Fix a warning about a bool > 0.

[qgit4/redivivus.git] / exception_manager.txt

                                      THE PROBLEM


Qt3 uses the call qApp->processEvents() (from now on PE) to let a program do not block while  
a long operation is occurring. This function passes the control to the application event loop
and any new pending event like GUI event, socket notifier(QProcess) and others are processed.

The problem with PE is that when the calling function returns a changed context could 
be found. Some objects could be born, other disappeared, some action incompatible
with the previous context could have been done, as example closing a window while 
processing or clearing a data container that the calling function is using.

How to deal with this? A way is to check some 'context flags' on return from
PE to search for a 'wrong context' and take appropriate actions.
But this is not a good general solution because implies that the function that calls, and
returns, from PE knows about the whole context.

As example, A() is a function that access some data (kept in any kind of container a list, a 
file, etc), B() is a function that makes a lengthy operation and uses PE.
If we have a code flow like the following

    A()
    |--->   B()
    |        |-----> qApp->processEvents()
    |        |-----> do something else
    |                   return
    |--->   ....
    |--->   data.access()

We should check inside of B() if the data is no more valid and eventually return an error code.
But B() knows nothing about the our data. Of course we should check in A(), just after the B() call,
but this is sub-optimal because it implies that we know for sure that B() does not accesses the data
_nor_ any other function called by B() before to return to A().

In real software the call chain between the function that uses a resource and the function that
pass the control to the application event loop and return from it can be very long and complex.



                                      INTRODUCING EXCEPTIONS

The standard way that C++ has to deal with this kind of problems is called exceptions handling.

If B() throws an exception, returning from PE, on a corrupted database, and the first catch clause
is in A(), no matter how long and complex the chain is, A() will catch the exception before anything
else is done.

This seems interesting, but has two drawbacks, one general, one depending on Qt.


-Exception resuming/termination models

What happens if B() is called on start up, on data refreshing, or, in general, in a context where database
consistency is not important or, worst, should not be checked? The exception will be thrown, no catch 
clause will take the exception, the default handler will be invoked and this means, at least in C++,
program termination [in C++ you can provide a custom set_terminate() function to handle uncaught 
exceptions but you cannot continue from here, just clean-up and exit].
The general problem has a name and is called 'C++ supports termination-model exceptions, not resumption'.

So we need a way to let B() throw an exception _only_ if the exception will be caught, IE
only if the abnormal condition is 'interesting' in the actual context.


-Exceptions trough signals/slots

Standard C++ try-throw-catch exception system is not compatible with Qt signals/slots. If a function B() is
called by A() by means of a signal the catch clause will not work, also if signals/slots in Qt3 are wrappers
to direct calls.
         
    A()
    |---> try
    |---> emit mySignal  --> slotMySignal() 
    |                           |-----> throw X
    |---> catch(x)
    |      |---> we will NOT catch X


It is possible to code to avoid signals/slots when exceptions are involved, but the _real_ problem is
that also PE is a barrier to exceptions propagation.

What happens is the following:

  A()
    |---> try
    |       B()
    |        |-----> qApp->processEvents()
    |        |                           |----> C()
    |        |                                   |----> database.clear()
    |        |                                throw databaseIsEmpty
    |        |<----- qApp->processEvents()
    |        | 
    | <----return
    | 
    |---> catch(databaseIsEmpty)
    |      |
    |      |---> we will NOT catch databaseIsEmpty


This is very unfortunate.



                                      INTRODUCING EXCEPTION MANAGER

If we rewrite the above scheme as follows:

  A()
    |---> try
    |       B()
    |        |-----> qApp->processEvents()
    |        |                           |----> C()
    |        |                                   |----> database.clear()
    |        |                                throw databaseIsEmpty
    |        |                                   |
    |        |<----- qApp->processEvents()<----return
    |        | 
    |        if (databaseIsEmpty is throwable)
    |              throw databaseIsEmpty
    | <----return
    | 
    |---> catch(databaseIsEmpty)
    |      |
    |      |---> NOW we will catch databaseIsEmpty



Two things have changed between the schemes.

      - The potential exception is checked to verify if it is among the throwables exceptions

      - The exception is thrown in the same region* of the catch clause


*[A 'region' is the code that executes between two calls of PE]


Class ExceptionManager does exactly this, checks the exception against a throwable set, wait until
the correct region is reached and finally throws the exception.

If we rewrite the above code to use ExceptionManager helper macros we have:

 A() {
      .....
      try {
          EM_REGISTER(databaseIsEmpty); // adds databaseIsEmpty to the throwable set

          .....
          B();
          .....

          EM_REMOVE(databaseIsEmpty); // removes databaseIsEmpty from the throwable set

      } catch (int i) {

            EM_REMOVE(databaseIsEmpty);

            if (i == databaseIsEmpty) {

                   .....handle the exception....

                   EM_CHECK_PENDING; // re-check any other pending exception
            }   
      }
      .....
}

B() {
      .....
      EM_BEFORE_PROCESS_EVENTS; // some magic occurs ;-)

      while(something_happens)
          qApp->processEvents();

      EM_AFTER_PROCESS_EVENTS; // throws the pending exceptions belonging to the current region
      .....
}

C() {
      .....
      database.clear();
      EM_RAISE(databaseIsEmpty); // checks if databaseIsEmpty is throwable and, in case,
      .....                      // flags it as 'raised'. In the latter case it will be 
      .....                      // thrown, but only when returning in the correct region.                       
}

 
With this scheme everything works as expected. There are some things to note:

1)  In B() there is no knowledge of 'databaseIsEmpty'. B() does not have to know about the general
    context at all.

2)  At the end of the catch clause any other pending exception will be thrown, so to allow for
    multiple raised exceptions.

3)  The same exception will be thrown as many times as has been registered. What it means is
    ExceptionManager supports nested try-catch blocks, also when looking for the same exception,
     in this case each catch clause will be called with the same exception and in the correct order.
 
4)  ExceptionManager has an internal multi-region stack. What it means is that try-catch blocks
    can be nested _across_ many PE calls: each catch clause will be called with the correct raised
    exception and in the correct time, when returning in the corresponding region from a PE call.
    No matter when the exceptions have been raised.



                                      TECHNICAL DETAILS

A 'region' is the code that executes between two calls of qApp->processEvents()
the code in a region has the stack property. Ie if fb() is called inside fa() then
fb() will return before fa().

A 'catch set' is a set of exceptions that are added at the beginning of the
same try block. Given a group of exceptions of the same catch set the following can occur:

1- No exception is raised -> the catch clause is not called.

2- Only one exception Ex of the set is raised -> the catch clause is called
   with Ex parameter.

3- More then one exception Ex1, Ex2,..Exn are raised -> the catch clause is
   called with Ex1 parameter, ie the first priority exception. The exception
   priority is given when adding the exceptions at the beginning of try block.
   The last added is the first priority.


The totalThrowableSet is a list of exceptions that can be raised with a call to
raise(excp).

The regionThrowableSet is a subset of totalThrowableSet and lists the exceptions
that can be thrown in the corresponding region.

The regionThrowableSet is saved before to call qApp->processEvents() and restored
on return.

A call to qApp->processEvents() trigger a region boundary. So a new and empty
regionThrowableSet must be used. To let ExceptionManager to know the region crossing time,
ie qApp->processEvents() call, we use the convention that the call to saveThrowableSet()
is done 'just before' the qApp->processEvents() call. Where 'just before' it means
that no others ExceptionManager method must be called between saveThrowableSet() and
processEvents().

        int currentRegionId = saveThrowableSet();

        .....(no more ExceptionManager calls).....

        qApp->processEvents();

        .....(no ExceptionManager calls).....

        restoreThrowableSet(currentRegionId);

The region throwable sets are saved in a list: throwableSetList

When a call to raise(excp) occurs totalThrowableSet is walked to find excp.
If the exception is found then the exception is tagged to be thrown.

In this case the flag  isRaised is set in _all_ the occurrences of excp in the
regionThrowableSet and in _all_ the occurrences of excp in throwableSetList.
This is because the exception will be thrown in each region upon re-entering,
not only in the current region. And in the same region will be thrown as many
times as are the occurrences of excp in the corresponding throwable set

Upon restoring the throwable set with restoreThrowableSet() it is safe to
throw any pending exception with:

        throwPending();


Method throwPending() walks _in order_ the regionThrowableSet ONLY to find all the
exceptions with the flag isRaised set.

This is because C++ throw-catch does not seem to be able to walk-back across
qApp->processEvents() boundaries, ie across regions. So _only_ the pending exceptions
of the current region will be thrown. The others will be eventually thrown later.

ExceptionManager throws ONLY ONE exception among the matching exceptions set. Then, in the
catch clause throwPending() is called again. This is to guarantee that exceptions are
thrown in the correct order.
Note that the catch clause is always in the same region of the throw command.


                The exception thrown is the last that has been added


-Removing exceptions from throwables list

Normally an exception is removed from throwables list by code when leaving try block. 
But thrown exceptions will by-pass try block and will go directly in the catch clause.
So all the exceptions added in try block must be removed in the catch clause, also if
the thrown exception is not handled there.

Note that in the catch clause all the exceptions of the catch set will be removed.
If there are two exceptions raised, the first will throw and the catch clause will
remove both. So the second exception will never be thrown. This is to take in account 
when adding exceptions in the try clause:

                        NO!!!                                           YES
        try {                                           try {
                EM_REGISTER(very_bad_one);                      EM_REGISTER(small_one);
                EM_REGISTER(small_one);                         EM_REGISTER(very_bad_one);


When a remove(excp) occurs regionThrowableSet and  totalThrowableSet are walked
in order from newest entry to oldest and the first occurrence of excp is removed.
So to take in account the case where the same exception is added twice:

        fa() {
                ......
                try {
                        EM_REGISTER(myExcp);
                        .......(no processEvents() here, same region)
                        fb();
                        .......(no processEvents() here, same region)
                        EM_REMOVE(myExcp);
                } catch ()

Where
        fb() {
                ......
                try {
                        EM_REGISTER(myExcp);
                        .......
                        EM_REMOVE(myExcp);
                } catch ()


In this case myExcp will be added twice and must be removed twice. And after the
first remove in fb() catch clause will be thrown again and caught in the fa() catch clause.

So at the end of the catch clause there must always be the throwPending() call.