[Gluster-devel] Can function arguments be modified ?

[Xavier Hernandez]

Hello,

I've a doubt about the way in which arguments received from upper or 
lower translators should be handled in case they need to be modified.

Simple arguments are passed by value and there is no problem, but all 
other data types are passed by reference (dict_t *, fd_t *, inode_t *, 
struct iatt *, ...). I know that for fd_t and inode_t the way to go is 
to attach context structures to them associated to my translator without 
modifying anything, but for dict_t and other structures I may need to 
make changes before continuing. For example, I need to add some keys 
into a dict before calling lower translators and I need to remove some 
keys before propagating response to upper translators. I also need to 
modify some fields from an struct iatt.

The question is what is the policy used for gluster development ? are 
translators allowed to modify arguments received from upper layers ? are 
translators allowed to modify arguments received from callbacks before 
propagating them ?

My translator can have multiple subvolumes, so if they are allowed to 
modify the parameters I send to them, I need to send a different copy to 
each one. Otherwise they can interfere with each other. On the other 
hand, if they are not allowed to modify my arguments, but I need to 
modify what I receive, I would need to make a copy before.

The same happens with arguments received from a callback. If 
translators can modify them, it's supposed that they can't access data 
after propagating it upper into the chain, otherwise they could see 
unknown or inconsistent data. If they aren't allowed to modify data, 
then a copy must be done before making any change.

Making copies is always safe but if it's unnecessary, it's a waste of 
time and a loss of resources and throughput.

What is the correct behaviour ?

Thank you very much for your help

Xavi