MySQL Bugs: #30444: 5.0 mysqldump silently allows wrong backup to be taken against a 4.0 database

Bug #30444	5.0 mysqldump silently allows wrong backup to be taken against a 4.0 database
Submitted:	15 Aug 2007 18:33	Modified:	24 Oct 2007 21:56
Reporter:	Mark Leith
Status:	Closed
Category:	Server: Backup	Severity:	S2 (Serious)
Version:	5.0.33+	OS:	Any
Assigned to:	Tatjana A. Nuernberg	Target Version:
Tags:	regression

Description:
It appears that the fix for Bug #19660 will cause silent corruption against a 4.0 database
if used with the --single-transaction and --master-data options.

Please note the comment above where the fix was applied in mysqldump.c:

    We use BEGIN for old servers. --single-transaction --master-data will fail
    on old servers, but that's ok as it was already silently broken (it didn't
    do a consistent read, so better tell people frankly, with the error).

    We want the first consistent read to be used for all tables to dump so we
    need the REPEATABLE READ level (not anything lower, for example READ
    COMMITTED would give one new consistent read per dumped table).

---

The comment is still the same, but doesn't reflect the code anymore. mysqldump really
ought to warn or exit if you attempt this combo against a 4.0 database, as it used to.

How to repeat:
mysqldump --single-transaction --master-data against a 4.0 database. 

Suggested fix:
Add a warning that this will cause inconsistencies when running against a 4.0 database.

Code is OK.  Instead of "read may not be consistent!", it might be better to say "backup
may not be consistent across all tables" or similar.  White space looks a bit funky, may
just be cut-n-paste artifacts.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/34867

ChangeSet@1.2675, 2007-10-04 08:27:03+02:00, tnurnberg@sin.intern.azundris.com +1 -0
  Bug #30444: 5.0 mysqldump silently allows wrong backup to be taken against a 4.0
database
  
  The combination of --single-transaction and --master-data requires
  START TRANSACTION WITH CONSISTENT SNAPSHOT which is available from
  mysqld 4.1 on. When trying this against an older server, print
  diagnostic, then, if --force is not given, abort.
  
  No test-case given since it would require a mysqld < 4.1.

pushed to 4.1.24, 5.0.52, 5.1 maint trees

Pushed into 5.1.23-beta

Pushed into 5.0.52

Pushed into 4.1.24

Noted in 4.1.24, 5.0.52, 5.1.23 changelogs.

Versions of mysqldump from MySQL 4.1 or higher tried to use START
TRANSACTION WITH CONSISTENT SNAPSHOT if the --single-transaction and
--master-data options were given, even with servers older than 4.1
that do not support consistent snapshots.