会不会出现commitlog文件写消息时候, maxBlank 小于8，导致写入异常

[chenzhixiu]

`public AppendMessageResult doAppend(final long fileFromOffset, final ByteBuffer byteBuffer, final int maxBlank,
final MessageExtBatch messageExtBatch, PutMessageContext putMessageContext) {
byteBuffer.mark();
//physical offset
long wroteOffset = fileFromOffset + byteBuffer.position();
// Record ConsumeQueue information
Long queueOffset = messageExtBatch.getQueueOffset();
long beginQueueOffset = queueOffset;
int totalMsgLen = 0;
int msgNum = 0;

        final long beginTimeMills = CommitLog.this.defaultMessageStore.now();
        ByteBuffer messagesByteBuff = messageExtBatch.getEncodedBuff();

        int sysFlag = messageExtBatch.getSysFlag();
        int bornHostLength = (sysFlag & MessageSysFlag.BORNHOST_V6_FLAG) == 0 ? 4 + 4 : 16 + 4;
        int storeHostLength = (sysFlag & MessageSysFlag.STOREHOSTADDRESS_V6_FLAG) == 0 ? 4 + 4 : 16 + 4;
        Supplier<String> msgIdSupplier = () -> {
            int msgIdLen = storeHostLength + 8;
            int batchCount = putMessageContext.getBatchSize();
            long[] phyPosArray = putMessageContext.getPhyPos();
            ByteBuffer msgIdBuffer = ByteBuffer.allocate(msgIdLen);
            MessageExt.socketAddress2ByteBuffer(messageExtBatch.getStoreHost(), msgIdBuffer);
            msgIdBuffer.clear();//because socketAddress2ByteBuffer flip the buffer

            StringBuilder buffer = new StringBuilder(batchCount * msgIdLen * 2 + batchCount - 1);
            for (int i = 0; i < phyPosArray.length; i++) {
                msgIdBuffer.putLong(msgIdLen - 8, phyPosArray[i]);
                String msgId = UtilAll.bytes2string(msgIdBuffer.array());
                if (i != 0) {
                    buffer.append(',');
                }
                buffer.append(msgId);
            }
            return buffer.toString();
        };

        messagesByteBuff.mark();
        int index = 0;
        while (messagesByteBuff.hasRemaining()) {
            // 1 TOTALSIZE
            final int msgPos = messagesByteBuff.position();
            final int msgLen = messagesByteBuff.getInt();

            totalMsgLen += msgLen;
            // Determines whether there is sufficient free space
            if ((totalMsgLen + END_FILE_MIN_BLANK_LENGTH) > maxBlank) {
                this.msgStoreItemMemory.clear();
                // 1 TOTALSIZE
                this.msgStoreItemMemory.putInt(maxBlank);
                // 2 MAGICCODE
                this.msgStoreItemMemory.putInt(CommitLog.BLANK_MAGIC_CODE);
                // 3 The remaining space may be any value
                //ignore previous read
                messagesByteBuff.reset();
                // Here the length of the specially set maxBlank
                byteBuffer.reset(); //ignore the previous appended messages
                byteBuffer.put(this.msgStoreItemMemory.array(), 0, 8);
                return new AppendMessageResult(AppendMessageStatus.END_OF_FILE, wroteOffset, maxBlank, msgIdSupplier, messageExtBatch.getStoreTimestamp(),
                    beginQueueOffset, CommitLog.this.defaultMessageStore.now() - beginTimeMills);
            }
            //move to add queue offset and commitlog offset
            int pos = msgPos + 20;
            messagesByteBuff.putLong(pos, queueOffset);
            pos += 8;
            messagesByteBuff.putLong(pos, wroteOffset + totalMsgLen - msgLen);
            // 8 SYSFLAG, 9 BORNTIMESTAMP, 10 BORNHOST, 11 STORETIMESTAMP
            pos += 8 + 4 + 8 + bornHostLength;
            // refresh store time stamp in lock
            messagesByteBuff.putLong(pos, messageExtBatch.getStoreTimestamp());

            putMessageContext.getPhyPos()[index++] = wroteOffset + totalMsgLen - msgLen;
            queueOffset++;
            msgNum++;
            messagesByteBuff.position(msgPos + msgLen);
        }

        messagesByteBuff.position(0);
        messagesByteBuff.limit(totalMsgLen);
        byteBuffer.put(messagesByteBuff);
        messageExtBatch.setEncodedBuff(null);
        AppendMessageResult result = new AppendMessageResult(AppendMessageStatus.PUT_OK, wroteOffset, totalMsgLen, msgIdSupplier,
            messageExtBatch.getStoreTimestamp(), beginQueueOffset, CommitLog.this.defaultMessageStore.now() - beginTimeMills);
        result.setMsgNum(msgNum);

        return result;
    }

}`