Hello guys,While looking at the https://github.com/avocado-framework/avocado-virt/pull/90 I noticed quite serious issue in the current multiplexer design and if I understand it correctly even in the ideal multiplexer design (avocado still does not support the full specification). I'm not sure whether I just don't see something obvious so please take a look at it.
The problem is, that in that PR Fam assigns the value of non-leaf item. That works well, the problem is when one tries to multiplex it, because he changes the non-leaf object to !mux and it adds the other branches as different mux domains.
Let's make it concrete. The idea is to expect `kvm = on|off` in `/plugins/virt/qemu/*` path. This path contains several children already (`/plugins/virt/qemu/paths` and `/plugins/virt/qemu/migrate`), which still works well (one can generate similar output with `avocado multiplex --system-wide ...` when avocado-virt is installed; note you can't use `-s` as it clashes with `--silent`, PR to fix it has been sent):
```
┗━━ plugins
┗━━ virt
┣━━ qemu
┃ ┣━━ paths
┃ ┃ → qemu_dst_bin: None
┃ ┃ → qemu_img_bin: None
┃ ┃ → kvm: on
┃ ┃ → qemu_bin: None
┃ ┗━━ migrate
┃ → kvm: on
┃ → timeout: 60.0
```
Now when you want to vary between different values, you might want to
inject something like:
```
plugins:
virt:
qemu: !mux
enabled:
kvm: on
disabled:
qemu: off
```
The problem is, that it affects the whole `/plugins/virt/qemu` node and
all it's children become multiplexed:
```
┗━━ plugins
┗━━ virt
┣━━ qemu
┃ ╠══ enabled
┃ ║ → kvm: on
┃ ╠══ disabled
┃ ║ → kvm: off
┃ ╠══ paths
┃ ║ → qemu_dst_bin: None
┃ ║ → qemu_img_bin: None
┃ ║ → qemu_bin: None
┃ ╚══ migrate
┃ → timeout: 60.0
```
The original multiplexer RFC wanted to always allow left side to be
optional. This currently works only for paths inside mux-paths (like
/run). Anyway let's assume it works and explore the consequences. The
query path would change to `*/plugins/virt/qemu/*` and we'd inject the
values on a different place:
```
┣━━ my
┃ ┗━━ kvm
┃ ┗━━ plugins
┃ ┗━━ virt
┃ ┗━━ qemu
┃ ╠══ enabled
┃ ║ → kvm: on
┃ ╚══ disabled
┃ → kvm: off
┗━━ plugins
┗━━ virt
┣━━ qemu
┃ ┣━━ paths
┃ ┃ → qemu_dst_bin: None
┃ ┃ → qemu_img_bin: None
┃ ┃ → qemu_bin: None
┃ ┗━━ migrate
┃ → timeout: 60.0
```
1. mux looks into mux-path (`/run`) first, can't find the
`kvm/plugins/virt/qemu` there
2. it looks into `/` and finds it in `/my/kvm/plugins/virt/qemu` -> successThe problem is, when we use defaults (eg. the way we define default values in avocado-virt):
```
┣━━ my
┃ ┗━━ kvm
┃ ┗━━ plugins
┃ ┗━━ virt
┃ ┗━━ qemu
┃ ╠══ enabled
┃ ║ → kvm: on
┃ ╚══ disabled
┃ → kvm: off
┗━━ plugins
┗━━ virt
┣━━ qemu
┃ ┣━━ paths
┃ ┃ → qemu_dst_bin: None
┃ ┃ → qemu_img_bin: None
┃ ┃ → kvm: Something_else
┃ ┃ → qemu_bin: None
┃ ┗━━ migrate
┃ → kvm: Something_else
┃ → timeout: 60.0
```
1. mux looks into mux-path (`/run`) first, can't find the
`kvm/plugins/virt/qemu` there
2. it looks into `/` and finds it in `/my/kvm/plugins/virt/qemu` and in
`/plugins/virt/qemu` -> failure
Yes, one could solve it by defining another `mux-path` to `/my` or even `/my/kvm`, but that just adds the complexity.
Let me also mention why do we like to extend nodes from right. Imagine we expect `disk_type` in `/virt/hw/disk/*`. The yaml file might look like this:
```
virt:
hw:
disk: !mux
virtio_blk:
disk_type: virtio_blk
virtio_scsi:
disk_type: virtio_scsi
```
Now the user develops `virtio_scsi_next` and he wants to compare them.
Today he simply merges this config with the above:
```
virt:
hw:
disk: !mux
virtio_scsi_debug:
disk_type: virtio_scsi
enable_next: True
```
and avocado produces 3 variants, where `params.get("disk_type",
"/virt/hw/disk/*")` reports the 3 defined variants. If we try to do the
same with `*/virt/hw/disk` we have to modify the first file:
```
!mux
virtio_blk:
virt:
hw:
disk:
disk_type: virtio_blk
virtio_scsi:
virt:
hw:
disk:
disk_type: virtio_scsi
```
One would want to prepend yet another node in front of it, because we
don't want to vary over disk types only, but also over other items (like
cpus, ...). The problem is, that the first category has to again be
unique to the whole multiplex tree in order to not clash with the other
items. And that is what the tree path was actually introduced, to get
rid of this global-namespace.
Right now the only solution I see is to change the way `!mux` works. Currently it multiplexes all the children, but (not sure if easily done) it should only define the children, which mix together. Therefor (back to the original example) one would be able to say:
```
plugins:
virt:
qemu:
enabled: !newmux
kvm: on
disabled: !newmux
kvm: off
paths:
qemu_dst_bin: None
qemu_img_bin: None
qemu_bin: None
migrate:
timeout: 60.0
```
which would produce:
```
┗━━ plugins
┗━━ virt
┣━━ qemu
┃ ╠══ enabled
┃ ║ → kvm: on
┃ ╠══ disabled
┃ ┃ → kvm: off
┃ ┣━━ paths
┃ ┃ → qemu_dst_bin: None
┃ ┃ → qemu_img_bin: None
┃ ┃ → qemu_bin: None
┃ ┗━━ migrate
┃ → timeout: 60.0
```
and in terms of variants:
```
Variant 1: /plugins/virt/qemu/enabled, /plugins/virt/paths,
/plugins/virt/migrate
Variant 2: /plugins/virt/qemu/disabled, /plugins/virt/paths,
/plugins/virt/migrate
```I'm looking forward to your suggestions and I hope I'm wrong and that the multiplexer (at least the full-spec) can handle this nicely.
Kind regards, Lukáš
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