Using `on_post` pathway without setting its delay chooses random delay value

[denisalevi]

Using something like

syn = Synapses(..., on_pre='...', on_post='...', delay=0*ms)

sets the delay for the pre pathway, but not for the post pathway (as it should according to brian2 docs). But for some reason the delay of the on_post pathway gets a random value, seems like uninitialized memory allocation. So I needed to set

syn.post.delay = 0*ms

explicitly to have no delays. The delay variable of the synapse object is also not set when using the on_post keyword.

[denisalevi]

Nope, setting the post delay still results in an allocated but uninitialized delay array... looks like the delay vector is resized wrong (according to the no_or_const_delay_mode of the pre pathway to size 1) and then the other delay values stay uninitialised.

[denisalevi]

And this seems only to happen when one Synapses object defines both, on_pre and on_post pathways.

Synapses(..., on_pre=...)
Synapses(..., on_post=...)

seems to work fine.

[denisalevi]

This might effect our STDP benchmarks, where both pathways are set in the Synapses constructor without any delay specification!