QGIS Server Debug Tip

A quick tip to debug segfaults in QGIS Server.

Sometimes is hard to debug segfaults appearing in QGIS Server when running in CGI mode. The classic approach is attaching a gdb to the running process. The problem is that there is not enough time to do it! A simple plugin filter, can provide you the time you need to attach the debugger:
from qgis.server import *
from qgis.core import *
import os
class DelayFilter(QgsServerFilter):
    def __init__(self, serverIface):
        super(DelayFilter, self).__init__(serverIface)    
    def responseComplete(self):        
        request = self.serverInterface().requestHandler()
        params = request.parameterMap()
        if params.get('DELAY', ''):
            QgsMessageLog.logMessage("PID: %s" % os.getpid())     
            import time
Calling the server with DELAY=1 will wait for 30 seconds and print the current PID in the server logs. This will give you enough time to fire gdb and attach it to the process.