builtin uncompress function ignored the four bytes length

[niedhui]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

mysql> select uncompress(unhex('02000000789CCB48CDC9C95728CF2FCA4901001A0B045D'));

2. What did you expect to see?

mysql> select uncompress(unhex('02000000789CCB48CDC9C95728CF2FCA4901001A0B045D'));
+---------------------------------------------------------------------+
| uncompress(unhex('02000000789CCB48CDC9C95728CF2FCA4901001A0B045D')) |
+---------------------------------------------------------------------+
| NULL                                                                |
+---------------------------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+--------------------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                                          |
+---------+------+--------------------------------------------------------------------------------------------------+
| Warning | 1258 | ZLIB: Not enough room in the output buffer (probably, length of uncompressed data was corrupted) |
+---------+------+--------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead?

+---------------------------------------------------------------------+
| uncompress(unhex('02000000789CCB48CDC9C95728CF2FCA4901001A0B045D')) |
+---------------------------------------------------------------------+
| hello world                                                         |
+---------------------------------------------------------------------+
1 row in set (0.01 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?

tidb_version(): Release Version: v2.1.0-rc.2-10-gb30dbd0
Git Commit Hash: b30dbd06acaab088a1e8fa004e189c651ca08bf1
Git Branch: master
UTC Build Time: 2018-09-17 01:20:48
GoVersion: go version go1.11 linux/amd64
Race Enabled: false
TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e
Check Table Before Drop: false

5. more info
   This is found when I try to implement builtin compress and uncompress function in TiKV (implement compress,uncompress,uncompressed_length tikv/tikv#3856)
   the first four bytes stored the length of the uncompress string, if it's less than the corrected length (maybe corrupted), MySQL return Null， TiDB just ignore it.

[zimulala]

@niedhui
Thanks for your feedback!
Would you like to file a PR to fix it? Refer to https://dev.mysql.com/doc/refman/5.7/en/encryption-functions.html#function_uncompress

[ziyi-yan]

I am trying to fix this issue.