test: add I2P test for a runaway SAM proxy

Add a regression test for https://github.com/bitcoin/bitcoin/pull/21407.

The test creates a socket that, upon read, returns some data, but never
the expected terminator `\n`, injects that socket into the I2P code and
expects `i2p::sam::Session::Connect()` to fail, printing a specific
error message to the log.

src/Makefile.test.include

@@ -90,6 +90,7 @@ BITCOIN_TESTS =\
   test/fs_tests.cpp \
   test/getarg_tests.cpp \
   test/hash_tests.cpp \
+  test/i2p_tests.cpp \
   test/interfaces_tests.cpp \
   test/key_io_tests.cpp \
   test/key_tests.cpp \

src/test/i2p_tests.cpp

@@ -0,0 +1,44 @@
+// Copyright (c) 2021-2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <i2p.h>
+#include <netaddress.h>
+#include <test/util/logging.h>
+#include <test/util/net.h>
+#include <test/util/setup_common.h>
+#include <threadinterrupt.h>
+#include <util/system.h>
+
+#include <boost/test/unit_test.hpp>
+
+#include <memory>
+#include <string>
+
+BOOST_FIXTURE_TEST_SUITE(i2p_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(unlimited_recv)
+{
+    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'));
+    };
+
+    CThreadInterrupt interrupt;
+    i2p::sam::Session session(GetDataDir() / "test_i2p_private_key", CService{}, &interrupt);
+
+    {
+        ASSERT_DEBUG_LOG("Creating SAM session");
+        ASSERT_DEBUG_LOG("too many bytes without a terminator");
+
+        i2p::Connection conn;
+        bool proxy_error;
+        BOOST_REQUIRE(!session.Connect(CService{}, conn, proxy_error));
+    }
+
+    CreateSock = CreateSockOrig;
+}
+
+BOOST_AUTO_TEST_SUITE_END()

src/test/util/net.h

@@ -5,7 +5,13 @@
 #ifndef BITCOIN_TEST_UTIL_NET_H
 #define BITCOIN_TEST_UTIL_NET_H
 
+#include <compat.h>
 #include <net.h>
+#include <util/sock.h>
+
+#include <cassert>
+#include <cstring>
+#include <string>
 
 struct ConnmanTestMsg : public CConnman {
     using CConnman::CConnman;
@@ -61,4 +67,67 @@ constexpr ConnectionType ALL_CONNECTION_TYPES[]{
     ConnectionType::ADDR_FETCH,
 };
 
+/**
+ * A mocked Sock alternative that returns a statically contained data upon read and succeeds
+ * and ignores all writes. The data to be returned is given to the constructor and when it is
+ * exhausted an EOF is returned by further reads.
+ */
+class StaticContentsSock : public Sock
+{
+public:
+    explicit StaticContentsSock(const std::string& contents) : m_contents{contents}, m_consumed{0}
+    {
+        // Just a dummy number that is not INVALID_SOCKET.
+        static_assert(INVALID_SOCKET != 1000);
+        m_socket = 1000;
+    }
+
+    ~StaticContentsSock() override { Reset(); }
+
+    StaticContentsSock& operator=(Sock&& other) override
+    {
+        assert(false && "Move of Sock into MockSock not allowed.");
+        return *this;
+    }
+
+    void Reset() override
+    {
+        m_socket = INVALID_SOCKET;
+    }
+
+    ssize_t Send(const void*, size_t len, int) const override { return len; }
+
+    ssize_t Recv(void* buf, size_t len, int flags) const override
+    {
+        const size_t consume_bytes{std::min(len, m_contents.size() - m_consumed)};
+        std::memcpy(buf, m_contents.data() + m_consumed, consume_bytes);
+        if ((flags & MSG_PEEK) == 0) {
+            m_consumed += consume_bytes;
+        }
+        return consume_bytes;
+    }
+
+    int Connect(const sockaddr*, socklen_t) const override { return 0; }
+
+    int GetSockOpt(int level, int opt_name, void* opt_val, socklen_t* opt_len) const override
+    {
+        std::memset(opt_val, 0x0, *opt_len);
+        return 0;
+    }
+
+    bool Wait(std::chrono::milliseconds timeout,
+              Event requested,
+              Event* occurred = nullptr) const override
+    {
+        if (occurred != nullptr) {
+            *occurred = requested;
+        }
+        return true;
+    }
+
+private:
+    const std::string m_contents;
+    mutable size_t m_consumed;
+};
+
 #endif // BITCOIN_TEST_UTIL_NET_H