Re: [PATCH 2/5] gdb/python: new gdb.architecture_names function

[Andrew Burgess <andrew.burgess@embecosm.com>]

* Simon Marchi <simon.marchi@polymtl.ca> [2021-10-22 09:02:04 -0400]:

> Hi Andrew,
> 
> Sorry to reply only after you have merged this.  I just have a question
> about the API.

Not a problem, better to get this sorted if something isn't right.

> 
> On 2021-10-13 17:59, Andrew Burgess wrote:
> > Add a new function to the Python API, gdb.architecture_names().  This
> > function returns a list containing all of the supported architecture
> > names within the current build of GDB.
> > 
> > The values returned in this list are all of the possible values that
> > can be returned from gdb.Architecture.name().
> 
> Did you consider having a `gdb.architectures()` function, that returns a
> list of gdb.Architecture objects?  And then, if you want the names, you
> use gdb.Architecture.name:
> 
>     for arch in gdb.architectures():
> 	print(arch.name)

Except I don't believe this would work.  A gdb.Architecture goes 1:1
with a gdbarch object, and we can have multiple gdbarch objects for
the same underlying bfd architecture.  For example, if two targets
have the same bfd-architecture, but different target descriptions,
you'll get different gdbarch objects, and different gdb.Architecture
objects.

So, in your above code you have to at least filter for duplicates.

Then, as I understand it, gdbarch objects are only created "on
demand", so in a multi-arch GDB, if I pass an x86 ELF, I don't
believe a risc-v gdb.Architecture is ever created.

So, in your above code, you'll only see the names of architectures
that the user has exposed to GDB.

That doesn't mean a gdb.architectures() method wouldn't be useful in
some other situation, I just don't think it did what I wanted - tell
me all the architectures this GDB knows about.

Maybe what I should do is add an architecture_created event, then I
could do:

  def do_something(arch):
    print(arch.name)

  for arch in gdb.architectures():
    do_something(arch)

  def handler(event):
    do_something(event.architecture)

  gdb.events.architecture_created.connect(handler)

> 
> Being able to get the gdb.Architecture objects instead of just the name
> sounds more flexible / future-proof to me.

I don't think I'm worried that adding this API will be something we
regret.  gdb.Architecture already has a .name method, so being able to
ask for the set of all possible names doesn't seem unreasonable.  For
that reason, I'd like to keep the existing method, even if you think
that the above would be a better API...

>                                             Like, we have
> `gdb.breakpoints()`, not `gdb.breakpoint_nums()`.  But there is perhaps
> a technical reason why this doesn't work or isn't a good idea.

The only different I see is that the names can exist before the
corresponding architectures, and we can have multiple architectures
per name, it would be like if gdb.breakpoint_nums() only returned the
numbers 1 -> 10 because we could only have that many breakpoints, but,
then we could actually have multiple breakpoints for each
number...... OK, this analogy got weird...

I've added the above additional APIs to my todo list, and I'll try to
get them implemented.

Thanks,
Andrew