Re: [igraph] releasing the GIL in the python extension

[Tamas Nepusz]

> So, would the following scenario result in memory corruption if the  
> GIL was released?:
>
> in python, create graph,
> make a full copy of the graph (not a weakref),
> in thread 1, with first graph, calculate something
> in thread 2 with the copy of the graph, calculate something
> thread 1 calculation fails,
> error function/destructor is called, ...
>
> thread 2's graph (the copy) gets freed?
Nope. The data structures created _during_ the calculation would  
possibly be freed in thread 2 (not the copy of the graph itself). So,  
for instance, let us assume that you are calculating betweenness  
centrality. Betweenness centrality requires a few temporary vectors to  
store partial result. What happens is:

1. In thread 1, the calculation starts with the first graph

2. Thread 1 allocates the temporary vectors and pushes the memory  
addresses down the stack

3. In thread 2, the calculation starts with the second graph

4. Thread 2 allocates the temporary vectors and pushes the memory  
addresses down the stack

5. Thread 1 finishes the calculation. Since thread 1 knows that it  
pushed, say, two addresses down the stack (for the temporary vectors  
-- this is just an example, so the numbers may be arbitrary), it pops  
the topmost two memory addresses from the stack. These addresses  
happen to belong to thread 2 since those were pushed later, so thread  
1 could happily free the memory address that is being used by thread 2.

--
Tamas