I'm using Poco 1.6.0 and the Util::ServerApplication structure.
At the start of int main(const ArgVec& args) in my main class, I redirect all the logging to a file:
Poco::AutoPtr<Poco::FileChannel> chanFile = new Poco::FileChannel;
chanFile->setProperty("path", "C:\\doesnotexist\\file.log");
Poco::Util::Application::instance().logger().setChannel(chanFile);
If the log file cannot be opened, this causes an exception to be thrown, which I catch, and return an error code from main(). The Application::run() code in Poco's Application.cpp then calls Application::uninitialize().
The implementation of Application::uninitialize()iterates through each SubSystem executing that subsystem's uninitialize().
But one of those is LogFile::uninitialize(), which causes the following message to be logged: Uninitializing subsystem: Logging Subsystem.
When it attempts to log that message, an exception is thrown since the log file could not be opened (for the same reason as before). That exception is caught somewhere in Poco's code and it attempts to log an error, which causes an exception, and that one finally terminates the program.
How should I deal with this issue? E.g. is it possible to tell the logging subsystem to not throw any exceptions?
There seems to be a greater issue too; if any subsystem uninitialize() throws, this will cause execution to leave the subsystem shutdown loop in Application.cpp , so other subsystems will not have a chance to shut down either.
You should make sure that the path exist before setting up the file channel, e.g.:
if (Poco::File("C:\\doesnotexist").exists())
{
Poco::AutoPtr<Poco::FileChannel> chanFile = new Poco::FileChannel;
chanFile->setProperty("path", "C:\\doesnotexist\\file.log");
Poco::Util::Application::instance().logger().setChannel(chanFile);
}
Application::unitialize() will loop through subsystems and log iterations as debug messages - the idea is to catch problems before release.
UPDATE: as pointed in the comments, the directory may exist at the time of the check but may not exist (or not be accessible) afterwards, when logging actually happens. There is nothing in Poco that shields user from that; so, you will have to make sure the directory exists and is accessible throughout the lifetime of the FileChannel using it. I have not found this to be an obstacle in practice. I did find the initial non-existence of a directory to be an annoying problem and there is a proposal for addition of such (optional/configurable) feature but it has not been scheduled yet for inclusion in upcoming releases.
Related
I'm starting to use Boost, so may be I'm messing something up.
I'm trying to set up http server with boost (ASIO). I've taken the code from docs: http://www.boost.org/doc/libs/1_54_0/doc/html/boost_asio/examples/cpp03_examples.html (HTTP Server, the first one)
The only difference from the example is I'm running server by my own method "run" and starting io_service in background thread, like in the docs: http://www.boost.org/doc/libs/1_54_0/doc/html/boost_asio/reference/io_service.html
boost::asio::io_service::work work(io_service_);
(Also I'm stopping io_service from my run method too.)
When I'm starting this modified server everything seems to be OK, run method is working fine. But then I'm trying to get a doc from the server the request hangs and control flow never comes to "request_handle" method.
Am I missing something?
UPD. Here is my code of run method:
void NetstreamServer::run()
{
LOG4CPLUS_DEBUG(logger, "NetstreamServer is running");
boost::asio::io_service::work work(io_service_);
try
{
while (true)
{
if (condition)
{
io_service_.stop();
break;
}
}
}
catch (std::exception const& e)
{
LOG4CPLUS_ERROR(logger, "NetstreamServer" << " caught exception: " << e.what());
}
}
You should call io_service_::run() - otherwise no one will dispatch the completion handlers of Asio objects serviced by io_service_.
Without including the code you changed, everyone here can only guess. Unfortunately you also do not include the compiler and the OS you are using. Even with boost claiming it is platform independent, you should always include this information, as it reality, platforms are different even with boost.
Let me do a guess. You use Microsoft Windows? How do you prevent the "main" function to exit? You moved the blocking "run" function out of it in another thread, the main function has no wait point anymore. Let me guess again, you used something like "getchar". With that, you can exit your server with only hitting the keyboard return key. If yes, the problem is the getchar, with unfortunately blocks every io of the asio socket implementation, but only on Windows based systems.
I would not need to guess if you would include the informations mentioned in your post. In particular all(!) changes you made to the code sample.
I have a unit test that creates an error condition. Normally, the class under test writes this error to the logs (using log4j in this case, but I don't think that matters). I can change the log level temporarily, using
Logger targetLogger = Logger.getLogger(ClassUnderTest.class);
Level oldLvl = targetLogger.getLevel();
targetLogger.setLevel(Level.FATAL);
theTestObject.doABadThing();
assertTrue(theTestObject.hadAnError());
targetLogger.setLevel(oldLvl);
but that also means that if an unrelated / unintended error occurs during testing, I won't see that information in the logs either.
Is there a best practice or common pattern I'm supposed to use here? I don't like prodding the log levels if I can help it, but I also don't like having a bunch of ERROR noise in the test output, which could scare future developers.
If your logging layer permits, it is a good practice to make an assertion on the error message. You can do it by implementing your own logger that just asserts on the message (without output), or by using a memory-buffer logger and then check on the contents of the log buffer.
Under no circumstances should the error message end up in the unit-test execution log. This will cause people to get used to errors in the log and mask other errors. In short, your options are:
Most preferred: Catch the message in the harness and assert on it.
Somewhat OK: Raise the level and ignore the message.
Not OK: Don't do anything and let the log message reach stderr/syslog.
The way I approach this assuming an XUnit style of unit testing (Junit,Pyunit, etc)
#Test(expected = MyException)
foo_1() throws Exception
{
theTestObject.doABadThing(); //MyException here
}
The issue with doing logging is that someone needs to go and actually parse the log file, this is time consuming and error prone. However the test will pass above if MyException is generated and fail if it isn't. This in turn allows you to fail the build automatically instead of hoping the tester read the logs correctly.
I have been trying to use c++/cx StorageFile::ReadAsync() to read a file in a store-apps, but it always return an invalid params exception no matter what
// "file" are returned from FileOpenPicker
IRandomAccessStream^ reader = create_task(file->OpenAsync(FileAccessMode::Read)).get();
if (reader->CanRead)
{
BitmapImage^ b = ref new BitmapImage();
const int count = 1000000;
Streams::Buffer^ bb = ref new Streams::Buffer(count);
create_task(reader->ReadAsync(bb, 1, Streams::InputStreamOptions::None)).get();
}
I have turn on all the manifest capabilities and added "file open picker" + "file type association" for Declarations. Any ideas ? thanks!
ps: most solutions I found is for C#, but the code structure are similar...
If this code is executing on the UI thread (or in any other Single Threaded Apartment, or STA), then the calls to .get() will throw if the tasks have not yet completed, because the call to .get() would block the thread. You must not block the UI thread or any other STA, and when compiling with C++/CX support enabled, the libraries enforce this.
If you turn on first chance exception handling in the debugger (Debug -> Exceptions..., check the C++ Exceptions check box), you should see that the first exception to be thrown is an invalid_operation exception, from the following line in <ppltasks.h>:
// In order to prevent Windows Runtime STA threads from blocking the UI, calling
// task.wait() task.get() is illegal if task has not been completed.
if (!_IsCompleted() && !_IsCanceled())
{
throw invalid_operation("Illegal to wait on a task in a Windows Runtime STA");
}
The "invalid parameter" you are reporting is the fatal error that is caused when this exception reaches the ABI boundary: the debugger is notified that the application is about to terminate because this exception was unhandled.
You need to restructure your code to use continuations, using task::then, as described in the article Asynchronous Programming in C++ Using PPL
Just to make sure you understand the async pattern, what is happening in your code is that you call create_task and immediately after that task has started you are trying to get the result with .get(). Calls to .get() will throw immediately if the task is still running or the file could not be found. Therefore, the correct way of structuring this is using a .then on your file task, ensuring that you have the result of this task before starting the next one.
create_task(file->OpenAsync(FileAccessMode::Read)).then([](IRandomAccessStream^ reader)
{
//do stuff with the reader
});
At that point the reader is available so you can do whatever you want to, even start a new task.
Also, it is possible that the call to OpenAsync is failing cause the file is empty, I would add a try catch block to the previous task, the one that gets the file, just to make sure that's not the problem.
I had been trying hard to figure out why the exceptions thrown from StartElement event handler are not being caught by my application which makes use of expat parser( in C). The application just terminates saying that it cannot find catch blocks, though I have all the catch blocks in place. It is just that since exceptions are being thrown from my own event handlers, XML_Parse API of expat is unable to pass it on to my code where I have all the catch blocks. One of the stackoverflow user with name 'Michael Anderson" suggested to rebuild expat lib with necessary gcc flags to make expat lib handle exceptions. Can someone let me know what flags are those? Or Suggest a better way out to handle errors in event handlers like startelement, endelement etc.
I somehow want XML_Parse API to return 0 if I encounter any exception in my registered event handlers. Please help. Thanks in advance.
Here is the code:
try
{
if( ! XML_Parse(.....) )
{
throw exception;
}
}
catch(...)
{
}
In the working scenario, if XML_Parse encounters a malformed xml file, it promptly returns zero, and I get into if block and throw exception, and it is caught fine.
But in the problematic case, the exceptions are being thrown from the event handlers but my application dumps core, and core stack says that it cannot find catch and finally calling std::terminate and abort.
Now, how do I make XML_Parse to return zero when I want to throw user defined exception from event handlers?
As per expat.h, you should invoke XML_StopParser(parser, 0) when you encounter an error in your handler that warrants aborting the parse.
XML_Parse will then return XML_FALSE. At that point you can invoke your application-specific error handling
I have a server application which I am debugging which basically parses scripts (VBscript, Python, Jscript and SQl) for the application that requests it.
This is a very critical application which, if it crashes causes havoc for a lot of users. The problem I am facing is how to handle exceptions so that the application can continue and the users know if something is wrong in their scripts.
An example: In the SQL scripts the application normally returns a set of values (Date, Number, String and Number). So the scripts have to have a statement at the end as such:
into dtDate, Number, Number, sString. These are values that are built into the application and the server application knows how to interpret these. These fields are treated in the server app as part of an array. The return values should normally be in a specific order as the indexes for these fields into the array are hardcoded inside the server application.
Now when a user writing a script forgets one of these fields, then the last field (normally string) throws an IndexOutofBoundsException.
The question is how does one recover from exceptions of this nature without taking down the application?
Another example is an error in a script for which no error parsing message can be generated. These errors just disappear in the background in the application and eventually cause the server app to crash. The scripts on which it fails don't necessarily fail to execute entirely, but part of it doesn't execute and the other parts do, which makes it look fairly odd to a user.
This server app is a native C++ application and uses COM technologies.
I was wondering if anyone has any ideas on what the best way is to handle exceptions such as the ones described above without crashing the application??
You can't handle problems like this with exceptions. You could have a top-level catch block that catches the exception and hope that not too much state of the program got irrecoverably munched to try to keep the program alive. Still doesn't make the user happy, that query she is waiting for still doesn't run.
Ensuring that changes don't destabilize a critical business app requires organization. People that sign-off on the changes and verify that they work as intended before it is allowed into production. QA.
since you talk about parsing different languages, you probably have something like
class IParser //parser interface
{
virtual bool Parse( File& fileToParse, String& errMessage ) = 0;
};
class VBParser : public Parser
class SQLParser : public Parser
Suppose the Parse() method throws an exception that is not handled, your entire app crashes. Here's a simplified example how this could be fixed at the application level:
//somewhere main server code
void ParseFileForClient( File& fileToParse )
{
try
{
String err;
if( !currentParser->Parse( fileToParse, err ) )
ReportErrorToUser( err );
else
//process parser result
}
catch( std::exception& e )
{
ReportErrorToUser( FormatExceptionMessage( err ) );
}
catch( ... )
{
ReportErrorToUser( "parser X threw unknown exception; parsing aborted" );
}
}
If you know an operation can throw an exception, then you need to add exception handling to this area.
Basically, you need to write the code in an exception safe manner which usually uses the following guidelines
Work on temporary values that can throw exceptions
Commit the changes using the temp values after (usually this will not throw an exception)
If an exception is thrown while working on the temp values, nothing gets corrupted and in the exception handling you can manage the situation and recover.
http://www.gotw.ca/gotw/056.htm
http://www.gotw.ca/gotw/082.htm
It really depends on how long it takes to start up your server application. It may be safer to let the application crash and then reload it. Or taking a cue from Chrome browser run different parts of your application in different processes that can crash. If you can safely recover an exception and trust that your application state is ok then fine do it. However catching std::exception and continuing can be risky.
There are simple to complex ways to baby sit processes to make sure if they crash they can be restarted. A couple of tools I use.
bluepill http://asemanfar.com/Bluepill:-a-new-process-monitoring-tool
pacemaker http://www.clusterlabs.org/
For simple exceptions that can happen inside your program due to user errors,
simply save the state that can be changed, and restore it like this:
SaveStateThatCanBeAlteredByScript();
try {
LoadScript();
} catch(std::exception& e){
RestoreSavedState();
ReportErrorToUser(e);
}
FreeSavedState();
If you want to prevent external code from crashing (possible untrustable code like plugins), you need an IPC scheme. On Windows, I think you can memory map files with OpenFile(). On POSIX-systems you can use sem_open() together with mmap().
If you have a server. You basically have a main loop that waits for a signal to start up a job. The signal could be nothing and your server just goes through a list of files on the file system or it could be more like a web server where it waits for a connection and executes the script provided on the connection (or any thing like that).
MainLoop()
{
while(job = jobList.getJob())
{
job.execute();
}
}
To stop the server from crashing because of the scripts you need to encapsulate the external jobs in a protected region.
MainLoop()
{
// Don't bother to catch exceptions from here.
// This probably means you have a programming error in the server.
while(job = jobList.getJob())
{
// Catch exception from job.execute()
// as these exceptions are generally caused by the script.
try
{
job.execute();
}
catch(MyServerException const& e)
{
// Something went wrong with the server not the script.
// You need to stop. So let the exception propagate.
throw;
}
catch(std::exception const& e)
{
log(job, e.what());
}
catch(...)
{
log(job, "Unknown exception!");
}
}
}
If the server is critical to your operation then just detecting the problem and logging it is not always enough. A badly written server will crash so you want to automate the recovery. So you should write some form of heartbeat processes that checks at regular intervals if the processes has crashed and if it has automatically restart it.