added backward compatible CRC check and extra explanatory comments (#6233)

weed/storage/needle/needle_read.go

@@ -74,7 +74,9 @@ func (n *Needle) ReadBytes(bytes []byte, offset int64, size Size, version Versio
 		checksum := util.BytesToUint32(bytes[NeedleHeaderSize+size : NeedleHeaderSize+size+NeedleChecksumSize])
 		newChecksum := NewCRC(n.Data)
 		if checksum != newChecksum.Value() && checksum != uint32(newChecksum) {
-			// the crc.Value() function is to be deprecated. this double checking is for backward compatible.
+			// the crc.Value() function is to be deprecated. this double checking is for backward compatibility
+			// with seaweed version using crc.Value() instead of uint32(crc), which appears in commit 056c480eb
+			// and switch appeared in version 3.09.
 			stats.VolumeServerHandlerCounter.WithLabelValues(stats.ErrorCRC).Inc()
 			return errors.New("CRC error! Data On Disk Corrupted")
 		}

weed/storage/volume_read.go

@@ -164,7 +164,10 @@ func (v *Volume) readNeedleDataInto(n *needle.Needle, readOption *ReadOption, wr
 		toWrite := min(count, int(offset+size-x))
 		if toWrite > 0 {
 			crc = crc.Update(buf[0:toWrite])
-			if offset == 0 && size == int64(n.DataSize) && int64(count) == size && (n.Checksum != crc) {
+			// the crc.Value() function is to be deprecated. this double checking is for backward compatibility
+			// with seaweed version using crc.Value() instead of uint32(crc), which appears in commit 056c480eb
+			// and switch appeared in version 3.09.
+			if offset == 0 && size == int64(n.DataSize) && int64(count) == size && (n.Checksum != crc && uint32(n.Checksum) != crc.Value()) {
 				// This check works only if the buffer is big enough to hold the whole needle data
 				// and we ask for all needle data.
 				// Otherwise we cannot check the validity of partially aquired data.
@@ -187,7 +190,9 @@ func (v *Volume) readNeedleDataInto(n *needle.Needle, readOption *ReadOption, wr
 		}
 	}
 	if offset == 0 && size == int64(n.DataSize) && (n.Checksum != crc && uint32(n.Checksum) != crc.Value()) {
-		// the crc.Value() function is to be deprecated. this double checking is for backward compatible.
+		// the crc.Value() function is to be deprecated. this double checking is for backward compatibility
+		// with seaweed version using crc.Value() instead of uint32(crc), which appears in commit 056c480eb
+		// and switch appeared in version 3.09.
 		stats.VolumeServerHandlerCounter.WithLabelValues(stats.ErrorCRC).Inc()
 		return fmt.Errorf("ReadNeedleData checksum %v expected %v for Needle: %v,%v", crc, n.Checksum, v.Id, n)
 	}