Re: [PATCH 3/3] test: Re-implement pasta NDP tests using tunbridge & exeter

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Oct 16, 2025 at 11:31:19PM +0200, Stefano Brivio wrote:
> On Fri, 10 Oct 2025 13:17:13 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Fri, Oct 10, 2025 at 01:20:23AM +0200, Stefano Brivio wrote:
> > > On Thu, 9 Oct 2025 15:47:01 +1100
> > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > >   
> > > > On Thu, Oct 09, 2025 at 01:02:48AM +0200, Stefano Brivio wrote:  
> > > > > On Wed, 8 Oct 2025 13:32:27 +1100
> > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > >     
> > > > > > On Tue, Oct 07, 2025 at 10:01:10PM +0200, Stefano Brivio wrote:    
> > > > > > > On Thu,  2 Oct 2025 17:57:08 +1000
> > > > > > > David Gibson <david@gibson.dropbear.id.au> wrote:
> > > > > > >       
> > > > > > > > Convert the pasta NDP tests from shell and our own DSL to Python using
> > > > > > > > the exeter test protocol and tunbridge network simulation library.
> > > > > > > > 
> > > > > > > > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > > > > > > > ---
> > > > > > > >  test/Makefile          |  2 +-
> > > > > > > >  test/pasta/dhcp        |  5 ++++
> > > > > > > >  test/pasta/ndp.py      | 59 ++++++++++++++++++++++++++++++++++++++++++
> > > > > > > >  test/run               |  6 +++--
> > > > > > > >  test/tasst/__init__.py |  4 +++
> > > > > > > >  test/tasst/pasta.py    | 40 ++++++++++++++++++++++++++++
> > > > > > > >  6 files changed, 113 insertions(+), 3 deletions(-)
> > > > > > > >  create mode 100755 test/pasta/ndp.py
> > > > > > > >  create mode 100644 test/tasst/pasta.py
> > > > > > > > 
> > > > > > > > diff --git a/test/Makefile b/test/Makefile
> > > > > > > > index f66c7e7e..95e3d75e 100644
> > > > > > > > --- a/test/Makefile
> > > > > > > > +++ b/test/Makefile
> > > > > > > > @@ -67,7 +67,7 @@ ASSETS = $(DOWNLOAD_ASSETS) $(LOCAL_ASSETS)
> > > > > > > >  
> > > > > > > >  EXETER_SH = smoke/smoke.sh build/static_checkers.sh
> > > > > > > >  EXETER_PYPATH = exeter/py3:tunbridge/:.
> > > > > > > > -EXETER_PYTHON = smoke/smoke.py build/build.py
> > > > > > > > +EXETER_PYTHON = smoke/smoke.py build/build.py pasta/ndp.py
> > > > > > > >  EXETER_BATS = $(EXETER_SH:%=%.bats) $(EXETER_PYTHON:%=%.bats)
> > > > > > > >  BATS_FILES = $(EXETER_BATS) \
> > > > > > > >  	podman/test/system/505-networking-pasta.bats
> > > > > > > > diff --git a/test/pasta/dhcp b/test/pasta/dhcp
> > > > > > > > index e1c66be6..61279fbf 100644
> > > > > > > > --- a/test/pasta/dhcp
> > > > > > > > +++ b/test/pasta/dhcp
> > > > > > > > @@ -18,6 +18,11 @@ test	Interface name
> > > > > > > >  nsout	IFNAME ip -j link show | jq -rM '.[] | select(.link_type == "ether").ifname'
> > > > > > > >  check	[ -n "__IFNAME__" ]
> > > > > > > >  
> > > > > > > > +# Bring up the interface
> > > > > > > > +ns	ip link set dev __IFNAME__ up
> > > > > > > > +# Wait for SLAAC & DAD to complete
> > > > > > > > +ns	while ! ip -j -6 addr show dev __IFNAME__ | jq -e '.[].addr_info.[] | select(.protocol == "kernel_ra")'; do sleep 0.1; done
> > > > > > > > +
> > > > > > > >  test	DHCP: address
> > > > > > > >  ns	/sbin/dhclient -4 --no-pid __IFNAME__
> > > > > > > >  nsout	ADDR ip -j -4 addr show|jq -rM '.[] | select(.ifname == "__IFNAME__").addr_info[0].local'
> > > > > > > > diff --git a/test/pasta/ndp.py b/test/pasta/ndp.py
> > > > > > > > new file mode 100755
> > > > > > > > index 00000000..8c7ce31e
> > > > > > > > --- /dev/null
> > > > > > > > +++ b/test/pasta/ndp.py
> > > > > > > > @@ -0,0 +1,59 @@
> > > > > > > > +#! /usr/bin/env python3
> > > > > > > > +#
> > > > > > > > +# SPDX-License-Identifier: GPL-2.0-or-later
> > > > > > > > +#
> > > > > > > > +# test/pasta/ndp.py - pasta NDP functionality
> > > > > > > > +#
> > > > > > > > +# Copyright Red Hat
> > > > > > > > +# Author: David Gibson <david@gibson.dropbear.id.au>
> > > > > > > > +
> > > > > > > > +import contextlib
> > > > > > > > +import dataclasses
> > > > > > > > +from typing import Iterator
> > > > > > > > +
> > > > > > > > +import exeter
> > > > > > > > +import tunbridge
> > > > > > > > +import tasst
> > > > > > > > +
> > > > > > > > +
> > > > > > > > +@dataclasses.dataclass
> > > > > > > > +class UnconfiguredScenario(exeter.Scenario):
> > > > > > > > +    """Tests for a pasta instance without --config-net"""
> > > > > > > > +
> > > > > > > > +    host: tunbridge.Site
> > > > > > > > +    guest: tunbridge.Site
> > > > > > > > +    ifname: str
> > > > > > > > +    addr6: tunbridge.ip.AddrMask6
> > > > > > > > +    gw6: tunbridge.ip.Addr6      
> > > > > > > 
> > > > > > > Until this point, it looks like stuff I can happily copy and paste,
> > > > > > > and grasp, even. But then:
> > > > > > >       
> > > > > > > > +    @exeter.scenariotest
> > > > > > > > +    def test_ifname(self) -> None:
> > > > > > > > +        ifs = tunbridge.ip.ifs(self.guest)
> > > > > > > > +        exeter.assert_eq(set(ifs), {'lo', self.ifname})      
> > > > > > > 
> > > > > > > ...why does a "Scenario" have a .ifname?      
> > > > > > 
> > > > > > Yeah, the readability of the Scenario mechanism was something I was
> > > > > > particularly concerned about.  I think the concept is valuable, but
> > > > > > I'm very open to different ways of naming or organising it, if we can
> > > > > > up with something better.    
> > > > > 
> > > > > From the description you give below, the name seems to fit.
> > > > >     
> > > > > > A "Scenario" (specifically a subclass of exeter.Scenario) is a group
> > > > > > of tests with a common set of parameters.  In this case
> > > > > > UnconfiguredScenario is a bunch of tests about the behaviour of pasta
> > > > > > without --config-net.  Each of those tests has access to the host and
> > > > > > guest sites, the expected interface name, address and gateway in the
> > > > > > guest - that is, the contents of an UncofiguredScenario instance.    
> > > > > 
> > > > > I'm not sure if I understand this correctly, but if each guest has a
> > > > > single interface, that sounds a bit limiting.    
> > > > 
> > > > Sorry, to be clear: a Scenario in the general sense can contain
> > > > whatever parameters you like.  This *particular* Scenario -
> > > > UnconfiguredScenario - has just those things, because those are all
> > > > that its tests require.  
> > > 
> > > Ah, okay. Still, if I now want to take UnconfiguredScenario and add a
> > > couple of dummy interfaces to it for a quick test, I guess I have the
> > > choice to either do that with some "external" hack, or... copy and
> > > rename it, so that it doesn't affect all the usages?  
> > 
> > No.  A Scenario instance isn't responsible for managing the simulated
> > environment - that's the setup function - it's just conveying the
> > information about it that the tests need.  So, you can make a setup
> > function that adds the dummy interfaces, and still yield an
> > UnconfiguredScenario.  It doesn't need to have information about the
> > dummy interfaces because the tests carried by UnconfiguredScenario
> > don't care about them.
> 
> Oh, sorry, it's a class, of course, I see now.
> 
> > The scenario mechanism does several things:
> >   1) Groups together some related (parameterized) tests
> >   2) Allows all of those tests to be registered at once
> >   3) Provides a mechanism for providing a bunch of information to
> >      those tests (without requiring them each to have a large set of
> >      direct parameters)
> > 
> > I'm aware that doing those things with the same construct may be
> > confusing - it's just ways of doing them separately also seem
> > confusing and/or awkward in their own ways.  Maybe there's a better
> > way, but I haven't spotted it yet.
> 
> It really is confusing to me, but the description above is rather clear
> so I'll try to propose something once I get to write some kind of setup
> function and test cases myself.

Makes sense.

> > > > > Actually, I think any abstraction that doesn't offer arbitrary sets of
> > > > > (and relationships between) the objects shown via netlink (or, at
> > > > > least, namespaces, links, routes, addresses, neighbours) might be
> > > > > limiting and not generic enough.    
> > > > 
> > > > Absolutely, and the abstraction does allow that.
> > > >   
> > > > > > That instance describes a real (simulated) environment in which we can
> > > > > > run those tests.
> > > > > > 
> > > > > > You use this by supplying a function which sets things up, then yields
> > > > > > an UnconfiguredScenario instance describing what it set up.  exeter
> > > > > > will run all of the UnconfiguredScenario tests on the environment the
> > > > > > setup function created, each one as a separate test case.    
> > > > > 
> > > > > This part is now clear to me, and I think it's not complicated to grasp
> > > > > the concept vaguely but enough to copy, paste, and modify code doing
> > > > > this.    
> > > > 
> > > > Ok.
> > > >   
> > > > > It would be even better to hide this entirely, because "yielding a
> > > > > scenario" is a Python thing. In general, there's an imperative part in
> > > > > all this (bordering functional programming, but still, not descriptive)
> > > > > which I struggle to see as beneficial.
> > > > > 
> > > > > Here the tasks at hand are, roughly:
> > > > > 
> > > > > 1. represent two network namespaces, with two interfaces each (loopback
> > > > >    and non-loopback), with pasta connecting one of the interfaces of the
> > > > >    inner one    
> > > > 
> > > > There's a bit more to it than that - we need to specify the host's
> > > > routing setup, because that will affect what pasta does.  That's what
> > > > simple_host() is about, creating a host with the single gateway
> > > > routing that's our easiest / most common case.  
> > > 
> > > Okay, sure, by "interfaces" I meant configured interfaces with
> > > addresses and a default route, too. But that doesn't really modify my
> > > point, that is:
> > >   
> > > > > 2. bring up one of the interfaces
> > > > > 
> > > > > 3. compare addresses
> > > > > 
> > > > > ...and doing 1. like that is simply not... intuitive, I think.    
> > > > 
> > > > I'm not really clear on what you're getting at here.  There is an
> > > > unavoidable tradeoff here between obviousness for a single case,
> > > > versus reuseability for multiple related cases.  Is it just that some
> > > > of the relevant setup is hidden inside simple_host() that's the
> > > > problem? Or is it something else?  
> > > 
> > > ...yes, one part is that it's hidden. Another part are, specifically,
> > > these lines:
> > > 
> > > 	host: tunbridge.Site
> > > 	guest: tunbridge.Site
> > > 	ifname: str
> > > 
> > > 	[...]
> > > 
> > > 	@exeter.scenariotest
> > > 	def test_ifname(self) -> None:
> > > 
> > > 	[...]
> > > 
> > > None of these clearly links to "two network namespaces: A, with
> > > interface a1 and address x1, ...".  
> > 
> > Fair.  This needs a docstring explaining the parameters / fields.
> 
> That might help a tiny bit but I think the syntax and notations are
> kind of self-explanatory.
> 
> My concern is at a more conceptual level, and it's better summarised
> below, but here, specifically, we're writing:
> 
>   host: tunbridge.Site
> 
> to say:
> 
>   give me the "host" network namespace

What's "me" in this sentence?

> and to say that, in my ideal world, I would probably go for something
> on the line(s) of:
> 
>   A

Not really following, I hope it will make more sense to me below.

> > > I understand this is probably very close to the bare minimum you can
> > > get by modelling this all with actual code, and that's why I think
> > > actual (imperative/functional) code is usually not used to
> > > model/describe things.  
> > 
> > Imperative/functional code as opposed to..?
> 
> ...declarative.

Ok, I've generally seen functional programming put under the
declarative heading.  Or as my exam invigilator famously said once
"Declaraytive Programming Paradijjums".

So, I guess that makes me unclear what does and doesn't count as
declarative for you.

[snip]
> > > > Syntax certainly isn't irrelevant, but so far I haven't grasped what
> > > > you dislike about the function syntax versus a specialized grammer.
> > > > Is it:
> > > >   - The irritating silly parentheses?
> > > >   - Longish (qualified) function names?
> > > >   - The indentation from the with syntax?
> > > >   - Something else?  
> > > 
> > > It's *also* the first two  
> > 
> > Ok.  First I can't easily change.  Second can be mitigated by handling
> > the imports differently.
> > 
> > > (the indentation looks actually convenient),  
> > 
> > Ok, good.  I also think this is useful because it conveys the lifetime
> > of each object, which will be important once we get to tests where you
> > need to change things part way through.
> > 
> > > but that's not my main point. My main point is that this isn't
> > > fundamentally declarative. You're turning it into something that
> > > resembles that, but the syntax is still from an imperative programming
> > > language.
> > > 
> > > And in my mind the main feature of a network (topology) simulator is
> > > that you describe the topology (and it will build it for you), not that
> > > you... have to build a description?
> > > 
> > > Using an example that's obviously familiar to you: think of taking a
> > > device tree for some system with a couple of USB and I²C busses and a
> > > flash controller, and writing all that in Python based on some form of
> > > "bus" module/component.  
> > 
> > I mean... old school Open Firmware kind of is this, but with Forth
> > instead of Python.
> 
> Okay, you can model data structures in Python, obviously, but that
> wasn't my point. Anyway, it's all in the example below.
> 
> > > Once one sees how practical device trees are for that, the Python
> > > version would look wrong, wouldn't it?  
> > 
> > That really depends on the context.  If I was making an API for
> > building a device tree, I'd probably come up with something pretty
> > like this.
> 
> ...an API for building one, yes. But not if you were writing a device
> tree itself.
> 
> > > Now, while I think that some single bits of DTS syntax are
> > > unnecessarily complicated, conceptually, a "networking" device tree
> > > would look more usable to me than the approach you're taking.
> > > 
> > > Of course, we need the whole "testing" / exeter part as well, and test
> > > cases are fundamentally sequential/imperative.
> > > 
> > > But (sorry, it's been a few years I don't touch these):
> > > 
> > > namespace@1 {
> > >   interfaces {
> > >     lo {
> > >       address = 127.0.0.1;
> > >     };
> > >     eth0 {
> > >       address = ...;
> > >     };
> > >   };
> > >   routes {
> > >     /* something simpler than ip -j ro sh ? */
> > >   };
> > > }
> > > 
> > > ...
> > > 
> > > link@... {
> > >   vxlan {
> > >     endpoints {
> > >       a {
> > >          ns = <&namespace@1>;
> > >       };
> > >       b ...
> > > 
> > > ...
> > >       
> > > this looks much more natural to me, as an input for a simulator (I
> > > would personally make the syntax much more "elastic" by just throwing a
> > > link into a namespace but I'm trying to keep it clean just for this
> > > example).  
> > 
> > Aha, I think I finally get what you're saying.  More below.
> > 
> > > Maybe tunbridge implements this somewhere and I missed it? Or would
> > > this be part of a "Scenario" description eventually?  
> > 
> > This is entirely unrelated to what Scenario is trying to accomplish.
> > That may cause you to reconsider whether "Scenario" is a good name,
> > which is ok.
> > 
> > 
> > So.  A declarative way of defining networks would be nice to have.
> 
> From my perspective that's fundamental, rather. I gave it for granted.
> 
> > I think doing it with the flexibility we want is much harder than you
> > estimate.
> 
> I'll pretend I'm not commenting on this line by... oops. :)

Heh.  Well, I'd love to be proved wrong on this one.

> > It looks easy for simple static situations like the
> > examples above, but:
> > 
> >  * If you want to describe a topology that changes partway through,
> >    that's a huge step up in complexity, and kind of necessarily
> >    reintroduces imperative elements.
> 
> But you can use JSON or a ton of other formats that allow for ordering
> of elements.

Just ordering network elements w.r.t. each other isn't enough.  If you
have a test where you want /this/ network topology - do some stuff -
then change the toplogy to /that/ - do some more stuff.  I'm not sure
how you encode that in a way that isn't imperative, or at least
pseudo-imperative.

> Alternatively, one could add attributes for lifecycle and
> timing (think of nftables sets) but it looks much less intuitive than
> the alternative.

I wasn't previously familiar with nftables sets.  I had a quick look
at the docs, but I don't see how it relates to the current proposal.

> >    Device tree absolutely suffers
> >    from this - that's what motivated the godawful runtime overlay
> >    mechanism, and right now, I'm struggling to find time to
> >    participate in the latest of many discussions about how to better
> >    handle devices which can be runtime added and removed.
> 
> I'm not suggesting that we use ANS Forth by the way.

Well, we agree on that at least :).

> >  * If you want to build complex scenarios out of simpler ones, you
> >    need what amounts to a macro system.
> 
> There are a ton of ways. You can also use a filesystem and includes. Or
> simply definitions of blocks, not necessarily macros,

You want to parameterise the blocks - at which point it's basically
macros.

> and JSON
> implicitly gives you all that.

No, it doesn't.  If you have a JSON sub-object that's repeated several
times through a tree, you have to write it out, in full, each time.
YAML does allow repeating, as do some other JSON extensions, but bare
JSON does not.  It still doesn't allow parameterisation of those
blocks.

> As it's nothing security relevant, I would actually go with something
> that's in theory more complicated but in practice more digestible such
> as YAML.

I don't terribly like YAML, because I think there are a bunch of edge
cases where it's not obvious reading it whether something is a list or
object or somethine else.  The typing and back-referencing would be
useful for this problem, I'll grant you.

> >    Again, a big leap up in
> >    complexity.  Device tree struggles with this too - it originated
> >    primarily as a machine->machine format, where having heaps of
> >    repeated information is fine.  As it transitioned to being a
> >    human->machine format, not so much.  Hence /include/, expression
> >    support and semi-standard running dts files through cpp before
> >    compilation.  It's still pretty clunky in this regard.
> 
> It absolutely is, but that's because it was designed for a different
> purpose.
> 
> > Plus.. I think the interpreter for this hypothetical declarative
> > language would need an internal structure pretty similar to what
> > tunbridge, so this is kind of already a first step towards it.
> 
> Okay, that's good to know.
> 
> I'm estimating I'm currently writing about 5-10 scripts per month,
> including pasta/iproute2 one-liners, setting up strange stuff, to
> reproduce / debug issues.
> 
> Given that this looks so fundamental for my usage I'm thinking that I
> could make at least part of this a priority of mine.
> 
> I realised I can implement netlink stuff and handling of networking
> configuration concepts quite fast with Rust and neli, so I'm pondering
> to write a proof of concept that can parse the example above (minus
> Forth notations, but with some kind of pointer) and make it create at
> least namespaces, links, addresses, and routes.
> 
> If it helps visualising how that could possibly look like with / in
> tunbridge itself, I'll take care of it soon rather than later.
> 
> The only little problem is that I'm much faster with Rust (because of
> neli) than I can possibly picture myself with Python, and that doesn't
> play along with tunbridge. But perhaps as a proof of concept it helps
> anyway?

It would.  At earlier points I did consider writing tunbridge (or
whatever I was calling the idea at the time) in Rust.  In principle at
least, the lifetime tracking would make a very natural way for
ensuring you bring up / pull down things in a sensible order.

In practice, however, I almost instantly ran into intractable borrow
checker problems.  Specifically, Rust's notorious difficulty with
references between parts of the same structure.  That arises almost
immediately once you start building composite objects out of smaller
components:

	struct Node { ... }
	struct Veth<'a> { node0: &'a Node, node1: &'a Node, ... }

Then you want something that represents two nodes with a veth between
them and you get:

	struct TwoNodes { node0: Node, node1: Node, veth: Veth<'??> }

There are ways around it, of course, but everything I found so far was
either hideously unergonomic, depended on obscure crates or both.
There are a bunch of language extensions that would help, but while
much discussed, none have landed yet.

If a way to solve this nicely appears, I'm not against moving
tunbridge to Rust.  I mean, slightly more against it than I was months
ago before I made a start in Python, but still.

> In general, do you think there's something in particular I could
> contribute at this stage, if I want to see my declarative dream come
> true?

Yes.  A bunch of pseudo-code examples - both the network declarations
and example tests that might go with them.

[snip]
> > > > > > Path I can easily add.  Version would require an extra invocation of
> > > > > > pasta, which I don't really want to do.    
> > > > > 
> > > > > Ah, right, never mind. The path will be good enough for that.
> > > > >     
> > > > > > > This part also looks
> > > > > > > quite readable and intuitive to me without having looked into tunbridge
> > > > > > > recently.      
> > > > > > 
> > > > > > Ok, that's promising.    
> > > > > 
> > > > > I mean, I think it's all usable for the moment, and perhaps a starting
> > > > > point for some other kind of... front-end? I'm not sure. As I mentioned
> > > > > I'm a bit worried about the potential for universal intuitiveness and
> > > > > usability.    
> > > > 
> > > > So am I, but I have to weigh it against being able to re-use both
> > > > tests and setups without having to re-express both in each case.  
> > > 
> > > I think setups written like that are reusable (or can be made
> > > reusable). My usability point is about other project/usages. For passt
> > > and pasta themselves, this level or reusability looks enough to me for
> > > the foreseeable future.
> > > 
> > > Even though, one day, I guess we might want to generate pseudo-random
> > > (fractal-tree-like?) topologies (and I was recently trying out a
> > > pasta-in-pasta-in-pasta-in-pasta-in-pasta setup to reproduce that
> > > HTTP/FIN issue). For that, a declarative approach would make things
> > > easier, I suppose.  
> > 
> > Declarative, or imperative?  I actually have something like that in
> > tunbridge's selftests: a function that builds a stack of N nested
> > namespaces.
> >     https://gitlab.com/dgibson/tunbridge/-/blob/main/tunbridge/unshare.py#L302
> 
> ...but they are all the same. Think, for example, of connecting every
> odd-numbered pair with veth tunnels, and every even-numbered pair with
> pasta. Say: n1 <-- veth --> n2 <-- pasta --> n3 <-- veth --> n4.

That would certainly be possible.  More complex, of course, but not
dramtically so.

> What's really well suited for this situation, in my experience, is a
> declarative description format that can be easily generated and
> manipulated by imperative code.

Ah, so there's both an imperative and declarative component.  The idea
in tunbridge is that you can do this, but rather than emit the
"declarative part" as concrete text for another parser, it's emitted
as a data structure (generally a bunch of wrappers around context
managers).

It is true that as currently designed, tunbridge builds the data
structure representation at the same time as building the actual
simulated network.  With a declarative language approach, building the
description (language fragment) is separate from instantiating the
simulation.  Is that something you see as valuable?  Or only a side
effect of the other things about the declarative approach you like?

> The name of this kind of "indirection" in computer science research
> currently escapes me, but I'm fairly sure there must be some theory
> about it. In any case, I can include something like this in my (now
> planned) proof of concept.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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