fuzz: implement unimplemented FuzzedSock methods

We want `Get()` to always return the same value, otherwise it will look
like the `FuzzedSock` implementation itself is broken. So assign
`m_socket` a random number in the `FuzzedSock` constructor.

There is nothing to fuzz about the `Get()` and `Release()` methods, so
use the ones from the base class `Sock`.

`Reset()` is just setting our socket to `INVALID_SOCKET`. We don't want
to use the base `Reset()` because it will close `m_socket` and given
that our `m_socket` is just a random number it may end up closing a real
opened file descriptor if it coincides with our random `m_socket`.

src/test/fuzz/util.h

@@ -558,33 +558,27 @@ class FuzzedSock : public Sock
 public:
     explicit FuzzedSock(FuzzedDataProvider& fuzzed_data_provider) : m_fuzzed_data_provider{fuzzed_data_provider}
     {
+          m_socket = fuzzed_data_provider.ConsumeIntegral<SOCKET>();
     }
 
     ~FuzzedSock() override
     {
+        // Sock::~Sock() will be called after FuzzedSock::~FuzzedSock() and it will call
+        // Sock::Reset() (not FuzzedSock::Reset()!) which will call CloseSocket(m_socket).
+        // Avoid closing an arbitrary file descriptor (m_socket is just a random number which
+        // may concide with a real opened file descriptor).
+        Reset();
     }
 
     FuzzedSock& operator=(Sock&& other) override
     {
-        assert(false && "Not implemented yet.");
+        assert(false && "Move of Sock into FuzzedSock not allowed.");
         return *this;
     }
 
-    SOCKET Get() const override
-    {
-        assert(false && "Not implemented yet.");
-        return INVALID_SOCKET;
-    }
-
-    SOCKET Release() override
-    {
-        assert(false && "Not implemented yet.");
-        return INVALID_SOCKET;
-    }
-
     void Reset() override
     {
-        assert(false && "Not implemented yet.");
+        m_socket = INVALID_SOCKET;
     }
 
     ssize_t Send(const void* data, size_t len, int flags) const override
@@ -667,6 +661,14 @@ public:
     {
         return m_fuzzed_data_provider.ConsumeBool();
     }
+
+    bool IsConnected(std::string& errmsg) const override {
+        if (m_fuzzed_data_provider.ConsumeBool()) {
+            return true;
+        }
+        errmsg = "disconnected at random by the fuzzer";
+        return false;
+    }
 };
 
 [[nodiscard]] inline FuzzedSock ConsumeSock(FuzzedDataProvider& fuzzed_data_provider)

src/util/sock.h

@@ -153,7 +153,7 @@ public:
      */
     virtual bool IsConnected(std::string& errmsg) const;
 
-private:
+protected:
     /**
      * Contained socket. `INVALID_SOCKET` designates the object is empty.
      */