Remove ReceiveSession::TerminalFailure

`TerminalFailure` was never a real state in the receiver state machine.
Instead, a SessionEvent::Closed simply results in an empty `()` next
state.

Author: spacebear21

payjoin-cli/src/app/v2/mod.rs

@@ -71,7 +71,6 @@ impl StatusText for ReceiveSession {
             | ReceiveSession::ProvisionalProposal(_) => "Processing original proposal",
             ReceiveSession::PayjoinProposal(_) => "Payjoin proposal sent",
             ReceiveSession::HasReplyableError(_) => "Session failure",
-            ReceiveSession::TerminalFailure => "Session failure",
         }
     }
 }
@@ -524,8 +523,6 @@ impl App {
                     self.send_payjoin_proposal(proposal, persister).await,
                 ReceiveSession::HasReplyableError(error) =>
                     self.handle_error(error, persister).await,
-                ReceiveSession::TerminalFailure =>
-                    return Err(anyhow!("Terminal receiver session")),
             }
         };
         res

payjoin-ffi/src/receive/mod.rs

@@ -79,7 +79,6 @@ pub enum ReceiveSession {
     ProvisionalProposal { inner: Arc<ProvisionalProposal> },
     PayjoinProposal { inner: Arc<PayjoinProposal> },
     HasReplyableError { inner: Arc<HasReplyableError> },
-    TerminalFailure,
 }
 
 impl From<payjoin::receive::v2::ReceiveSession> for ReceiveSession {
@@ -108,7 +107,6 @@ impl From<payjoin::receive::v2::ReceiveSession> for ReceiveSession {
                 Self::PayjoinProposal { inner: Arc::new(inner.into()) },
             ReceiveSession::HasReplyableError(inner) =>
                 Self::HasReplyableError { inner: Arc::new(inner.into()) },
-            ReceiveSession::TerminalFailure => Self::TerminalFailure,
         }
     }
 }

payjoin/src/core/receive/v2/mod.rs

@@ -122,7 +122,7 @@ fn short_id_from_pubkey(pubkey: &HpkePublicKey) -> ShortId {
 }
 
 /// Represents the various states of a Payjoin receiver session during the protocol flow.
-/// Each variant parameterizes a `Receiver` with a specific state type, and [`ReceiveSession::TerminalFailure`] which indicates the session has ended or is invalid.
+/// Each variant parameterizes a `Receiver` with a specific state type.
 ///
 /// This provides type erasure for the receive session state, allowing for the session to be replayed
 /// and the state to be updated with the next event over a uniform interface.
@@ -139,7 +139,6 @@ pub enum ReceiveSession {
     ProvisionalProposal(Receiver<ProvisionalProposal>),
     PayjoinProposal(Receiver<PayjoinProposal>),
     HasReplyableError(Receiver<HasReplyableError>),
-    TerminalFailure,
 }
 
 impl ReceiveSession {
@@ -204,8 +203,6 @@ impl ReceiveSession {
                         ReceiveSession::ProvisionalProposal(r) => r.session_context,
                         ReceiveSession::PayjoinProposal(r) => r.session_context,
                         ReceiveSession::HasReplyableError(r) => r.session_context,
-                        ReceiveSession::TerminalFailure =>
-                            return Ok(ReceiveSession::TerminalFailure),
                     },
                 })),