Client floods server with requests after a topup

[devium]

This is very much notable in an on-chain test of test_session_topup:
After topping up, the client will resume paying the server on the topped up channel. The server does not accept the topup yet for a few more blocks. The response includes a RDN-Invalid-Amount header, causing the client to adopt the server's last known "valid" amount and retrying the payment immediately again.

The client cannot differentiate between a balance signature update/recovery (which also returns a RDN-Invalid-Amount header) and an unconfirmed topup.

So there are two possible solutions for this:

• Introduce a new error header RDN-Insufficient-Funds (which exists but is currently unused) or something along those lines, to indicate that this is a different error than an outdated balance signature or wrong price paid. This could be used in combination with RDN-Insufficient-Confirmations.
• Introduce some client-side-only logic that tries to detect situations like this (for example by checking if the exact price requested has already been paid and the channel has recently been topped up). Though I think this might be too unnecessarily complex.

[pcppcp]

New error header will do it. Much better than wild client-side guess which may introduce new errors.