
 }
 
 func findStartPart(cfg multiObjectReaderConfig) (index int, offset uint64) {
-	return findPartByPosition(cfg.off, cfg.parts)
+	position := cfg.off
+	for i, part := range cfg.parts {
+		// Strict inequality when searching for start position to avoid reading zero length part.
+		if position < part.Size {
+			return i, position
+		}
+		position -= part.Size
+	}
+
+	return -1, 0
 }
 
 func findEndPart(cfg multiObjectReaderConfig) (index int, length uint64) {
-	return findPartByPosition(cfg.off+cfg.ln, cfg.parts)
-}
-
-func findPartByPosition(position uint64, parts []partObj) (index int, positionInPart uint64) {
-	for i, part := range parts {
+	position := cfg.off + cfg.ln
+	for i, part := range cfg.parts {
+		// Non-strict inequality when searching for end position to avoid out of payload range error.
 		if position <= part.Size {
 			return i, position
 		}

api/layer/multi_object_reader_test.go

@@ -90,6 +90,16 @@ func TestMultiReader(t *testing.T) {
 			off:  parts[0].Size - 4,
 			ln:   parts[1].Size + 8,
 		},
+		{
+			name: "second part",
+			off:  parts[0].Size,
+			ln:   parts[1].Size,
+		},
+		{
+			name: "second and third",
+			off:  parts[0].Size,
+			ln:   parts[1].Size + parts[2].Size,
+		},
 		{
 			name: "offset out of range",
 			off:  uint64(len(fullPayload) + 1),