fix(qgisserver): add retry when socket seems dead

src/server/qgsfcgiserverresponse.cpp

@@ -134,14 +134,32 @@ void QgsSocketMonitoringThread::run()
     }
     else
     {
-      // socket closed, nothing can be read
-      QgsDebugMsgLevel( QStringLiteral( "FCGIServer %1: remote socket has been closed! errno: %2, x: %3" ) //
-                          .arg( threadId )
-                          .arg( errno )
-                          .arg( x ),
-                        1 );
-      mFeedback->cancel();
-      break;
+      // double check...
+      ssize_t x2 = 0;
+      for ( int i = 0; !mShouldStop.load() && x2 == 0 && i < 10; i++ )
+      {
+        x2 = recv( mIpcFd, &c, 1, MSG_PEEK | MSG_DONTWAIT ); // see https://stackoverflow.com/a/12402596
+        std::this_thread::sleep_for( 50ms );
+      }
+      if ( x2 == 0 )
+      {
+        // socket closed, nothing can be read
+        QgsDebugMsgLevel( QStringLiteral( "FCGIServer %1: remote socket has been closed! errno: %2, x: %3" ) //
+                            .arg( threadId )
+                            .arg( errno )
+                            .arg( x2 ),
+                          1 );
+        mFeedback->cancel();
+        break;
+      }
+      else
+      {
+        QgsDebugMsgLevel( QStringLiteral( "FCGIServer::run %1: remote socket is not closed in fact! errno: %2, x: %3" ) //
+                            .arg( threadId )
+                            .arg( errno )
+                            .arg( x2 ),
+                          1 );
+      }
     }
 
     // If lock is acquired this means the response has finished and we will exit the while loop