Fix possible CHECKSUM_DOESNT_MATCH during replicated fetches

Previously HTTP server does not tries to send exception if something had
been written to the client already, but after rewriting to "zero-copy"
HTTP implementation (back in #19516) this got broken, and you can get
pretty "dangerous" at least at first glance errors, like:
- Malformed chunked encoding
- or most likely to CHECKSUM_DOESNT_MATCH error

This is not crucial because it will never pass checksum checks, but
still icky. So let's fix it.

Signed-off-by: Azat Khuzhin <a.khuzhin@semrush.com>

src/Server/HTTP/HTTPServerResponse.cpp

@@ -62,6 +62,7 @@ std::shared_ptr<WriteBufferFromPocoSocket> HTTPServerResponse::send()
         stream = std::make_shared<WriteBufferFromPocoSocket>(session.socket(), write_event);
     }
 
+    send_started = true;
     return stream;
 }
 
@@ -96,9 +97,16 @@ std::pair<std::shared_ptr<WriteBufferFromPocoSocket>, std::shared_ptr<WriteBuffe
     else
         stream = std::make_shared<WriteBufferFromPocoSocket>(session.socket(), write_event);
 
+    send_started = true;
     return std::make_pair(header_stream, stream);
 }
 
+void HTTPServerResponse::beginWrite(std::ostream & ostr) const
+{
+    HTTPResponse::beginWrite(ostr);
+    send_started = true;
+}
+
 void HTTPServerResponse::sendBuffer(const void * buffer, std::size_t length)
 {
     setContentLength(static_cast<int>(length));

src/Server/HTTP/HTTPServerResponse.h

@@ -209,6 +209,10 @@ public:
     /// or redirect() has been called.
     std::pair<std::shared_ptr<WriteBufferFromPocoSocket>, std::shared_ptr<WriteBufferFromPocoSocket>> beginSend();
 
+    /// Override to correctly mark that the data send had been started for
+    /// zero-copy response (i.e. replicated fetches).
+    void beginWrite(std::ostream & ostr) const;
+
     /// Sends the response header to the client, followed
     /// by the contents of the given buffer.
     ///
@@ -229,7 +233,7 @@ public:
     /// according to the given realm.
 
     /// Returns true if the response (header) has been sent.
-    bool sent() const { return !!stream; }
+    bool sent() const { return send_started; }
 
     /// Sets the status code, which must be one of
     /// HTTP_MOVED_PERMANENTLY (301), HTTP_FOUND (302),
@@ -251,6 +255,7 @@ private:
     ProfileEvents::Event write_event;
     std::shared_ptr<WriteBufferFromPocoSocket> stream;
     std::shared_ptr<WriteBufferFromPocoSocket> header_stream;
+    mutable bool send_started = false;
 };
 
 }