Add Recovery composite

[corot]

Retry a failed main task after attempting one or more recovery behaviors.

This is similar to the recovery node implemented in Nav2.

I know @stonier is more in the purist side and won't like this so much, but I tried to implement the same behavior with existing components, but it's much harder than it sounds.

⚠️ This is a though one. I didn't go deep into the internals of py_trees, so I relied heavily on cGPT. Thus, I ask for a thorough and interactive review to improve and fix any issue. Particularly I have no idea about how .dot works. The file I add is at cGPT created it. I guess we need to tune somehow so this composite appears with its own color in the viewer.

Thanks!

[corot]

Hi @sea-bass , sorry for asking you for help directly. On this PR I'm waiting for your support (or any other maintainer, but you look like the only one active). What I ask

• green / red light to proceed or not with the change
• help on integrating in the GUI (and decide a new color)
• thorough review
• some guiding for updating the documentation

Thanks!

[sea-bass]

Hi @corot, my high-level though is: Instead of being a special case for the GUI etc., is there a way to treat it as another of the composites?

I guess this is where my BehaviorTree.CPP comes out, in that there the colors of the GUI correspond go leaf nodes, control nodes, and decorators only.

So maybe what we should do here is pick a new color to correspond to "other" (custom) control nodes and go that way? This would make it easy to later integrate more custom control nodes. How about purple? Really, any color that is not here

Otherwise, looks fine to me to add the new functionality.

[alisys-robotics]

😆 green
But cannot see why not to add a color for Recovery,,,, as all other composites have their own 🤔
I don't expect other "official" composites to get added to py_trees repos. I guess most other particular needs will be addressed with private implementations. I PR Recovery because I found the same limitation consistently across 3 different BT implementations!

Nice — since your Recover composite sounds like a safety or fallback mechanism (something that activates after failure), the color should suggest recovery, stability, and alertness — but still stand out from the rest.

Here are a few good options:

Color	Hex	Meaning
🟩 Emerald Green	#2ECC71	Suggests recovery, health, and successful restoration — fits the “recover” semantics perfectly.
🟦 Steel Blue	#4682B4	Conveys calm control, reliability, and system stability — great for a composite that restores order.
🟪 Slate Purple	#9370DB	Distinct yet not alarming; visually differentiates recovery logic from functional composites.
🟧 Amber	#FFBF00	A transition color between orange (sequence) and yellow (parallel), suggesting “in-between recovery mode.”

✅ Recommended choice:

'Recover': '#2ECC71'  # Emerald green for restoration/success

It’s visually distinct from your existing set and semantically intuitive (green = recover / OK).

[sea-bass]

Aren't red, green, and blue used to denote success/failure/running in the lines? I'm kinda still thinking purple for that reason. But some visuals of a real tree ticking may help us decide.

[alisys-robotics]

Aren't red, green, and blue used to denote success/failure/running in the lines? I'm kinda still thinking purple for that reason. But some visuals of a real tree ticking may help us decide.

Ah, true. Let's try purple then

Adding the color there is the only required change?