Attempting to use MultiExec with CAS fails about predis

[ebernhardson]

Also wanted to note that if i add run the commands from top of that example:

SELECT 15
ZADD zset 1 a
ZADD zset 2 b
ZADD zset 3 c

I can run the script 3 times without error, the 4th time when zset is empty the error occurs

[ebernhardson]

One more note to add, If you switch the client profile from dev to 2.0 in the example, the error is also 'DISCARD without MULTI'. I have a feeling the fix for the bug above will also fix this one, but basically the error that should have been output is 'The current profile does not support WATCH and UNWATCH commands'

[nrk]

Thanks for the detailed report, this is a nasty bug introduced with commit cdfd0b9 while I was trying to fix other weird issues with Predis\MultiExecContext in a couple of corner cases (it slipped through simply because this particular case doesn't seem to be covered by the test suite).

A quick explanation of the cause: DISCARD is automatically used by Predis in order to reset a transaction if client-side errors occur during a MULTI/EXEC, but this is wrong if the exception occur before MULTI when using check-and-set operations since DISCARD will not work and the server will return a DISCARD without MULTI error. Also, Predis tries to avoid sending MULTI if there are no pending commands (empty transaction), but when using check-and-set we still need to reset pending WATCH__ed keys, but again it was using __DISCARD to do that.

I implemented a fix (see commit 85c1a7a) that uses UNWATCH instead of DISCARD for the two cases mentioned above. For empty transactions it is probably better to just fire an EXEC, this will reduce useless complexity. This is a clear evidence of the fact that Predis\MultiExecContext definitely needs some heavy refactoring before releasing Predis 0.7.0 as stable.

Thanks again for the bug report!