Segfault logic with two threads - c++

I have an application with main thread and additional (detached) process created in it.
In that process we are running network server which sends logs from queue through the network.
The question is: is it possible to do something in segfault handler to wait/finish for sending that log queue. So I want almost 100% delivery of that queue.

While it is possible to write a segfault handler, I highly recommend against it. First off, it's very easy to get your program into a "won't terminate" state due to a segfault in the segfault handler.
Second, as dan3 mentions, the memory of the process is likely in a corrupt state, making it hard to know what will and won't work.
Finally, you lose the opportunity to use the coredump from the process to help track down the problem.
While it's not recommended, it is possible.
My recommendation is to write a small program that avoids memory allocation and the use of pointers as much as possible. Perhaps create buffers as global arrays and only ever access them with limited code that can be reviewed by several skilled developers and tested thoroughly (stress testing is great here). Keep in mind, though, that the message could still get lost by the sender or receiver if they crash, so it may not be worth the effort.
By the way - when Netscape first wrote a version of their browser for Linux, I ran it and it kept getting into a locked-up state. Using the strace program, I quickly found that it was in an infinite segfault loop. Very frustrating, and leading to almost 100% cpu wasted.

You can wait() for a process and pthread_wait() for a thread to finish (you didn't specify clearly which one you use).
Remember that if you are in segfault handler, your memory is messed up (avoid malloc() and free()) and your FILE * could also be borked.

Related

Terminating QWebSocketServer with connected sockets

I debug console multithreaded application written in C++/Qt 5.12.1. It is running on Linux Mint 18.3 x64.
This app has SIGINT handler, QWebSocketServer and QWebSocket table. It uses close() QWebSocketServer and call abort()/deleteLater() for items in QWebSocket table to handle the termination.
If the websocket client connects to this console app, then termination fails because of some running thread (I suppose it's internal QWebSocket thread).
Termination is successful if there were no connections.
How to fix it? So that the app gracefully exits.
To gracefully quit the socket server we can attempt:
The most important part is to allow the main thread event loop to run and wait on QWebSocketServer::closed() so that the slot calls QCoreApplication::quit().
That can be done even with:
connect(webSocketServer, &QWebSocketServer::closed,
QCoreApplication::instance(), &QCoreApplication::quit);
If we don't need more detailed reaction.
After connecting that signal before all, proceed with pauseAccepting() to prevent more connections.
Call QWebSocketServer::close.
The below may not be needed if the above sufficient. You need to try the above first, and only if still have problems then deal with existing and pending connections. From my experience the behavior was varying on platforms and with some unique websocket implementations in the server environment (which is likely just Qt for you).
As long as we have some array with QWebSocket instances, we can try to call QWebSocket::abort() on all of them to immediately release. This step seem to be described by the question author.
Try to iterate pending connections with QWebSocketServer::nextPendingConnection() and call abort() for them. Call deleteLater, if that works as well.
There is no need to do anything. What do you mean by "graceful exit"? As soon as there's a request to terminate your application, you should terminate it immediately using exit(0) or a similar mechanism. That's what "graceful exit" should be.
Note: I got reformed. I used to think that graceful exits were a good thing. They are most usually a waste of CPU resources and usually indicate problems in the architecture of the application.
A good rationale for why it should be so written in the kj framework (a part of capnproto).
Quoting Kenton Varda:
KJ_NORETURN(virtual void exit()) = 0;
Indicates program completion. The program is considered successful unless error() was
called. Typically this exits with _Exit(), meaning that the stack is not unwound, buffers
are not flushed, etc. -- it is the responsibility of the caller to flush any buffers that
matter. However, an alternate context implementation e.g. for unit testing purposes could
choose to throw an exception instead.
At first this approach may sound crazy. Isn't it much better to shut down cleanly? What if
you lose data? However, it turns out that if you look at each common class of program, _Exit()
is almost always preferable. Let's break it down:
Commands: A typical program you might run from the command line is single-threaded and
exits quickly and deterministically. Commands often use buffered I/O and need to flush
those buffers before exit. However, most of the work performed by destructors is not
flushing buffers, but rather freeing up memory, placing objects into freelists, and closing
file descriptors. All of this is irrelevant if the process is about to exit anyway, and
for a command that runs quickly, time wasted freeing heap space may make a real difference
in the overall runtime of a script. Meanwhile, it is usually easy to determine exactly what
resources need to be flushed before exit, and easy to tell if they are not being flushed
(because the command fails to produce the expected output). Therefore, it is reasonably
easy for commands to explicitly ensure all output is flushed before exiting, and it is
probably a good idea for them to do so anyway, because write failures should be detected
and handled. For commands, a good strategy is to allocate any objects that require clean
destruction on the stack, and allow them to go out of scope before the command exits.
Meanwhile, any resources which do not need to be cleaned up should be allocated as members
of the command's main class, whose destructor normally will not be called.
Interactive apps: Programs that interact with the user (whether they be graphical apps
with windows or console-based apps like emacs) generally exit only when the user asks them
to. Such applications may store large data structures in memory which need to be synced
to disk, such as documents or user preferences. However, relying on stack unwind or global
destructors as the mechanism for ensuring such syncing occurs is probably wrong. First of
all, it's 2013, and applications ought to be actively syncing changes to non-volatile
storage the moment those changes are made. Applications can crash at any time and a crash
should never lose data that is more than half a second old. Meanwhile, if a user actually
does try to close an application while unsaved changes exist, the application UI should
prompt the user to decide what to do. Such a UI mechanism is obviously too high level to
be implemented via destructors, so KJ's use of _Exit() shouldn't make a difference here.
Servers: A good server is fault-tolerant, prepared for the possibility that at any time
it could crash, the OS could decide to kill it off, or the machine it is running on could
just die. So, using _Exit() should be no problem. In fact, servers generally never even
call exit anyway; they are killed externally.
Batch jobs: A long-running batch job is something between a command and a server. It
probably knows exactly what needs to be flushed before exiting, and it probably should be
fault-tolerant.

How to release resources if a program crashes

I have a program that uses services from others. If the program crashes, what is the best way to close those services? At server side, I would define some checkers that monitor if a client is invalid periodically. But can we do any thing at client? I am not the sure if the normal RAII still works at this case. My code is written in C and C++.
If your application experiences a hard crash, then no, your carefully crafted cleanup code will not run, whether it is part of an RAII paradigm or a method you call at the end of main. None of an application's cleanup code runs after a crash that causes the application to be terminated.
Of course, this is not true for exceptions. Although those might eventually cause the application to be terminated, they still trigger this termination in a controlled way. Generally, the runtime library will catch an unhandled exception and trigger termination. Along the way, your RAII-based cleanup code will be executed, unless it also throws an exception. Then you're back to being unceremoniously ripped out of memory.
But even if your application's cleanup code can't run, the operating system will still attempt to clean up after you. This solves the problem of unreleased memory, handles, and other system objects. In general, if you crash, you need not worry about releasing these things. Your application's state is inconsistent, so trying to execute a bunch of cleanup code will just lead to unpredictable and potentially erroneous behavior, not to mention wasting a bunch of time. Just crash and let the system deal with your mess. As Raymond Chen puts it:
The building is being demolished. Don't bother sweeping the floor and emptying the trash cans and erasing the whiteboards. And don't line up at the exit to the building so everybody can move their in/out magnet to out. All you're doing is making the demolition team wait for you to finish these pointless housecleaning tasks.
Do what you must; skip everything else.
The only problem with this approach is, as you suggest in this question, when you're managing resources that are not controlled by the operating system, such as a remote resource on another system. In that case, there is very little you can do. The best scenario is to make your application as robust as possible so that it doesn't crash, but even that is not a perfect solution. Consider what happens when the power is lost, e.g. because a user's cat pulled the cord from the wall. No cleanup code could possibly run then, so even if your application never crashes, there may be termination events that are outside of your control. Therefore, your external resources must be robust in the event of failure. Time-outs are a standard method, and a much better solution than polling.
Another possible solution, depending on the particular use case, is to run consistency-checking and cleanup code at application initialization. This might be something that you would do for a service that is intended to run continuously and will be restarted promptly after termination. The next time it restarts, it checks its data and/or external resources for consistency, releases and/or re-initializes them as necessary, and then continues on as normal. Obviously this is a bad solution for a typical application, because there is no guarantee that the user will relaunch it in a timely manner.
As the other answers make clear, hoping to clean up after an uncontrolled crash (i.e., a failure which doesn't trigger the C++ exception unwind mechanism) is probably a path to nowhere. Even if you cover some cases, there will be other cases that fail and you are building in a serious vulnerability to those cases.
You mention that the source of the crashes is that you are "us[ing] services from others". I take this to mean that you are running untrusted code in-process, which is the potential source of crashes. In this case, you might consider running the untrusted code "out of process" and communicating back to your main process through a pipe or shared memory or whatever. Then you isolate the crashes this child process, and can do controlled cleanup in your main process. A separate process is really the lightest weight thing you can do that gives you the strong isolation you need to avoid corruption in the calling code.
If forking a process per-call is performance-prohibitive, you can try to keep the child process alive for multiple calls.
One approach would be for your program to have two modes: normal operation and monitoring.
When started in a usual way, it would :
Act as a background monitor.
Launch a subprocess of itself, passing it an internal argument (something that wouldn't clash with normal arguments passed to it, if any).
When the subprocess exists, it would release any resources held at the server.
When started with the internal argument, it would:
Expose the user interface and "act normally", using the resources of the server.
You might look into atexit, which may give you the functionality you need to release resources upon program termination. I don't believe it is infallible, though.
Having said that, however, you should really be focusing on making sure your program doesn't crash; if you're hitting an error that is "unrecoverable", you should still invest in some error-handling code. If the error is caused by a Seg-Fault or some other similar OS-related error, you can either enable SEH exceptions (not sure if this is Windows-specific or not) to enable you to catch them with a normal try-catch block, or write some Signal Handlers to intercept those errors and deal with them.

Catching Signals c++

I have a boost threadpool which I use to do certain tasks. I also have a Sensor class that has the pure virtual function doWork(int total) = 0;. Whenever it is requested, my main process gets the necessary Sensor pointer and tells the threadpool to run Sensor::doWork(int total).
threadpool->schedule(boost::bind(&Sensor::doWork,this,123456));
I am dynamically loading libraries of type Sensor, thus it is out of my control if someone else has faulty coding which results in SEGFAULTS and such. So is there a way for me to (in my main process) handle any errors thrown by Sensor::doWork(int total), clean up the thread, delete that sensor object and notify the console what and where the error has occurred?
Really the only way to handle a segmentation fault here is to run Sensor::doWork in a completely separate process.
In UNIX, this involves using fork (or some other similar means), running Sensor::doWork in the child process, and then somehow shuttling the results back to the parent process.
I assume similar means are available in Windows.
EDIT: I thought I'd flesh out a bit some of the things you can do.
Solution #1: you can work with processes in the same fashion as you would threads. For example, you could create process pool that sit there in a loop of
Wait for a task to be passed in over a pipe or queue or some similar object
Perform the task
Return the results over a pipe or queue or some similar object
And since you're executing the tasks in the other processes, you're protected against them crashing. The main difficulty with this solution is actually communicating between processes; maybe boost's interprocess library will help with that. I've mainly done this sort of thing in python, which has a standard multiprocessing module that handles this stuff for you.
Solution #2: You could divide your application into "safe" and "risky" portions that run in different processes. The "risky" portion executes the Sensor::doWork methods and anything else you might want to do in that process -- but only work that is acceptable to be spontaneously lost if it crashes. The "safe" portion deals with any precious information that you cannot afford to lose, and monitors the "risky" portion, performing some recovery operations when the child crashes. And, of course, whatever other work you decide you want to do in the safe part.
If you got a SIGSEGV, even if you caught it you have no guarantee about your program state so there's pretty much no way to recover.
If you're working with 3rd party libraries, and they're buggy, and the library maintainer won't fix it (and you don't have the source) then your only recourse is to run the third party library from within a totally separate binary that talks to the main binary by some means. See for example firefox and plugin-container.
You might want to register a function callback to catch SIGSEV. In C this can be done using signal. Be aware, however, there is not much you can do, when the OS sends you a SIGSEV (note that it isn't required to). You don't really know in what state your program is in, I'd guess. If for example the heap got corrupt, new and delete operations may fail, so even a plain simple
std::cout << std::string("hello world") << std::endl;
statement, might not work since memory from the heap needs to be allocated.
Best, Christoph

Application crash with no explanation

I'd like to apologize in advance, because this is not a very good question.
I have a server application that runs as a service on a dedicated Windows server. Very randomly, this application crashes and leaves no hint as to what caused the crash.
When it crashes, the event logs have an entry stating that the application failed, but gives no clue as to why. It also gives some information on the faulting module, but it doesn't seem very reliable, as the faulting module is usually different on each crash. For example, the latest said it was ntdll, the one before that said it was libmysql, the one before that said it was netsomething, and so on.
Every single thread in the application is wrapped in a try/catch (...) (anything thrown from an exception handler/not specifically caught), __try/__except (structured exceptions), and try/catch (specific C++ exceptions). The application is compiled with /EHa, so the catch all will also catch structured exceptions.
All of these exception handlers do the same thing. First, a crash dump is created. Second, an entry is logged to a new file on disk. Third, an entry is logged in the application logs. In the case of these crashes, none of this is happening. The bottom most exception handler (the try/catch (...)) does nothing, it just terminates the thread. The main application thread is asleep and has no chance of throwing an exception.
The application log files just stop logging. Shortly after, the process that monitors the server notices that it's no longer responding, sends an alert, and starts it again. If the server monitor notices that the server is still running, but just not responding, it takes a dump of the process and reports this, but this isn't happening.
The only other reason for this behavior that I can come up with, aside from uncaught exceptions, is a call to exit or similar. Searching the code brings up no calls to any functions that could terminate the process. I've also made sure that the program isn't terminating normally (i.e. a stop request from the service manager).
We have tried running it with windbg attached (no chance to use Visual Studio, the overhead is too high), but it didn't report anything when the crash occurred.
What can cause an application to crash like this? We're beginning to run out of options and consider that it might be a hardware failure, but that seems a bit unlikely to me.
If your app is evaporating an not generating a dump file, then it is likely that an exception is being generated which your app doesnt (or cant) handle. This could happen in two instances:
1) A top-level exception is generated and there is no matching catch block for that exception type.
2) You have a matching catch block (such as catch(...)), but you are generating an exception within that handler. When this happens, Windows will rip the bones from your program. Your app will simply cease to exist. No dump will be generated, and virtually nothing will be logged, This is Windows' last-ditch effort to keep a rogue program from taking down the entire system.
A note about catch(...). This is patently Evil. There should (almost) never be a catch(...) in production code. People who write catch(...) generally argue one of two things:
"My program should never crash. If anything happens, I want to recover from the exception and continue running. This is a server application! ZOMG!"
-or-
"My program might crash, but if it does I want to create a dump file on the way down."
The former is a naive and dangerous attitude because if you do try to handle and recover from every single exception, you are going to do something bad to your operating footprint. Maybe you'll munch the heap, keep resources open that should be closed, create deadlocks or race conditions, who knows. Your program will suffer from a fatal crash eventually. But by that time the call stack will bear no resemblance to what caused the actual problem, and no dump file will ever help you.
The latter is a noble & robust approach, but the implementation of it is much more difficult that it might seem, and it fraught with peril. The problem is you have to avoid generating any further exceptions in your exception handler, and your machine is already in a very wobbly state. Operations which are normally perfectly safe are suddenly hand grenades. new, delete, any CRT functions, string formatting, even stack-based allocations as simple as char buf[256] could make your application go >POOF< and be gone. You have to assume the stack and the heap both lie in ruins. No allocation is safe.
Moreover, there are exceptions that can occur that a catch block simply can't catch, such as SEH exceptions. For that reason, I always write an unhandled-exception handler, and register it with Windows, via SetUnhandledExceptionFilter. Within my exception handler, I allocate every single byte I need via static allocation, before the program even starts up. The best (most robust) thing to do within this handler is to trigger a seperate application to start up, which will generate a MiniDump file from outside of your application. However, you can generate the MiniDump from within the handler itself if you are extremely careful no not call any CRT function directly or indirectly. Basically, if it isn't an API function you're calling, it probably isn't safe.
I've seen crashes like these happen as a result of memory corruption. Have you run your app under a memory debugger like Purify to see if that sheds some light on potential problem areas?
Analyze memory in a signal handler
http://msdn.microsoft.com/en-us/library/xdkz3x12%28v=VS.100%29.aspx
This isn't a very good answer, but hopefully it might help you.
I ran into those symptoms once, and after spending some painful hours chasing the cause, I found out a funny thing about Windows (from MSDN):
Dereferencing potentially invalid
pointers can disable stack expansion
in other threads. A thread exhausting
its stack, when stack expansion has
been disabled, results in the
immediate termination of the parent
process, with no pop-up error window
or diagnostic information.
As it turns out, due to some mis-designed data sharing between threads, one of my threads would end up dereferencing more or less random pointers - and of course it hit the area just around the stack top sometimes. Tracking down those pointers was heaps of fun.
There's some technincal background in Raymond Chen's IsBadXxxPtr should really be called CrashProgramRandomly
Late response, but maybe it helps someone: every Windows app has a limit on how many handles can have open at any time. We had a service not releasing a handle in some situation, the service would just disappear, after a few days, or at times weeks (depending on the usage of the service).
Finding the leak was great fun :D (use Task Manager to see thread count, handles count, GDI objects, etc)

Detecting application hang

I have a very large, complex (million+ LOC) Windows application written in C++. We receive a handful of reports every day that the application has locked up, and must be forcefully shut down.
While we have extensive reporting about crashes in place, I would like to expand this to include these hang scenarios -- even with heavy logging in place, we have not been able to track down root causes for some of these. We can clearly see where activity stopped - but not why it stopped, even in evaluating output of all threads.
The problem is detecting when a hang occurs. So far, the best I can come up with is a watchdog thread (as we have evidence that background threads are continuing to run w/out issues) which periodically pings the main window with a custom message, and confirms that it is handled in a timely fashion. This would only capture GUI thread hangs, but this does seem to be where the majority of them are occurring. If a reply was not received within a configurable time frame, we would capture a memory and stack dump, and give the user the option of continuing to wait or restarting the app.
Does anyone know of a better way to do this than such a periodic polling of the main window in this way? It seems painfully clumsy, but I have not seen alternatives that will work on our platforms -- Windows XP, and Windows 2003 Server. I see that Vista has much better tools for this, but unfortunately that won't help us.
Suffice it to say that we have done extensive diagnostics on this and have been met with only limited success. Note that attaching windbg in real-time is not an option, as we don't get the reports until hours or days after the incident. We would be able to retrieve a memory dump and log files, but nothing more.
Any suggestions beyond what I'm planning above would be appreciated.
The answer is simple: SendMessageTimeout!
Using this API you can send a message to a window and wait for a timeout before continuing; if the application responds before timeout the is still running otherwise it is hung.
One option is to run your program under your own "debugger" all the time. Some programs, such as GetRight, do this for copy protection, but you can also do it to detect hangs. Essentially, you include in your program some code to attach to a process via the debugging API and then use that API to periodically check for hangs. When the program first starts, it checks if there's a debugger attached to it and, if not, it runs another copy of itself and attaches to it - so the first instance does nothing but act as the debugger and the second instance is the "real" one.
How you actually check for hangs is another whole question, but having access to the debugging API there should be some way to check reasonably efficiently whether the stack has changed or not (ie. without loading all the symbols). Still, you might only need to do this every few minutes or so, so even if it's not efficient it might be OK.
It's a somewhat extreme solution, but should be effective. It would also be quite easy to turn this behaviour on and off - a command-line switch will do or a #define if you prefer. I'm sure there's some code out there that does things like this already, so you probably don't have to do it from scratch.
A suggestion:
Assuming that the problem is due to locking, you could dump your mutex & semaphore states from a watchdog thread. With a little bit of work (tracing your call graph), you can determine how you've arrived at a deadlock, which call paths are mutually blocking, etc.
While a crashdump analysis seems to provide a solution for identifying the problem, in my experience this rarely bears much fruit since it lacks sufficient unambiguous detail of what happened just before the crash. Even with the tool you propose, it would provide little more than circumstantial evidence of what happened. I bet the cause is unprotected shared data, so a lock trace wouldn't show it.
The most productive way of finding this—in my experience—is distilling the application's logic to its essence and identifying where conflicts must be occurring. How many threads are there? How many are GUI? At how many points do the threads interact? Yep, this is good old desk checking. Leading suspect interactions can be identified in a day or two, then just convince a small group of skeptics that the interaction is correct.