Streamlined processing state of the contribution requests

src/replica/ingest/IngestRequest.cc

@@ -347,11 +347,10 @@ void IngestRequest::_processStart() {
     string const context = ::context_ + string(__func__) + " ";
     replica::Lock const lock(_mtx, context);
 
-    if (_processing) {
+    if (_processing.exchange(true)) {
         throw logic_error(context + "the contribution request " + to_string(_contrib.id) +
                           " is already being processed or has been processed.");
     }
-    _processing = true;
 
     bool const failed = true;
     auto const databaseServices = serviceProvider()->databaseServices();

src/replica/ingest/IngestRequest.h

@@ -242,19 +242,19 @@ class IngestRequest : public std::enable_shared_from_this<IngestRequest>, public
     /// parameters of the request.
     TransactionContribInfo _contrib;
 
-    // These variables are set after completing parameter validation
+    // These variables are set by the constructors after completing parameter validation.
     std::unique_ptr<http::Url> _resource;
     csv::Dialect _dialect;
 
     /// The flag is set by method process(), and once it's set it's never
     /// reset. The flag is used for coordinating state change with other methods
     /// of the class. In particular, setting this flag would prevent executing
     /// the request more than one time.
-    bool _processing = false;
+    std::atomic<bool> _processing{false};
 
-    // Setting the flag will interrupt request processing (if the one is
-    // still going on).
-    std::atomic<bool> _cancelled{false};  ///< Set by calling the public method cancel()
+    /// Set by calling the public method cancel(). Setting the flag will interrupt
+    /// request processing (if the one is still going on).
+    std::atomic<bool> _cancelled{false};
 };
 
 }  // namespace lsst::qserv::replica