Merge bitcoin/bitcoin#28077: I2P: also sleep after errors in Accept() & destroy the session if we get an unexpected error

5c8e15c451 i2p: destroy the session if we get an unexpected error from the I2P router (Vasil Dimov)
762404a68c i2p: also sleep after errors in Accept() (Vasil Dimov)

Pull request description:

  ### Background

  In the `i2p::sam::Session` class:

  `Listen()` does:
  * if the session is not created yet
    * create the control socket and on it:
    * `HELLO`
    * `SESSION CREATE ID=sessid`
    * leave the control socked opened
  * create a new socket and on it:
  * `HELLO`
  * `STREAM ACCEPT ID=sessid`
  * read reply (`STREAM STATUS`), `Listen()` only succeeds if it contains `RESULT=OK`

  Then a wait starts, for a peer to connect. When connected,

  `Accept()` does:
  * on the socket from `STREAM ACCEPT` from `Listen()`: read the Base64 identification of the connecting peer

  ### Problem

  The I2P router may be in such a state that this happens in a quick succession (many times per second, see https://github.com/bitcoin/bitcoin/issues/22759#issuecomment-1609907115): `Listen()`-succeeds, `Accept()`-fails.

  `Accept()` fails because the I2P router sends something that is not Base64 on the socket: `STREAM STATUS RESULT=I2P_ERROR MESSAGE="Session was closed"`

  We only sleep after failed `Listen()` because the assumption was that if `Accept()` fails then the next `Listen()` will also fail.

  ### Solution

  Avoid filling the log with "Error accepting:" messages and sleep also after a failed `Accept()`.

  ### Extra changes

  * Reset the error waiting time after one successful connection. Otherwise the timer will remain high due to problems that have been solved long time in the past.

  * Increment the wait time less aggressively.

  * Handle the unexpected "Session was closed" message more gracefully (don't log stupid messages like `Cannot decode Base64: "STREAM STATUS...`) and destroy the session right way.

ACKs for top commit:
  achow101:
    ACK 5c8e15c451
  jonatack:
    re-ACK 5c8e15c451

Tree-SHA512: 1d47958c50eeae9eefcb668b8539fd092adead93328e4bf3355267819304b99ab41cbe1b5dbedbc3452c2bc389dc8330c0e27eb5ccb880e33dc46930a1592885

src/net.cpp

@@ -2928,6 +2928,13 @@ void CConnman::ThreadI2PAcceptIncoming()
     bool advertising_listen_addr = false;
     i2p::Connection conn;
 
+    auto SleepOnFailure = [&]() {
+        interruptNet.sleep_for(err_wait);
+        if (err_wait < err_wait_cap) {
+            err_wait += 1s;
+        }
+    };
+
     while (!interruptNet) {
 
         if (!m_i2p_sam_session->Listen(conn)) {
@@ -2935,12 +2942,7 @@ void CConnman::ThreadI2PAcceptIncoming()
                 RemoveLocal(conn.me);
                 advertising_listen_addr = false;
             }
-
-            interruptNet.sleep_for(err_wait);
-            if (err_wait < err_wait_cap) {
-                err_wait *= 2;
-            }
-
+            SleepOnFailure();
             continue;
         }
 
@@ -2950,11 +2952,14 @@ void CConnman::ThreadI2PAcceptIncoming()
         }
 
         if (!m_i2p_sam_session->Accept(conn)) {
+            SleepOnFailure();
             continue;
         }
 
         CreateNodeFromAcceptedSocket(std::move(conn.sock), NetPermissionFlags::None,
                                      CAddress{conn.me, NODE_NONE}, CAddress{conn.peer, NODE_NONE});
+
+        err_wait = err_wait_begin;
     }
 }