Fixes: #15016 - Catch AssertionError from cable trace and throw ValidationError

[DanSheps]

Fixes: #15016 - Catch AssertionError from cable trace and throw ValidationError

• Catch any assertion errors thrown by the Cable save signal and raise a validation error
• Catch validation error in ObjectEditView and report error in UI while re-rendering form

[jeremystretch]

The use of assert statements in this logic was primarily for sanity-checking during initial development; they shouldn't be relied upon as true validation checks. (They aren't even run when __debug__ is False.)

If the use case in #15016 is not supported, we need to add specific validation logic to check for and report on it.

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[jeremystretch]

I'm afraid this still doesn't address my primary concern: Validation of cable state needs to be performed prior to the point where we're generating cable paths. (Again, the original assertions were just sanity checks.) This should ideally be performed on each model under clean().

Additionally, as a rule generic views should not check for and handle any model-specific exceptions.

[DanSheps]

I'm afraid this still doesn't address my primary concern: Validation of cable state needs to be performed prior to the point where we're generating cable paths. (Again, the original assertions were just sanity checks.) This should ideally be performed on each model under clean().

The issue is there isn't really a way to validate the cable state prior to the cable being saved, without effectively duplicating all the work done in the cable path generation.

The main reason for this is because a lot of these assertion checks are dependant on sibling cable state further down the path, not just part of the existing cable save, which isn't known until you generate the full path.

We would effectively be running the from_origin twice in this instance, once to validate the cable state and once to save it.

Take for example this path, where Switch 1, interface1 is what you are connecting (the rest is fully formed):

                     /----[FP1-1]                       [FP2-1]----[FP3-1]                      [FP4-1]---- 
[Switch1][Interface1]             [PP1][RP]----[RP][PP2]                   [PP3][RP]----[RP][PP4]                 
                     \----[FP1-2]                       [FP2-2]----[FP3-2]                      [FP4-2]----

You can validate FP1-1 and FP1-2 because they are connected.

But you can't run any of the peer validations on FP2-1 and FP2-2 because FP2-1 doesn't know that FP2-2 will be part of that same path until Interface1 is connect and that path is fully formed.

I can take another pass at it, and see if I can have it run this in clean() first, but most of the parts of this rely on objects existing in the database already as well, so this would likely need a complete re-write, which I was trying to avoid doing.

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