You don’t want your software to crash, do you? This post describes my experiences in making desktop Mac and Windows software crash less.
Know thy crashes
The most important step to fixing your crashes is to know about them.
Given how complex and varied our desktop operating systems are (3 major version of Windows in active use, thousands of minor and major ways that each installation of Windows or Mac can be different) our ability to comprehensively test software is not good.
Sure, if you’re Microsoft or Adobe you can reinvest some of the revenue to hire an army or testers, setup compatibility labs etc. but for a small company or a single developer this is not realistic.
Bugs most often lurk in untested code and even a very good testing effort is unlikely to encounter all the many things that can go wrong in real life.
Get the crash reports automatically
Very few people bother to tell software vendors about crashes. They just shrug and restart. The only realistic way to be informed about crashes is to automatically gather crash reports without user involvement.
This is a proven idea. Microsoft was one of the pioneers of using this technique for Windows OS and they publicly praise it for letting them fix the most frequent crashes and increase stability of Windows. Many clued in organization do it as well: Apple, Mozilla, Google.
Getting the crashes - the mechanics
Regardless of the platform, the solution involves two parts:
- a server, which accepts crash reports from the software
- code inside the software itself. It gets activated when a crash happens and sends crash report to the server
The server part is simple. You can use any technology to write it, any protocol you want.
Personally I use App Engine, re-use HTTP POST protocol and run on standard HTTP port (80) for maximum compatibility with client-side firewall software.
It’s literally few lines of code to parse incoming POST requests, store them in the database and provide a basic web interface for easy browsing. As an additional bonus, App Engine is free if your traffic is small enough,
If you write in PHP and run on a small VPS, it’ll work just as well.
Client side on Windows in C# apps
In C# I setup a global exception handler (in WPF it means setting up handlers for App.DispatcherUnhandledException and AppDomain.CurrentDomain.UnhandledException).
I then use HttpWebRequest class to send exception message and callstack as HTTP POST (using multipart/form-data).
The code is less than hundred lines and took mere hours to write.
Client side on Windows in C++ apps
In C++ everything is substantially harder. The big picture is similar:
- install global handler for unhandled exceptions (with SetUnhandledExceptionFilter())
- in that handler generate the crash report and HTTP submit it to the server
The details are substantially more complicated. The code is more than a thousand lines and took me days to perfect.
The biggest issue was that, unlike in C#, you don’t get readable callstack in native code. You need symbols (.pdb files) for that. My solution is convoluted, but works:
- during build process, I archive .pdb files on the server (I use S3, but any web server will do)
- in the crash handler I download the symbols locally, on demand
- I create crash report containing human-readable callstacks of all threads and some other useful information
- I HTTP POST it to the server
Client side on Mac OS X
The good thing about Mac is that it already creates human-readable crash reports for you. They are stored in ~/Library/Logs/CrashReporter/ directory.
There’s no need to handle crashes yourself. I just check at startup if there’s a new crash report for my app (the files are named after your application) from a previous run. If there is, I submit it to the server (and delete so that it’s not sent multiple times).
While the general idea is always the same, there are different ways of implementing it.
On Windows a simpler solution is to capture so-called minidumps (using MiniDumpWriteDumpProc() Windows API) instead of going to the trouble of generating human-readable crash reports client side.
I did that too. The problem with that approach is that you have to inspect each crash dump manually in the debugger (e.g. WinDBG). I wrote a python script that automated the process (you can script it by launching cdb debugger with the right parameters and making it run !analyze -v)).
Unfortunately, cdb is buggy and was hanging on some dump files. It’s probably possible to work around with a timeout in the python script, but at that point I stopped caring.
Windows provides native support for minidumps. Google took minidump design and provided cross-platform implementation for Windows, Mac and Linux, as part of breakpad project.
Breakpad is the crash reporting system used by Google for Chrome and Mozilla for Firefox. It contains both client and server parts for native (C/C++ or Objective C) code.
I used it once for a Mac app. For Objective C I prefer the approach described above as it’s simpler to implement, but I’m sure that’s a solid and well tested approach.
On Windows, crash reports from your app are already sent to Microsoft as part of Windows Error Reporting. Apparently, it’s possible to for third party developers to get access to those reports but I never did that, so don’t know what’s involved in the process.
The SumatraPDF experience
So how well does it work in practice?
We’ve implemented the system described here in Sumatra 1.5. Sumatra is a rather complicated piece of C++ code and quite popular (several thousand of downloads per day).
Before 1.5 we had a system where we would save the minidump to a disk and after a crash we would ask the user to report it in our bug tracker and attach minidump to the bug report.
It became obvious that almost no one did that. We’ve only gotten few crash reports from users in few months. Our automated system was sending us tens of crash reports per day.
Once we knew about the problems, we could attempt to fix them. Some problems we could fix just by looking at crash report. Some required writing stress tests to make them easier to reproduce locally. Some of them we can’t fix (e.g. because they are caused by buggy printer drivers or other software that injects buggy dlls into our process).
We do know that we fixed some of the bugs. We can see that a new release generates less crashes and by looking at crash reports we can tell that some crashes that happened frequently in previous releases do not happen anymore.
Building automated crash reporting system was the best investment we could have made for improving reliability of SumatraPDF.
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