spec_exceptionHandling.dox

// put the namespace around the doxygen block so we don't have to give it all the time in the code to get links
namespace ChimeraTK {
/**
\page spec_execptionHandling Technical specification: Exception handling for device runtime errors

<b>DRAFT VERSION, WRITE-UP IN PROGRESS!</b>

\section spec_execptionHandling_intro Introduction

Exceptions are handled by ApplicationCore in a way that the application developer does not need to care much about it.

ChimeraTK::runtime_error exceptions are caught by the framework and are reported to the DeviceModule. The DeviceModule handles this exception and periodically tries to open the device. Communication with the faulty device is frozen or delayed until the device is functional again. In case of several devices only the faulty device is frozen. Faulty devices do not prevent the application from starting, only the parts of the application that depend on the fault device are waiting for the device to come up.

Input variables of ApplicationModules which cannot be read due to a faulty device will set and propagate the DataValidity::faulty flag (see also \link spec_dataValidityPropagation \endlink).

When the device is functional, it be (re)initialised by using application-defined initialisation handlers and also recover the last known values of its process variables.

\subsection spec_exceptionHandling_intro_terminology Special terminology used in this document

- An read operation might be "skipped". It means, the operation will not take place at all. Instead, the data is marked as DataValidity::faulty. Note: This term is also used, if the a running operation is interrupted by an exception.
- An read operation might be "frozen". This means, the function called will not return until the fault state is resolved and the operation is executed. Freezing always happens before the actual operation is executed and hence will always act on pre-existing fault states only.
- An write operation might be "delayed". This means, the operation will not be executed immediately and the calling thread continues. The operation will be asynchronosuly executed when the fault state is resolved. Note that the VersionNumber specified in the write operation will be retained and also used for the delayed write operation.
- Whenever a write operation or a call to write() is mentioned, destructive writes via writeDestructively() are included. The destructive write optimisation makes no difference for the exception handling.


\section spec_execptionHandling_behaviour A. Behavioural description

- 1. All ChimeraTK::runtime_error exceptions thrown by device register accessors are handled by the framework and are never exposed to user code in ApplicationModules.
  - 1.1 ChimeraTK::logic_error exceptions are left unhandled and will terminate the application. These errors may only occur in the initialisation phase (up to the point where all devices are opened and initialised) and point to a severe configuration error which is not recoverable. (*)
  - 1.2 Exception handling and DataValidity flag propagation is implemented such that it is transparent to a module whether it is directly connected to a device, or whether a fanout or another application module is in between.

- 2. When an exception has been received by the framework (thrown by a device register accessor):
  - 2.1 The exception status is published as a process variable together with an error message.
    - 2.1.1 The variable Devices/<alias>/status contains a boolean flag whether the device is in an error state
    - 2.1.2 The variable Devices/<alias>/message contains an error message, if the device is in an error state, or an empty string otherwise.
  - 2.2 Read operations will propagate the DataValidity::faulty flag to the owning module / fan out (without changing the actual value):
    - 2.2.1 The normal module algorithm code will be continued, to allow this flag to propagate to the outputs in the same way as if it had been received through the process variable itself (c.f. 1.2).
    - 2.2.2 The DataValidity::faulty flag resulting from the fault state is propagated once, even if the variable had the a DataValidity::faulty flag already set previously for another reason.
    - 2.2.3 Read operations without AccessMode::wait_for_new_data are skipped.
    - 2.2.4 Read operations with AccessMode::wait_for_new_data will be skipped once for each accessor to propagate the DataValidity::faulty flag (which counts as new data, i.e. readNonBlocking() will return true). In the following:
      - non-blocking read operations (readNonBlocking() and readLatest()) are skipped and return false, until new data has arrived from the device, and
      - blocking read operations (read()) will freeze until new data has arrived from the device.
      - Note: The device may start sending data already before the recovery procedure (cf. 3.1) is complete. If this is not acceptable, a device specific handshake mechanism has to be implemented in the application to control when the device is allowed to send updates again. (*)
    - 2.2.5 If the fault state had been resolved in between two read operations (regardless of the type) and the device had become faulty again before the second read is executed, it is not defined whether the second operation will frozen/skipped (depending on the type) or not. The second operation might behave either like it is a new exception or like the same fault state would still prevail. (*)
  - 2.3 Write operations will be delayed. In case of a fault state (new or persisting), the actual write operation will take place asynchronously when the device is recovering. The same mechanism as used for 3.1.2 is used here, hence the order of write operations is guaranteed across accessors, but only the latest written value of each accessor prevails. (*)
    - 2.3.1 The return value of write() indicates whether data was lost in the transfer. If the write has to be delayed due to an exception, the return value will be true, if a previously delayed and not-yet written value is discarded in the process, false otherwise.
    - 2.3.2 When the delayed value is finally written to the device during the recovery procedure, it is guaranteed that no data loss happens (writes with data loss will be retried).
    - 2.3.3 It is guaranteed that the write takes place before the device is considered fully recovered again and other transfers are allowed (cf. 3.1).
  - 2.4 In case of exceptions, there is no guaranteed realtime behaviour, not even for "non-blocking" transfers. (*)

- 3. The framework tries to resolve an exception state by periodically re-opening the faulty device.
  - 3.1 After successfully re-opening the device, a recovery procedure is executed before allowing any read/write operations from the AppliactionModules and FanOuts again. This recovery procedure involves:
    - 3.1.1 the execution of so-called initialisation handlers (see 3.2), and
    - 3.1.2 restoring all registers that have been written since the start of the application with their latest values. The register values are restored in the same order they were written. (*)
    - 3.1.3 Finally, Devices/<alias>/deviceBecameFunctional is written to inform any module subscribing this variable about the finished recovery. (*)
  - 3.2 Any number of initialisation handlers can be added to the DeviceModule in the user code. Initialisation handlers are callback function which will be executed when a device is opened for the first time and after a device recovers from an exception, before any process variables are written. See DeviceModule::addInitialisationHandler().

- 4. The behaviour at application start (when all devices are still closed at first) is similar to the case of a later received exception. The only differences are mentioned in 4.2.
  - 4.1 Even if some devices are initially in a persisting error state, the part of the application which does not interact with the faulty devices starts and works normally.
  - 4.2 Initial values are correctly propagated after a device is opened. See \link spec_initialValuePropagation \endlink. Especially, all read operations (even readNonBlocking/readLatest) will be frozen until an initial value has been received. (*)

- 5. Any ApplicationModule can explicitly report a problem with the device by calling DeviceModule::reportException(). This allows the reinitialisation of a device e.g. after a reboot of the device which didn't result in a exception (e.g. because it was too quick to be noticed, or rebooting the device takes place without interrupting the communication).


\subsection spec_execptionHandling_behaviour_comments (*) Comments

- 1.1 In future, maybe logic_errors are also handled, so configuration errors can nicely be presented to the control system. This may be important especially since logic_errors may depend also on the configuration of external components (devices). If e.g. a device is changed (e.g. device is another control system application which has been modified), logic_errors may be thrown in the recovery phase, despite the device had been successfully initialsed previously.

- 2.2.4 Preventing the device to send data before the recovery is complete is not trivial in the general case for asynchronous transfers (i.e. wait_for_new_data). Race conditions might occur if the transport layer does not guarantee the order of packets (e.g. UDP), in which case unsubscribing a variable might not guarantee that no more data arrives which has been sent before unsubscribing. Hence it was decided not to specify a mechanism which would guarantee that no asychronous data transfers take place before the recovery has completed.

- 2.2.5 Not defining the behaviour here avoids a conflict with 1.2 without requiring a complicated implementation which does not block in this case. Implementing this would not present any gain for the application. If there are many exceptions on the same device in a short period of time, the number of faulty data updates seen by the application modules will always depend on the speed the module is attempting to read data (unless we require every exception to be visible to every module, but this will have complex effects, too). It might break consistency of the number of updates sent through different paths in an application, but applications should anyway not rely on that and use a DataConsistencyGroup to synchronise instead. Hence, the implementation will block always if a blocking read sees a known exception

- 2.3 / 3.1.3 If timing is important for write operations (e.g. must not write a sequence of registers too fast), or if multiple values need to be written to the same register in sequence, the application cannot fully rely on the framework's recovery procedure. The framework hence provides the process variable Devices/<alias>/deviceBecameFunctional for each device, which will be written each time the recovery procedure is completed (cf. 3.1.3). ApplicationModules which implement such timed sequence need to receive this variable and restart the entire sequence after the recovery.

- 2.4 Even non-blocking read and write operations are not truely non-blocking, since they are still synchronous. The "non-blocking" guarantee only means that the operation does not block until new data has arrived, and that it is not frozen until the device is recovered. For the duration of the recovery procedure and of course for timeout periods these operations may still block.

- 3.1.2 For some applications, the order of writes may be important, e.g. if firmware expects this. Please note that the VersionNumber is insufficient as a sorting criteria, since many writes may have been done with the same VersionNumber (in an ApplicationModule, the VersionNumber used for the writes is determined by the largest VersionNumber of the inputs).

- 4.2 DataValidity::faulty is initially set by default, so there is no need to propagate this flag initially. To prevent race conditions and undefined behaviour, it even needs to be made sure that the flag is not propagated unnecessarily. The behaviour of non-blocking reads presents a slight asymmetry between the initial device opening and a later recovery. This will in particular be visible when restarting a server while a device is offline. If a module only uses readLatest()/readNonBlocking() (= read() for poll-type inputs) for the offline device, the module was still running before the server restart using the last known values for the dysfunctional registers (and flagging all outputs as faulty). After the restart, the module has to wait for the initial value and hence will not run until the device becomes functional again. To make this behaviour symmetric, one would need to persist the values of device inputs. Since this only affects a corner case in which anyway no usable output is produced, this slight inconsistency is considered acceptable.

\section spec_execptionHandling_high_level_implmentation B. Implementation

A so-called ExceptionHandlingDecorator is placed around all device register accessors (used in ApplicationModules and FanOuts). It is responsible for catching the exceptions and implementing most of the behaviour described in A.2. It has to work closely with the DeviceModule and there is a complex syncronsiation and locking scheme, which is described here, together with the according interface functions of the DeviceModule. The sequence executed in the DeviceModule is described in \ref spec_execptionHandling_high_level_implmentation_deviceModule.


\subsection spec_execptionHandling_high_level_implmentation_interface B.4 Internal interface between ExceptionHandlingDecorator and DeviceModule

FIXME: NUMBERING

Note: This section defines the internal interface on a low level. Helper functions, like getters and setters, are intenionally not mentioned here, since those are (in this context) unimportant details which can be chosen at will to structure the code conveniently. The entire interface between the ExceptionHandlingDecorator and the DeviceModule should be protected and the two classes should be friends, to prevent interference with the interface from other entities. Only DeviceModule::reportException() is public, see A.5.

- 4.1 The boolean flag DeviceModule::deviceHasError
  - 4.1.1 is used by the RecoveryAccessor to detect prevailing error conditions, to know when transfers have to be skipped, frozen or delayed (cf. 1.2 and 1.4).
  - 4.1.2 The access is protected by the DeviceModule::errorMutex:
    - shared lock allows to read
    - unique lock allows to read and write

- 4.2 The atomic DeviceModule::transferCounter (*)
  - 4.2.1 tracks the number of on-going (synchronous) transfers, and
  - 4.2.2 is used by the DeviceModule to wait until they are all terminated (2.3.15).

- 4.3 The DeviceModule::recoveryHelpers list elements
  - 4.3.1 are used to delay write operations and to restore the last-written values during recovery.
  - 4.3.2 are protected by the DeviceModule::recoveryMutex:
    - shared lock allows to update the application buffer of RecoveryHelper::accessor and to change the RecoveryHelper::versionNumber (*)
    - unique lock allows to call RecoveryHelper::accessor.write() and to read the RecoveryHelper::versionNumber

- 4.4 The cppext::future_queue DeviceModule::errorQueue
  - 4.4.1 is used by the RecoveryAccessor to inform the DeviceModule about new exceptions.
  
- 4.6 The following mutexes govern critical sections (besides variable access listed above):
  - 4.6.1 DeviceModule::errorMutex protects (*)
    - the (positive) decision to start a transfer followed by incrementing the DeviceModule::transferCounter in 1.2.1 to 1.2.3, against
    - setting DeviceModule::deviceHasError flag in 1.6.1.

  - 4.6.2 DeviceModule::recoveryMutex protects (*)
    - writing the DeviceModule::recoveryHelpers to the device and clearing the DeviceModule::deviceHasError flag in 2.3.5 to 2.3.8, against
    - updating the DeviceModule::recoveryHelpers in 1.3.

  - 4.6.3 DeviceModule::initialValueMutex protects (*)
    - the start of a read operation in 1.4.4, against
    - the setup phase of a device until it has been opened and recovered for the very first time in 2.1 to 2.9.


\subsubsection spec_execptionHandling_high_level_implmentation_interface_comments (*) Comments

- 4.2 Reason for not using an (exclusive) lock: Incrementing and decrementing the counter is done in the ExceptionHandlingDecorator for each operation, even if there is no exception or error state. Concurrent operations must not exclude each other, to allow lockfree operation in the no-exception case (if the backend supports it) and to avoid priority inversion, if different application threads have different priority.

- 4.3.2 A shared lock (in contrast to an exclusive lock) is used for the same reasons as in 4.2.

- 4.6.1 This prevents a race condition in 2.3.15. If a (synchronous) transfer might be started after DeviceModule::deviceHasError has been set, the barrier for new transfers in 2.3.15 would not be effective and the transfer might be even executed only after the device has been re-openend (2.3.1) but before the recovery is complete.

- 4.6.2 This prevents data loss due to a race condition. If the ExceptionHandlingDecorator would update the corresponding DeviceModule::RecoveryHelpers list entry only after it has been written to the device in 2.3.5, but the ExceptionHandlingDecorator would decide not to execute the write operation (1.2) because the DeviceModule thread is still before 2.3.8, the data would not be written to the device at all.

- 4.6.3 This implements freezing reads until the initial value can be read, cf. 4.2.


\subsection spec_execptionHandling_high_level_implmentation_decorator B.1 ExceptionHandlingDecorator

- 1.1 A second, undecorated copy of each writeable device register accessor (*) is used as a so-called recoveryAccessor by the ExceptionHandlingDecorator and the DeviceModule. These recoveryAccessor are used to set the initial values of registers when the device is opened for the first time and to recover the last written values during the recovery procedure.
  - 1.1.1 The recoveryAccessor is stored by the DeviceModule with additional meta data in a so-called RecoveryHelper data structure, which contains:
    - the recoveryAccessor itself,
    - the VersionNumber of the (potentially unwritten) data stored in the accessor,
    - an ordering parameter which determines the order of write opereations during recovery.
    - an atomic flag which indicates whether the value in the recoveryAccessor has already been written to data. (*)
  - 1.1.2 Ordering can be done per device (*), hence each DeviceModule has one 64-bit atomic counter which is incremented for each write operation and the value is stored in the ordering parameter for the recoveryAccessor.
  - 1.1.3 The RecoveryHelper object may be accessed only under a lock to prevent concurrent access during recovery. The lock shall be shared to allow concurrent write operations of different registers - only the DeviceModule needs to obtain an exclusive lock during recovery. The lock is obained by the ExceptionHandlingDecorators via DeviceModule::getRecoverySharedLock().
  
- 1.3 In doPreWrite() the recoveryAccessor with the version number and ordering parameter is updated, and the written flag is cleared. This has to happen while holding the shared recovery lock.
  - 1.3.0 This step needs to be done unconditionally at the very beginning of doPreWrite(), before 1.2 and before delegating preWrite(). (*)
  - 1.3.1 If the written flag was previously not set, the return value of doWriteTransfer() must be forced to true (data lost).
  - 1.3.2 The check wheterh to skip the transfer (cf. 1.2) has to be done without releasing the lock between the write to the recoveryAccessor and the check. (*)

- 1.2 In doPreRead()/doPreWrite(), it must be decided whether to execute xxxTransferYyy(). This part requires a shared lock on the DeviceModule::errorMutex.
  - 1.2.1 xxxTransferYyy() is <i>not</i> executed, if DeviceModule::deviceHasError == true and either:
    - it is a write transfer (cf. A.2.3), or
    - it is a read transfer and AccessMode::wait_for_new_data is not set (cf. A.2.2.3), or
    - it is a read transfer and AccessMode::wait_for_new_data is set and ExceptionHandlingDecorator::previousReadFailed == false (cf. 1.5.1, 1.6.3.1 and A.2.2.4).
    Otherwise xxxTransferYyy() is executed (potentially after it is frozen, see 1.4).
  - 1.2.2 If xxxTransferYyy() is not executed, none of the pre/transfer/post functions must be delegated to the target accessor.
  - 1.2.3 If xxxTransferYyy() is executed, and it is <i>not</i> a read transfer with AccessMode::wait_for_new_data set, the DeviceModule::transferCounter must be incremented.

- 1.4 In doPreRead() certain read operations are frozen in case of a fault state, i.e. startTransfer() returned false (see A.2.2):
  - 1.4.1 The shared lock on the DeviceModule::errorMutex acquired in 1.2 is still kept.
  - 1.4.2 Decide, whether freezing is done (don't freeze yet). Freezing is done if no initial value has been read yet (getCurretVersion() == {nullptr}) and DeviceModule::deviceHasError == true (cf. A.4.2). (*)
  - 1.4.3 Release the DeviceModule::errorMutex.
  - 1.4.4 If the read should be frozen, acquire a shared lock on the DeviceModule::initialValueMutex. (*)

- 1.5 In doPostRead()/doPostWrite():
  - 1.5.0 Delegate postRead() / postWrite() (see 1.6)
  - 1.5.1 If there was no exception, set ExceptionHandlingDecorator::previousReadFailed = false (cf. 1.2.1 and 1.6.3.1).
  - 1.5.3 In doPostWrite() the recoveryAccessor's written flag is set if the write was successful (no exception thrown; data lost flag does not matter here). (*)
  - 1.5.4 In doPostRead(), if no exception was thrown, end overriding the DataValidity returned by the accessor (cf. 1.6.2).
  - 1.5.2 If the DeviceModule::transferCounter was incremented in 1.2.3, decrement it. (*)

- 1.6 In doPostRead()/doPostWrite(), any runtime_error exception thrown by the delegated postRead()/postWrite() is caught (*). The following actions are in case of an exception:
  - 1.6.1 The error is reported to the DeviceModule via DeviceModule::reportException(). This automatically sets DeviceModule::deviceHasError to true. From this point on, no new transfers will be started.(*)
  - 1.6.2 For readable accessors: the DataValidity returned by the accessor is overridden to faulty until next successful read operation (cf. 1.5.4).
    - 1.6.2.1 The code instantiating the decorator (Application::createDeviceVariable()) has to make sure that the ExceptionHandlingDecorator is "inside" the MetaDataPropagatingRegisterDecorator, so the overriden DataValidity flag in case of an exception is properly propagated to the owning module/fan out.
  - 1.6.3 Action depending on the calling operation:
    - 1.6.3.1 All read operations: The ExceptionHandlingDecorator remembers that it is in an exception state by setting ExceptionHandlingDecorator::previousReadFailed = true (cf. 1.2.1 and 1.5.1)
    - 1.6.3.1 read (push-type inputs): return immediately (*)
    - 1.6.3.2 readNonBlocking / readLatest / read (poll-type inputs): Just return (true in readLatest() by definition in poll type). The calling module thread will continue and propagate the DataValidity::faulty flag (cf. 1.6.2).
    - 1.6.3.3 write: Do not block. Write will be later executed by the DeviceModule (see 1.1)
    
- 1.7 In the constructor of the decorator, put the name of the register to DeviceModule::listOfReadRegisters resp. DeviceModule::listOfWriteRegisters depending on the direction the accessor is used.

\subsubsection spec_execptionHandling_high_level_implmentation_decorator_comments (*) Comments

- 1.1 Possible future change: Output accessors can have the option not to have a recovery accessor. This is needed for instance for "trigger registers" which start an operation on the hardware. Also void registers don't have recovery accessors (once the void data type is supported).

- 1.1.1 The written flag cannot be replaced by comparing the version number of the recoveryAccessor and the version number stored in the RecoveryHelper, because normal writes (without exceptions) would not update the version number of the recoveryAccessor.
- 1.1.1 The flag is atomic so it can be set without getting the recoveryLock again in doPostRead(). This has to happen before calling DeviceModule::stopTransfer() to ensure the DeviceModule() does not start the recovery yet.
  When clearing it in doPreRead(), and setting it in the DeviceModule during recovery, the recoveryLock must be held.

- 1.1.2 The ordering guarantee cannot work across DeviceModules anyway. Different devices may go offline and recover at different times. Even in case of two DeviceModules which actually refer to the same hardware device there is no synchronisation mechanism which ensures the recovering procedure is done in a defined order.

- 1.3.0 Updating the recoveryHelper first ensures that no data is lost, even if the write operation attempt is concurrent with a recovery. See 4.6.2.

- 1.3.2 Extending the duration of the lock until the decision whether to skip the transfer will prevent unncessary duplicate writes, which otherwise could occur if the DeviceModule went through the whole critical section 2.3.2 to 2.3.10 in between.

- 1.2.5 The cppext::future_queue in the TransferFuture is a notification queue and hence of the type void. So we don't have to "invent" any value. Also this injection of values is legal, since the queue is multi-producer but single-consumer. This means, potentially concurrent injection of values while the actual accessor might also write to the queue is allowed. Also, the application is the only receiver of values of this queue, so injecting values cannot disturb the backend in any way.

- 1.4.2 In A.2.2.4 it was stated that also in case AccessMode::wait_for_new_data is set blocking read transfers are frozen on the second operation. Nothing is to be implemented for this case, the freezing simply relies on having an empty queue in the accessor. Once the device sends data again, the operation is intrinsically unfrozen.

- 1.4.4 The transferCounter is already incremeted at this point. It is acceptable to freeze anyway in this case by waiting on the initialValueMutex, because the DeviceModule release the mutex after the first successful recovery and never obtains it again, and this happens before it waits for the transferCounter to become 0 in 2.3.15.

- 1.5.2 The state of DeviceModule::deviceHasError does not matter here. The counter always MUST be decreased after a transfer (if it has been incremented in the corresponding preXxx()), whether the transfer failed or not. Also, this must happen after 1.5.3 ===> why? DeviceModule::transferCounter > 0 prevents the DeviceModule from starting the recovery, but during the recovery the written flag will also just be set and not read. The written flag is merely used to determine in the next write whether data has been lost (which is the case if the written flag is not set).

- 1.5.3 The written flag for the recoveryAccessor is used to report loss of data. If the loss of data is already reported directly, it should not later be reported again. Hence the written flag is set even if there was a loss of data in this context.

- 1.6 Remember: exceptions from other phases are redirected to the post phase by the TransferElement base class.

- 1.6.1 No transfers will be started in any of the accessors of the device, including this one. This is important to avoid the race condition described in the comment to 4.1.3

- 1.6.3.1 The freezing is done in doPreRead(), see 1.4.


\subsection spec_execptionHandling_high_level_implmentation_deviceModule B.2 DeviceModule

- 2.1 The application always starts with all devices as closed. For each device, the initial value for Devices/<alias>/status is set to 1 and the initial value for Devices/<alias>/message is set to an error that the device has not been opened yet (the message will be overwritten with the real error message if the first attempt to open fails, see 2.3.1).
- 2.2 The DeviceModule takes care that ExceptionHandlingDecorators initally do not perform any read or write operations, but freeze (cf. 1.4). This happens before running any prepare() of an ApplicationModule, where the first write calls to ExceptionHandlingDecorators might be done.
- 2.3 In the DeviceModule thread, the following procedure is executed (in a loop until termination):
  - 2.3.1 The DeviceModule tries to open the device until it succeeds and isFunctional() returns true.
    - 2.3.1.1 If the very first attempt to open the device after the application start fails, the error message of the exception is used to overwrite the content of Devices/<alias>/message. Otherwise error messages of exceptions thrown by Device::open() are not visible.
  - New position for 2.3.6 The queue of reported exceptions is cleared. (*)
  - 2.3.3 Check that all registers on DeviceModule::listOfReadRegisters are isReadable() and all registers on DeviceModule::listOfWriteRegisters are isWriteable().
    - 2.3.3.1 This involves obtaining an accessor for the register first, which is discarded after the check.
    - 2.3.3.2 If there is an exception, update Devices/<alias>/message with the error message and go back to 2.3.1.
    - 2.3.3.3 If one of the accessors does not meet this condition, throw a ChimeraTK::logic_error.
  - 2.3.4 Device is initialised by iterating initialisationHandlers list.
    - 2.3.4.1 If there is an exception, update Devices/<alias>/message with the error message and go back to 2.3.1.
  - New positon of 2.3.2 Obtain lock for accessing recoveryAccessors.
  - 2.3.5 All valid recoveryAccessors are written in the same order they were originally written.
    - 2.3.5.1 A recoveryAccessor is considered "valid", if it has already received a value, i.e. its current version number is not {nullptr} any more.
    - 2.3.5.2 If there is an exception, update Devices/<alias>/message with the error message, release the lock and go back to 2.3.1.
  - 2.3.7 Devices/<alias>/status is set to 0 and Devices/<alias>/message is set to an empty string.
  - 2.3.8 DeviceModule allows ExceptionHandlingDecorators to execute reads and writes again (cf. 2.3.14)
  - 2.3.9 All frozen read operations (cf. 1.4.4) are notified via DeviceModule::errorIsResolvedCondVar. 
  - 2.3.10 Release lock for recoveryAccessors.
  - 2.3.11 The DeviceModuleThread waits for the next reported exception. The call to reportException in the other thread has already set deviceHasError to true (*). From this point on, no new transfers will be started.
  - 2.3.12 An exception is received.
  - 2.3.13 Devices/<alias>/status is set to 1 and Devices/<alias>/message is set to the first received exception message.
  - 2.3.15 The device module waits until all running read and write operations of ExceptionHandlingDecorators have ended (wait until DeviceModule::activeTransfers == 0). (*)
  - 2.3.16 The thread goes back to 2.3.1 and tries to re-open the device.

\subsubsection spec_execptionHandling_high_level_implmentation_deviceModule_comments (*) Comments

- 2.3.6 The exact place when this is done does not matter, as long as it is done after 2.3.15 (no ongoing synchronous transfers) and before 2.3.8 (resetting deiveHasError). As soon as deviceHasError is cleared new exceptions can be reported, which would be lost if the list was cleared afterwards. Moving it as early as possible after the device has been reopenend has the (slight) advantage, that exceptions which might be reported by asynchronous transfers during the recovery are not discarded, even if the recovery itself does't catch them for some reason. Since exceptions reported by asynchronous transfers are subject to race conditions with the recovery procedure, there cannot be strict guarantees about the behaviour. The optimal place where to reset the queue (to minimise unnecessary recoveries while minimising the probability of rejecting true errors which then need to be found instead later by other transfers) might need to be found in real-life experiments later.

- 2.3.11 Setting the DeviceModule::deviceHasError flag has to be done in the application thread which has caught the exception. If you just send a message and let the device module do both setting and clearing of the flag you can have a race condition: A blocking read would inform the DeviceModule about an exception and continue. The next call to the blocking read is supposed to freeze, but pre-read might not detect this because the device module thread has not woken up yet to set the error flag.

- 2.3.15 The backend has to take care that all operations, also the blocking/asynchronous reads with "waitForNewData", terminate when an exception is thrown, so recovery can take place (see DeviceAccess TransferElement specification).


\subsection spec_execptionHandling_high_level_implmentation_reportException B.2 DeviceModule::reportException()

FIXME missing


\section spec_execptionHandling_known_issues Known issues - OUTDATED (numbers don't even match)

<strike>
- 11.1 In step 2.1: The initial value of deviceError is not set to 1.

- 11.2 In step 2.2.3: is not correctly fulfilled as we are only waiting for device to be opened and don't wait for it to be correctly initialised. The lock 4.2.3 is not implemented at all.

- 11.3 In step 2.3.5: is currently being set before initialisationHandlers and writeAfterOpen.

- 11.4 Check the documentation of DataValidity. ...'Note that if the data is distributed through a triggered FanOut....'

- 11.5 Data validity is currently propagated through the "owner", which conceptually does not always work. A DataFaultCounter needs to be introduced and used at the correct places.

- 11.6 In comment to 1.g: recovery accessors are not optional at the moment.

- 11.7 In 1.c: Currently data is transported even if the "value after construction" is still in.
- 11.8 In 1.i, 6: ThreadedFanout and TriggerFanout do not use non-blocking write because it does not exist yet
- 11.9 In 1.j, 2.5.3: Not implemented like that. The first read blocks, and a special mechanism to propagate the flags is triggered only in the next module.

- 11.10 In 2.3: The device module has a special "first opening" sequence. This is not necessary any more. The "writeAfterOpen" list is obsolete. You can always use the recovery accessors.
- 11.11 In 2.3.4: Recovery accessors are always written. It is not checked whether there is valid data (not "value after construction")
- 11.12 In 2.4.1.1: Write probably re-executed after recovery. This should not happen because the recovery accessor has already done it.
- 11.13 In 2.5.3: The non-blocking read functions always block on exceptions. They should not (only if there is no initial value).
- 11.14 In 2.5.2, 5.1: writeWithoutErrorBlocking is not implemented yet
- 11.15 Asynchronous reads are not working with the current implementation, incl. readAny.

- 11.16 In 3: DeviceAccess : RegisterAccessors throw in doReadTransfer now.
- 11.17 In 4.2.1: reportException does block (should not)
- 11.18 In 4.2.2: blocking wait function does not exist (not needed in current implementation as reportException blocks)

- 11.19 In 5.2.1: Exceptions are caught in doXxxTransfer instead of doPostXxx.
- 11.20 In 5.3.1.2, 5.3.2.1: Decoration of doXxxTransfer does not acquire the lock (which does not even exist yet, see 4.2.3)

- 11.21 In 3.2: Decorators might have to try-catch because they usually can only do their task after calling the delegated postXxx.
- 11.22 In 3.4: The TransferType is not known. Needs to be implemented in TransferElement
- 11.23 In 3.5: PostRead is currently skipped if readNonBlocking or readLatest does not have new data
- 11.24 In 3.6: The waitForNewData calls in the DoocsBackend (using zmq) are currently not interruptible
</strike>


*/

} // end of namespace ChimeraTK