fix(FTPManager): fall back to non-burst read when a burst cannot compete

src/Vehicle/FTPManager.cc

@@ -910,9 +910,36 @@ void FTPManager::_burstReadFileAckOrNak(const MavlinkFTP::Request* ackOrNak)
     }
 }
 
+void FTPManager::_recordMissingTail(void)
+{
+    if ((_downloadState.fileSize == 0) || (_downloadState.expectedOffset >= _downloadState.fileSize)) {
+        return;
+    }
+
+    MissingData_t missingData;
+    missingData.offset          = _downloadState.expectedOffset;
+    missingData.cBytesMissing   = _downloadState.fileSize - _downloadState.expectedOffset;
+    _downloadState.rgMissingData.append(missingData);
+
+    qCDebug(FTPManagerLog) << "_recordMissingTail: offset:cBytesMissing" << missingData.offset << missingData.cBytesMissing;
+}
+
 void FTPManager::_burstReadFileTimeout(void)
 {
     if (++_downloadState.retryCount > _maxRetry) {
+        // A burst streams packets until the file ends, with no way for us to ask for less. Links
+        // which can't absorb that (a router with a shallow forwarding queue, for example) deliver
+        // only the first few packets of every burst, so retrying the burst makes the same small
+        // amount of progress each time and then gives up. Fall back to the non-burst read used to
+        // repair holes instead: it fetches one chunk per request and never has more than a single
+        // message in flight.
+        _recordMissingTail();
+        if (!_downloadState.rgMissingData.isEmpty()) {
+            qCDebug(FTPManagerLog) << "_burstReadFileTimeout: retries exceeded, falling back to non-burst read";
+            _advanceStateMachine();
+            return;
+        }
+
         qCDebug(FTPManagerLog) << QString("_burstReadFileTimeout retries exceeded");
         _downloadComplete(tr("Download failed"));
     } else {

src/Vehicle/FTPManager.h

@@ -220,6 +220,8 @@ private slots:
     void    _fillRequestDataWithString(MavlinkFTP::Request* request, const QString& str);
     void    _fillMissingBlocksWorker    (bool firstRequest);
     void    _burstReadFileWorker        (bool firstRequest);
+    /// Records anything not yet received as missing so the non-burst read can fetch it.
+    void    _recordMissingTail          (void);
     void    _listDirectoryWorker        (bool firstRequest);
     bool    _parseURI                   (uint8_t fromCompId, const QString& uri, QString& parsedURI, uint8_t& compId);
     void    _listDirectoryCompleteNoError(void) { _listDirectoryComplete(QString()); }