i2p: destroy the session if we get an unexpected error from the I2P router

From https://geti2p.net/en/docs/api/samv3:

  If SILENT=false was passed, which is the default value, the SAM bridge
  sends the client a ASCII line containing the base64 public destination
  key of the requesting peer

So, `Accept()` is supposed to receive a Base64 encoded destination of
the connecting peer, but if it receives something like this instead:

  STREAM STATUS RESULT=I2P_ERROR MESSAGE="Session was closed"

then destroy the session.

src/i2p.cpp

@@ -12,6 +12,7 @@
 #include <netaddress.h>
 #include <netbase.h>
 #include <random.h>
+#include <sync.h>
 #include <tinyformat.h>
 #include <util/fs.h>
 #include <util/readwritefile.h>
@@ -153,27 +154,59 @@ bool Session::Listen(Connection& conn)
 
 bool Session::Accept(Connection& conn)
 {
-    try {
-        while (!*m_interrupt) {
-            Sock::Event occurred;
-            if (!conn.sock->Wait(MAX_WAIT_FOR_IO, Sock::RECV, &occurred)) {
-                throw std::runtime_error("wait on socket failed");
-            }
+    AssertLockNotHeld(m_mutex);
 
-            if (occurred == 0) {
-                // Timeout, no incoming connections or errors within MAX_WAIT_FOR_IO.
-                continue;
-            }
+    std::string errmsg;
+    bool disconnect{false};
 
-            const std::string& peer_dest =
-                conn.sock->RecvUntilTerminator('\n', MAX_WAIT_FOR_IO, *m_interrupt, MAX_MSG_SIZE);
-
-            conn.peer = CService(DestB64ToAddr(peer_dest), I2P_SAM31_PORT);
-
-            return true;
+    while (!*m_interrupt) {
+        Sock::Event occurred;
+        if (!conn.sock->Wait(MAX_WAIT_FOR_IO, Sock::RECV, &occurred)) {
+            errmsg = "wait on socket failed";
+            break;
         }
-    } catch (const std::runtime_error& e) {
-        Log("Error accepting: %s", e.what());
+
+        if (occurred == 0) {
+            // Timeout, no incoming connections or errors within MAX_WAIT_FOR_IO.
+            continue;
+        }
+
+        std::string peer_dest;
+        try {
+            peer_dest = conn.sock->RecvUntilTerminator('\n', MAX_WAIT_FOR_IO, *m_interrupt, MAX_MSG_SIZE);
+        } catch (const std::runtime_error& e) {
+            errmsg = e.what();
+            break;
+        }
+
+        CNetAddr peer_addr;
+        try {
+            peer_addr = DestB64ToAddr(peer_dest);
+        } catch (const std::runtime_error& e) {
+            // The I2P router is expected to send the Base64 of the connecting peer,
+            // but it may happen that something like this is sent instead:
+            // STREAM STATUS RESULT=I2P_ERROR MESSAGE="Session was closed"
+            // In that case consider the session damaged and close it right away,
+            // even if the control socket is alive.
+            if (peer_dest.find("RESULT=I2P_ERROR") != std::string::npos) {
+                errmsg = strprintf("unexpected reply that hints the session is unusable: %s", peer_dest);
+                disconnect = true;
+            } else {
+                errmsg = e.what();
+            }
+            break;
+        }
+
+        conn.peer = CService(peer_addr, I2P_SAM31_PORT);
+
+        return true;
+    }
+
+    Log("Error accepting%s: %s", disconnect ? " (will close the session)" : "", errmsg);
+    if (disconnect) {
+        LOCK(m_mutex);
+        Disconnect();
+    } else {
         CheckControlSock();
     }
     return false;

src/i2p.h

@@ -105,7 +105,7 @@ public:
      * completion the `peer` member will be set to the address of the incoming peer.
      * @return true on success
      */
-    bool Accept(Connection& conn);
+    bool Accept(Connection& conn) EXCLUSIVE_LOCKS_REQUIRED(!m_mutex);
 
     /**
      * Connect to an I2P peer.

src/test/i2p_tests.cpp

@@ -16,14 +16,34 @@
 #include <memory>
 #include <string>
 
-BOOST_FIXTURE_TEST_SUITE(i2p_tests, BasicTestingSetup)
+/// Save the log level and the value of CreateSock and restore them when the test ends.
+class EnvTestingSetup : public BasicTestingSetup
+{
+public:
+    explicit EnvTestingSetup(const ChainType chainType = ChainType::MAIN,
+                             const std::vector<const char*>& extra_args = {})
+        : BasicTestingSetup{chainType, extra_args},
+          m_prev_log_level{LogInstance().LogLevel()},
+          m_create_sock_orig{CreateSock}
+    {
+        LogInstance().SetLogLevel(BCLog::Level::Trace);
+    }
+
+    ~EnvTestingSetup()
+    {
+        CreateSock = m_create_sock_orig;
+        LogInstance().SetLogLevel(m_prev_log_level);
+    }
+
+private:
+    const BCLog::Level m_prev_log_level;
+    const std::function<std::unique_ptr<Sock>(const CService&)> m_create_sock_orig;
+};
+
+BOOST_FIXTURE_TEST_SUITE(i2p_tests, EnvTestingSetup)
 
 BOOST_AUTO_TEST_CASE(unlimited_recv)
 {
-    const auto prev_log_level{LogInstance().LogLevel()};
-    LogInstance().SetLogLevel(BCLog::Level::Trace);
-    auto CreateSockOrig = CreateSock;
-
     // Mock CreateSock() to create MockSock.
     CreateSock = [](const CService&) {
         return std::make_unique<StaticContentsSock>(std::string(i2p::sam::MAX_MSG_SIZE + 1, 'a'));
@@ -40,9 +60,69 @@ BOOST_AUTO_TEST_CASE(unlimited_recv)
         bool proxy_error;
         BOOST_REQUIRE(!session.Connect(CService{}, conn, proxy_error));
     }
+}
 
-    CreateSock = CreateSockOrig;
-    LogInstance().SetLogLevel(prev_log_level);
+BOOST_AUTO_TEST_CASE(listen_ok_accept_fail)
+{
+    size_t num_sockets{0};
+    CreateSock = [&num_sockets](const CService&) {
+        // clang-format off
+        ++num_sockets;
+        // First socket is the control socket for creating the session.
+        if (num_sockets == 1) {
+            return std::make_unique<StaticContentsSock>(
+                // reply to HELLO
+                "HELLO REPLY RESULT=OK VERSION=3.1\n"
+                // reply to DEST GENERATE
+                "DEST REPLY PUB=WnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqFpxji10Qah0IXVYxZVqkcScM~Yccf9v8BnNlaZbWtSoWnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqFpxji10Qah0IXVYxZVqkcScM~Yccf9v8BnNlaZbWtSoWnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqFpxji10Qah0IXVYxZVqkcScM~Yccf9v8BnNlaZbWtSoWnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqLE4SD-yjT48UNI7qiTUfIPiDitCoiTTz2cr4QGfw89rBQAEAAcAAA== PRIV=WnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqFpxji10Qah0IXVYxZVqkcScM~Yccf9v8BnNlaZbWtSoWnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqFpxji10Qah0IXVYxZVqkcScM~Yccf9v8BnNlaZbWtSoWnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqFpxji10Qah0IXVYxZVqkcScM~Yccf9v8BnNlaZbWtSoWnGOLXRBqHQhdVjFlWqRxJwz9hxx~2~wGc2Vplta1KhacY4tdEGodCF1WMWVapHEnDP2HHH~b~AZzZWmW1rUqLE4SD-yjT48UNI7qiTUfIPiDitCoiTTz2cr4QGfw89rBQAEAAcAAOvuCIKTyv5f~1QgGq7XQl-IqBULTB5WzB3gw5yGPtd1p0AeoADrq1ccZggLPQ4ZLUsGK-HVw373rcTfvxrcuwenqVjiN4tbbYLWtP7xXGWj6fM6HyORhU63GphrjEePpMUHDHXd3o7pWGM-ieVVQSK~1MzF9P93pQWI3Do52EeNAayz4HbpPjNhVBzG1hUEFwznfPmUZBPuaOR4-uBm1NEWEuONlNOCctE4-U0Ukh94z-Qb55U5vXjR5G4apmBblr68t6Wm1TKlzpgFHzSqLryh3stWqrOKY1H0z9eZ2z1EkHFOpD5LyF6nf51e-lV7HLMl44TYzoEHK8RRVodtLcW9lacVdBpv~tOzlZERIiDziZODPETENZMz5oy9DQ7UUw==\n"
+                // reply to SESSION CREATE
+                "SESSION STATUS RESULT=OK\n"
+                // dummy to avoid reporting EOF on the socket
+                "a"
+            );
+        }
+        // Subsequent sockets are for recreating the session or for listening and accepting incoming connections.
+        if (num_sockets % 2 == 0) {
+            // Replies to Listen() and Accept()
+            return std::make_unique<StaticContentsSock>(
+                // reply to HELLO
+                "HELLO REPLY RESULT=OK VERSION=3.1\n"
+                // reply to STREAM ACCEPT
+                "STREAM STATUS RESULT=OK\n"
+                // continued reply to STREAM ACCEPT, violating the protocol described at
+                // https://geti2p.net/en/docs/api/samv3#Accept%20Response
+                // should be base64, something like
+                // "IchV608baDoXbqzQKSqFDmTXPVgoDbPAhZJvNRXXxi4hyFXrTxtoOhdurNApKoUOZNc9WCgNs8CFkm81FdfGLiHIVetPG2g6F26s0CkqhQ5k1z1YKA2zwIWSbzUV18YuIchV608baDoXbqzQKSqFDmTXPVgoDbPAhZJvNRXXxi4hyFXrTxtoOhdurNApKoUOZNc9WCgNs8CFkm81FdfGLiHIVetPG2g6F26s0CkqhQ5k1z1YKA2zwIWSbzUV18YuIchV608baDoXbqzQKSqFDmTXPVgoDbPAhZJvNRXXxi4hyFXrTxtoOhdurNApKoUOZNc9WCgNs8CFkm81FdfGLiHIVetPG2g6F26s0CkqhQ5k1z1YKA2zwIWSbzUV18YuIchV608baDoXbqzQKSqFDmTXPVgoDbPAhZJvNRXXxi4hyFXrTxtoOhdurNApKoUOZNc9WCgNs8CFkm81FdfGLlSreVaCuCS5sdb-8ToWULWP7kt~lRPDeUNxQMq3cRSBBQAEAAcAAA==\n"
+                "STREAM STATUS RESULT=I2P_ERROR MESSAGE=\"Session was closed\"\n"
+            );
+        } else {
+            // Another control socket, but without creating a destination (it is cached in the session).
+            return std::make_unique<StaticContentsSock>(
+                // reply to HELLO
+                "HELLO REPLY RESULT=OK VERSION=3.1\n"
+                // reply to SESSION CREATE
+                "SESSION STATUS RESULT=OK\n"
+                // dummy to avoid reporting EOF on the socket
+                "a"
+            );
+        }
+        // clang-format on
+    };
+
+    CThreadInterrupt interrupt;
+    i2p::sam::Session session(gArgs.GetDataDirNet() / "test_i2p_private_key",
+                              CService{in6_addr(IN6ADDR_LOOPBACK_INIT), /*port=*/7656},
+                              &interrupt);
+
+    i2p::Connection conn;
+    for (size_t i = 0; i < 5; ++i) {
+        ASSERT_DEBUG_LOG("Creating persistent SAM session");
+        ASSERT_DEBUG_LOG("Persistent SAM session" /* ... created */);
+        ASSERT_DEBUG_LOG("Error accepting");
+        ASSERT_DEBUG_LOG("Destroying SAM session");
+        BOOST_REQUIRE(session.Listen(conn));
+        BOOST_REQUIRE(!session.Accept(conn));
+    }
 }
 
 BOOST_AUTO_TEST_SUITE_END()