DM-42681: Fixed a race condition in qhttp response processing

src/qhttp/Response.cc

@@ -145,32 +145,36 @@ void Response::sendStatus(Status status) {
 }
 
 void Response::send(std::string const& content, std::string const& contentType) {
+    if (!_startTransmit()) return;
+
     headers["Content-Type"] = contentType;
     headers["Content-Length"] = std::to_string(content.length());
 
     _responseBuf = _headers() + "\r\n" + content;
-    _startTransmit();
     asio::async_write(*_socket, asio::buffer(_responseBuf.data(), _responseBuf.size()),
                       [self = shared_from_this()](boost::system::error_code const& ec, std::size_t sent) {
                           self->_finishTransmit(ec, sent);
                       });
 }
 
 void Response::sendFile(fs::path const& path) {
-    _bytesRemaining = fs::file_size(path);
-    auto ct = contentTypesByExtension.find(path.extension().string());
-    headers["Content-Type"] = (ct != contentTypesByExtension.end()) ? ct->second : "text/plain";
-    headers["Content-Length"] = std::to_string(_bytesRemaining);
-
     // Try to open the file. Throw if we hit a snag; exception expected to be caught by
     // top-level handler in Server::_dispatchRequest().
-    _fileName = path.string();
-    _inFile.open(_fileName, std::ios::binary);
+    _inFile.open(path.string(), std::ios::binary);
     if (!_inFile.is_open()) {
-        LOGLS_ERROR(_log, logger(_server) << logger(_socket) << "open failed for " << _fileName << ": "
+        LOGLS_ERROR(_log, logger(_server) << logger(_socket) << "open failed for " << path << ": "
                                           << std::strerror(errno));
         throw(boost::system::system_error(errno, boost::system::generic_category()));
     }
+    if (!_startTransmit()) {
+        _inFile.close();
+        return;
+    }
+    _fileName = path.string();
+    _bytesRemaining = fs::file_size(path);
+    auto ct = contentTypesByExtension.find(path.extension().string());
+    headers["Content-Type"] = (ct != contentTypesByExtension.end()) ? ct->second : "text/plain";
+    headers["Content-Length"] = std::to_string(_bytesRemaining);
 
     // Make the initial allocation of the response buffer. For smaller files
     // the buffer should be large enough to accomodate both the header and
@@ -189,7 +193,6 @@ void Response::sendFile(fs::path const& path) {
     }
 
     // Start reading the file payload into the buffer and transmitting a series of records.
-    _startTransmit();
     std::string::size_type const pos = hdrSize;
     std::size_t const size = std::min(_bytesRemaining, _responseBuf.size() - pos);
     _sendFileRecord(pos, size);
@@ -214,11 +217,13 @@ std::string Response::_headers() const {
     return headerStream.str();
 }
 
-void Response::_startTransmit() {
-    if (_transmissionStarted.test_and_set()) {
+bool Response::_startTransmit() {
+    const bool multipleResponses = _transmissionStarted.test_and_set();
+    if (multipleResponses) {
         LOGLS_ERROR(_log, logger(_server)
                                   << logger(_socket) << "handler logic error: multiple responses sent");
     }
+    return !multipleResponses;
 }
 
 void Response::_finishTransmit(boost::system::error_code const& ec, std::size_t sent) {

src/qhttp/Response.h

@@ -75,7 +75,12 @@ class Response : public std::enable_shared_from_this<Response> {
              DoneCallback const& doneCallback, std::size_t const maxResponseBufSize);
 
     std::string _headers() const;
-    void _startTransmit();
+
+    //----- Mark a start of transmission. Return 'false' if multiple transmission attempts
+    //      were detected. Duplicate transmission requests will be ignored. An error message
+    //      may be logged. The ongoing transmission won't be affected.
+
+    bool _startTransmit();
     void _finishTransmit(boost::system::error_code const& ec, std::size_t sent);
     void _sendFileRecord(std::string::size_type pos, std::size_t size);