From: Christopher Powers
Date: October 15 2008 7:45pm
Subject: Re: Hypothetical on failing to thaw chilled record
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More detail...

RecordVersion::thaw() returns a record with a null data pointer.

It first tries to thaw from the Serial Log, but the transaction is 
commited, so it tries a record fetch. In each case, Transaction::state 
== Committed, writePending == false, dependencies > 0.

Section::fetchRecord() fails because there is no data page assigned to 
the record:

bool Section::fetchRecord(int32 recordNumber, Stream *stream, TransId 
transId)
[...]
RecordIndex *index = locatorPage->elements + slot;

// If there isn't a page number, there isn't a record

if (index->page == 0)
     {
     bdb->release(REL_HISTORY);
     return false;
     }

This happens reliably with the SystemQA chill/thaw stress test, so far 
only during an index scan:

RecordVersion::thaw()
Table::fetchForUpdate(Transaction * transaction=0x040a3e70, Record * 
source=0x07d14b60, bool usingIndex=false)  Line 3552
StorageDatabase::nextRow(StorageTable * storageTable=0x04869398, int 
recordNumber=135, bool lockForUpdate=true)  Line 288
StorageTable::next(int recordNumber=135, bool lockForUpdate=true)  Line 158
StorageInterface::rnd_next(unsigned char * buf=0x06e3e9f0)  Line 613 + 
0x31 bytes C++
rr_sequential(READ_RECORD * info=0x0777e400)  Line 390 + 0x1f bytes C++
mysql_update()
mysql_execute_command(THD * thd=0x06db6180)
mysql_parse()
...

If Ann's theory is correct, then a failed thaw may legitimately occur 
but should be handled correctly.



Ann W. Harrison wrote:
> Here's the timeline ...  all transactions on separate connections and
> concurrent except as noted.
> 
> Assume table t1 (f1 integer) with an index on f1.
> 
> 
> T1:  insert into table1 values 3;
> 
> T1:  commit;
> 
> T2:  start;
> 
> T3:  start;
> 
> T3:  update t1 set f1 = 5 where f1 = 3;
> 
> (at this point the index on f1 has entries for values 3 and 5, both
> with record 1. There's a record version object with the new and
> old versions in the record cache.  The old index entry has been
> processed, and there's a new index entry in T3's deferred index)
> 
> System chills T3 changes - new version of record 1 is chilled, old
> version is left in memory as back version.
> 
> T3:  commit;
> 
> T2:  commit;
> 
> T4:  select * from table1 where f1 > 1 and f1 < 4; (index bitmap
> built, with hit for 3);
> 
> System removes obsolete version of record 1 (f1 = 3), from cache
> and disk, and index.  Index lookup find a hit for 3 on record 1.
> 
> T3 becomes write complete, T4 has a dependency on T3
> 
> Thaw fails to find record with that value
> Fetch fails to find record with that value
>