Merge #21387: p2p: Refactor sock to add I2P fuzz and unit tests

40316a37cb test: add I2P test for a runaway SAM proxy (Vasil Dimov)
2d8ac77970 fuzz: add tests for the I2P Session public interface (Vasil Dimov)
9947e44de0 i2p: use pointers to Sock to accommodate mocking (Vasil Dimov)
82d360b5a8 net: change ConnectSocketDirectly() to take a Sock argument (Vasil Dimov)
b5861100f8 net: add connect() and getsockopt() wrappers to Sock (Vasil Dimov)
5a887d49b2 fuzz: avoid FuzzedSock::Recv() repeated errors with EAGAIN (Vasil Dimov)
3088f83d01 fuzz: extend FuzzedSock::Recv() to support MSG_PEEK (Vasil Dimov)
9b05c49ade fuzz: implement unimplemented FuzzedSock methods (Vasil Dimov)

Pull request description:

  Change the networking code and the I2P code to be fully mockable and use `FuzzedSocket` to fuzz the I2P methods `Listen()`, `Accept()` and `Connect()`.

  Add a mocked `Sock` implementation that returns a predefined data on reads and use it for a regression unit test for the bug fixed in https://github.com/bitcoin/bitcoin/pull/21407.

ACKs for top commit:
  practicalswift:
    Tested ACK 40316a37cb
  MarcoFalke:
    Concept ACK 40316a37cb
  jonatack:
    re-ACK 40316a37cb reviewed `git range-diff 01bb3afb 23c861d 40316a3` and the new unit test commit, debug built, ran unit tests, ran bitcoind with an I2P service and network operation with seven I2P peers (2 in, 5 out) is looking nominal
  laanwj:
    Code review ACK 40316a37cb

Tree-SHA512: 7fc4f129849e16e0c7e16662d9f4d35dfcc369bb31450ee369a2b97bdca95285533bee7787983e881e5a3d248f912afb42b4a2299d5860ace7129b0b19623cc8

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