Document that proxies should not raise specific exceptions `AlreadyRegisteredTokenAddress` etc.

[pirapira]

I thought something like below, but there is a reason not to do that according to @rakanalh and @hackaugusto. This issue keeps track of documenting the reason why proxies are not supposed to raise specific exceptions such as AlreadyRegisteredTokenAddress.

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In Python, programmers can define new exceptions. So at the cause of the exception, the programmer can raise a specific exception that describes the situation. Currently in Raiden, sometimes the API function catches a generic RaidenRecoverableError and converts that to more specific AlreadyRegisteredTokenAddress and so on.

This issue keeps track of stopping that, by

• raising specific exceptions from the proxies, and
• classifying these specific exceptions as RaidenRecoverableError (not RaidenError).

[rakanalh]

Raising specific exceptions from the proxies is not something we want to do (that was decided when @hackaugusto and I introduced BrokenPreconditionError). The line you're referring to is an oversight that should be fixed by just re-raising the RaidenRecoverableError.

[pirapira]

@rakanalh I'll document that decision in the proxy guide in the documentation.

Do you have the log for the decision?

[hackaugusto]

Document that proxies should not raise specific exceptions

@pirapira There are currently 229 raise statements in the proxies grep raise *.py | wc -l. Each of this exceptions is covering a distinct scenario. Having a single exception for each possibility would simple be too much.

With the above said, even if we did use a separate exception type for each error, AlreadyRegisteredTokenAddress would not be the correct exception. Currently the exception AlreadyRegisteredTokenAddress is used for input validation, in the modules raiden.api.rest and raiden.api.python. Validation errors are not unrecoverable, they are just validation issues that must return a human readable message, and not crash the client.

[pirapira]

Of course AlreadyRegisteredTokenAddress is recoverable. So I'm doing class AlreadyRegisteredTokenAddress(RaidenRecoverableError). https://github.com/raiden-network/raiden/pull/4879/files#diff-c48bfeb29bf18bd2ec0add25841f1ce8R155

[hackaugusto]

This should be a ValidationError

raiden/raiden/exceptions.py

Lines 44 to 49 in 1a690cf

	class RaidenValidationError(RaidenRecoverableError):
	"""Exception raised when an input value is invalid.
	This exception must be raised on the edges of the system, to inform the
	caller one of the provided values is invalid.
	"""

, I would still keep these hierarchies disjoint, they are not used for the same thing.

[pirapira]

The user provided a valid address. It's not the problem with the value. It's the problem with the chain that has registered the address already.

[pirapira]

Then I'll first update the docstring for RaidenValidationError.

[pirapira]

#4952 I extended the docstring a bit.

[pirapira]

Whether or not proxies raise AlreadyRegisteredTokenAddress, this exception already exists in the codebase because raiden/api/python.py raises it. So I don't see a reason why a proxy should not raise it.