Async Payments #2298

Async-receive:

• Support being the async receiver’s channel counterparty (eg LSP) per the spec PR
  • Support store-and-forward mailbox for onion messages Onion Message Mailbox #2950, Support intercepting onion messages for offline peers #2973
  • LDK Node support Add onion mailbox for async recipients ldk-node#632
  • Fix rare race condition between peer disconnection and onion message mailbox forward Onion Message Mailbox #2950
• Support being the always-online (for now) sender
  • Notify the receiver that an HTLC is being held for them Support paying static invoices #3140
  • Send HTLC when notified by the receiver Support paying static invoices #3140, Include invoice requests in async payment onions #3207
  • Support manual handling for static invoices
  • Time out async payments prior to invoice expiry (cc Support paying static invoices #3140 (comment)) add expiry_time to PendingOutboundPayment::StaticInvoiceReceived #3918
  • Don't take() invoice requests on retry (cc Support paying static invoices #3140 (comment)): Add static invoice creation utils to ChannelManager #3408
• Support being the async receiver
  • Support receiving keysend payments to blinded paths Support keysend to blinded paths #2935, Verify blinded keysend PaymentSecrets #2956, Verify blinded keysend payments #3383
  • Support creating a AP BOLT12 offer + keysend invoice that signals that the recipient is often-offline + their LSP’s contact info Add static invoice creation utils to ChannelManager #3408
  • Support receiving async payments to static invoices Async receive prefactor  #3517, Support receiving async payments #3440
  • Spec the above Support async payments in BOLT 12 lightning/bolts#1149
    • Update the spec for static invoices having a different default relative expiry than non-static
    • Update the spec for how the async sender sets the reply path to held_htlc_available (that terminates at the sender's always-online channel counterparty)
  • Authenticate async payment blinded payment paths, cc Add static invoice creation utils to ChannelManager #3408 (comment): fixed in Authenticate blinded payment paths #3435
  • Time out held_htlc_available OM paths, cc Add static invoice creation utils to ChannelManager #3408 (comment)
• Fuzz StaticInvoice message Fuzz StaticInvoice #4005
• Support interactively building a static invoice, async recipient <> static invoice server
  • Async recipient side Async recipient-side of static invoice server  #3618
  • Comments from review Async recipient-side of static invoice server  #3618 (review): #3618 followups + expose async receive feature #3999
  • Limit the number of messages sent to the server Async recipient-side of static invoice server  #3618, #3618 followups + expose async receive feature #3999
  • Server side support for persisting + replying with StaticInvoices Static invoice server prefactor #3667, Static invoice server #3628
  • Server side support for forwarding invoice requests to payee Always-online node forward invoice request #4049
  • Authenticate static invoice server messages: Introduce ReceiveAuthKey #3917
  • LDK Node support Static invoice server ldk-node#621
• Follow-ups to static invoice server MVP
  • Move more code into async_receive_offer_cache.rs
  • Consider supporting async receive offers for a specified amount
  • Support the server configuring how many invoices they'll store for the user
  • Invalidate offers/static invoices when a channel opens/closes/fees change
  • Figure out how to not persist the cache in ChannelManager
  • bLIP for the protocol
    • link to bLIP in LDK code

Async-send:

• Simple hold Async Send Simple Hold #3957, Async send always-online counterparty side #4045, Support multi-hop blinded paths for ReleaseHeldHtlc  #4066
• Trampoline Trampoline #2299
• Router for LSP clients that routes to LSP node as the direct Trampoline hop
• Parse AP BOLT12 invoices
• Support sending an async HTLC and corresponding OM Async send: sender-side  #4046
• Support retrying the OM or spec out+implement an ACK OM
• Generate event when an async payment is pending (see Support paying static invoices #3140 (comment))
• Add RecentPaymentDetails variant for async payments (see Support paying static invoices #3140 (comment))
• Support shortening the HTLC’s CLTV at sending-time