Foolscap Failure Reporting
==========================

Signalling Remote Exceptions
----------------------------

The ``remote_`` -prefixed methods which Foolscap invokes, just like their
local counterparts, can either return a value or raise an exception. Foolscap
callers can use the normal Twisted conventions for handling asyncronous
failures: ``callRemote`` returns a Deferred object, which will eventually
either fire its callback function (if the remote method returned a normal
value), or its errback function (if the remote method raised an exception).

There are several reasons that the Deferred returned by ``callRemote`` might
fire its errback:

- local outbound schema violation: the outbound method arguments did not
  match the ``RemoteInterface`` that is in force. This is an optional form of
  typechecking for remote calls, and is activated when the remote object
  describes itself as conforming to a named ``RemoteInterface`` which is also
  declared in a local class. The local constraints are checked before the
  message is transmitted over the wire. A constraint violation is indicated
  by raising ``foolscap.schema.Violation`` , which is delivered through the
  Deferred's errback.
- network partition: if the underlying TCP connection is lost before the
  response has been received, the Deferred will errback with a
  ``foolscap.ipb.DeadReferenceError`` exception. Several things can cause
  this: the remote process shutting down (intentionally or otherwise), a
  network partition or timeout, or the local process shutting down
  (``Tub.stopService`` will terminate all outstanding remote messages before
  shutdown).
- remote inbound schema violation: as the serialized method arguments were
  unpacked by the remote process, one of them violated that processes inbound
  ``RemoteInterface`` . This check serves to protect each process from
  incorrect types which might either confuse the subsequent code or consume a
  lot of memory. These constraints are enforced as the tokens are read off
  the wire, and are signalled with the same ``Violation`` exception as above
  (but this may be wrapped in a ``RemoteException`` : see below).
- remote method exception: if the ``remote_`` method raises an exception, or
  returns a Deferred which subsequently fires its errback, the remote side
  will send the caller that an exception occurred, and may attempt to provide
  some information about this exception. The caller will see an errback that
  may or may not attempt to replicate the remote exception. This may be
  wrapped in a ``RemoteException`` . See below for more details.
- remote outbound schema violation: as the remote method's return value is
  serialized and put on the wire, the values are compared against the
  return-value constraint (if a ``RemoteInterface`` is in effect). If it does
  not match the constraint, a Violation will be raised (but may be wrapped in
  a ``RemoteException`` ).
- local inbound schema violation: when the serialized return value arrives on
  the original caller's side of the wire, the return-value constraint of any
  effective ``RemoteInterface`` will be applied. This protects the caller's
  response code from unexpected values. Any mismatches will be signalled with
  a Violation exception.

Distinguishing Remote Exceptions
--------------------------------

When a remote call fails, what should you do about it? There are several
factors to consider. Raising exceptions may be part of your remote API:
easy-to-use exceptions are a big part of Python's success, and Foolscap
provides the tools to use them in a remote-calling environment as well.
Exceptions which are not meant to be part of the API frequently indicate
bugs, sometimes as precondition assertions (of which schema Violations are a
subset). It might be useful to react to the specific type of remote
exception, and/or it might be important to log as much information as
possible so a programmer can find out what went wrong, and in either case it
might be appropriate to react by falling back to some alternative code path.

Good debuggability frequently requires at least one side of the connection to
get lots of information about errors that indicate possible bugs. Note that
the ``Tub.setOption("logLocalFailures", True)`` and
``Tub.setOption("logRemoteFailures", True)`` options are relevant: when these
options are enabled, exceptions that are sent over the wire (in one direction
or the other) are recorded in the Foolscap log stream. If you use exceptions
as part of your regular remote-object API, you may want to consider disabling
both options. Otherwise the logs may be cluttered with perfectly harmless
exceptions.

Should your code pay attention to the details of a remote exception (other
than the fact that an exception happened at all)? There are roughly two
schools of thought:

- Distrust Outsiders: assume, like any sensible program which connects to the
  internet, that the entire world is out to get you. Use external services to
  the extent you can, but don't allow them to confuse you or trick you into
  some code path that will expose a vulnerability. Treat all remote
  exceptions as identical.
- "E" mode: treat external code with the same level of trust or distrust that
  you would apply to local code. In the "E" programming language (which
  inspires much of Foolscap's feature set), each object is a separate trust
  domain, and the only distinction made between "local" and "remote" objects
  is that the former may be called synchronously, while the latter may become
  partitioned. Treat remote exceptions just like local ones, interpreting
  their type as best you can.

From Foolscap's point of view, what we care about is how to handle exceptions
raised by the remote code. When operating in the first mode, Foolscap will
merge all remote exceptions into a single exception type named
``foolscap.api.RemoteException`` , which cannot be confused with regular
Python exceptions like ``KeyError`` and ``AttributeError`` . In the second
mode, Foolscap will try to convert each remote exception into a corresponding
local object, so that error-handling code can catch e.g. ``KeyError`` and use
it as part of the remote API.

To tell Foolscap which mode you want to use, call
``tub.setOption("expose-remote-exception-types", BOOL)`` , where BOOL is
either True (for the "E mode") or False (for the "Distrust Outsiders" mode).
The default is True.

In "Distrust Outsiders" mode, a remote exception will cause the caller's
errback handler to be called with a regular ``Failure`` object which contains
a ``foolscap.api.RemoteException`` , effectively hiding all information about
the nature of the problem except that it was caused by some other system.
Caller code can test for this with ``f.check`` and ``f.trap`` as usual. If
the caller's code decides to investigate further, it can use
``f.value.failure`` to obtain the ``CopiedFailure`` (see below) that arrived
from the remote system. Note that schema Violations which are caught on the
local system are reported normally, whereas Violations which are caught on
the remote system are reported as RemoteExceptions.

In "E mode", a remote exception will cause the errback handler to be called
with a ``CopiedFailure`` object. This ``CopiedFailure`` will behave as much
as possible like the corresponding Failure from the remote side, given the
limitations of the serialization process (see below for details). In
particular, if the remote side raises e.g. a standard Python ``IndexError`` ,
the local side can use ``f.trap(IndexError)`` to catch it. However, this same
f.trap call would also catch locally-generated IndexErrors, which could be
confusing.

Examples: Distrust Outsiders
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Since Deferreds can be chained, it is quite common to see remote calls
sandwiched in the middle of two (possibly asynchronous) local calls. The
following snippet performs a local processing step, then asks a remote server
for information, then adds that information into a local database. All three
steps are asynchronous.

.. code-block:: python

    
    # Example 1
    def get_and_store_record(name):
        d = local_db.getIDNumber(name)
        d.addCallback(lambda idnum: rref.callRemote("get_record", idnum))
        d.addCallback(lambda record: local_db.storeRecord(name))
        return d

To motivate an examination of error handling, we'll extend this example to
use two separate servers for the record: if one of them doesn't have it, we
ask the other. The first server might raise ``KeyError`` to tell us it can't
find the record, or it might experience some other internal error, or we
might lose the connection to that server before it can get us an answer: all
three cases should prompt us to talk to the second server.

.. code-block:: python

    
    # Example 2
    from foolscap.api import Tub, RemoteException
    t = Tub()
    t.setOption("expose-remote-exception-types", False) # Distrust Outsiders
    ...
    
    def get_and_store_record(name):
        d = local_db.getIDNumber(name)
        def get_record(idnum):
            d2 = server1.callRemote("get_record", idnum) # could raise KeyError
            def maybe_try_server2(f):
                f.trap(RemoteException)
                return server2.callRemote("get_record", idnum) # or KeyError
            d2.addErrback(maybe_try_server2)
            return d2
        d.addCallback(get_record)
        d.addCallback(lambda record: local_db.storeRecord(name))
        return d

In this example, only a failure that occurs on server1 will cause the code to
attempt to use server2. A locally-triggered error will be trapped by the
first line of ``maybe_try_server2`` and will not proceed to the second
``callRemote`` . This allows a more complex control flow like the following:

.. code-block:: python

    
    # Example 3
    def get_and_store_record(name):
        d = local_db.getIDNumber(name) # could raise IndexError
    
        def get_record(idnum):
            d2 = server1.callRemote("get_record", idnum) # or KeyError
            def maybe_try_server2(f):
                f.trap(RemoteException)
                return server2.callRemote("get_record", idnum) # or KeyError
            d2.addErrback(maybe_try_server2)
            return d2
        d.addCallback(get_record)
    
        d.addCallback(lambda record: local_db.storeRecord(name))
    
        def ignore_unknown_names(f):
            f.trap(IndexError)
            print "Couldn't get ID for name, ignoring"
            return None
        d.addErrback(ignore_unknown_names)
    
        def failed(f):
            print "didn't get data!"
            if f.check(RemoteException):
                if f.value.failure.check(KeyError):
                    print "both servers claim to not have the record"
                else:
                    print "both servers had error"
            else:
               print "local error"
            print "error details:", f
        d.addErrback(failed)
    
        return d

The final ``failed`` method will catch any unexpected error: this is the
place where you want to log enough information to diagnose a code bug. For
example, if the database fetch had returned a string, but the RemoteInterface
had declared ``get_record`` as taking an integer, then the ``callRemote``
would signal a (local) Violation exception, causing control to drop directly
to the ``failed()`` error handler. On the other hand, if the first server
decided to throw a Violation on its inbound argument, the ``callRemote``
would signal a RemoteException (wrapping a Violation), and control would flow
to the ``maybe_try_server2`` fallback.

It is usually best to put the errback as close as possible to the call which
might fail, since this provides the highest "signal to noise ratio" (i.e. it
reduces the number of possibilities that the error-handler code must handle).
But it is frequently more convenient to place the errback later in the
Deferred chain, so it can be useful to distinguish between the local
``IndexError`` and a remote exception of the same type. This is the same
decision that needs to be made with synchronous code: whether to use lots of
``try:/except:`` blocks wrapped around individual method calls, or to use one
big block around a whole sequence of calls. Smaller blocks will catch an
exception sooner, but larger blocks are less effort to write, and can be more
appropriate, especially if you do not expect exceptions to happen very often.

Note that if this example had used "E mode" and the first remote server
decided (perhaps maliciously) to raise ``IndexError`` , then the client could
be tricked into following the same ignore-unknown-names code path that was
meant to be reserved for a local database miss.

To examine the type of failure more closely, the error-handling code should
access the ``RemoteException`` 's ``.value.failure`` attribute. By making the
following change to ``maybe_try_server2`` , the behavior is changed to only
query the second server in the specific case of a remote ``KeyError`` . Other
remote exceptions (and all local exceptions) will skip the second query and
signal an error to ``failed()`` . You might want to do this if you believe
that a remote failure like ``AttributeError`` is worthy of error-logging
rather than fallback behavior.

.. code-block:: python

    
    # Example 4
            def maybe_try_server2(f):
                f.trap(RemoteException)
                if f.value.failure.check(KeyError):
                    return server2.callRemote("get_record", idnum) # or KeyError
                return f

Note that you should probably not use ``f.value.failure.trap`` , since if the
exception type does not match, that will raise the inner exception (i.e. the
``KeyError`` ) instead of the ``RemoteException`` , potentially confusing
subsequent error-handling code.

Examples: E Mode
~~~~~~~~~~~~~~~~

Systems which use a lot of remote exceptions as part of their inter-process
API can reduce the size of the remote-error-handling code by switching modes,
at the expense of risking confusion between local and remote occurrences of
the same exception type. In the following example, we use "E Mode" and look
for ``KeyError`` to indicate a remote ``get_record`` miss.

.. code-block:: python

    
    # Example 5
    from foolscap.api import Tub
    t = Tub()
    t.setOption("expose-remote-exception-types", True) # E Mode
    ...
    
    def get_and_store_record(name):
        d = local_db.getIDNumber(name)
    
        def get_record(idnum):
            d2 = server1.callRemote("get_record", idnum) # or KeyError
            def maybe_try_server2(f):
                f.trap(KeyError)
                return server2.callRemote("get_record", idnum) # or KeyError
            d2.addErrback(maybe_try_server2)
            return d2
        d.addCallback(get_record)
    
        d.addCallback(lambda record: local_db.storeRecord(name))
    
        def ignore_unknown_names(f):
            f.trap(IndexError)
            print "Couldn't get ID for name, ignoring"
            return None
        d.addErrback(ignore_unknown_names)
    
        def failed(f):
            print "didn't get data!"
            if f.check(KeyError):
                # don't bother showing details
                print "both servers claim to not have the record"
            else:
                # show details by printing "f", the Failure instance
                print "other error", f
        d.addErrback(failed)
    
        return d

In this example, ``KeyError`` is part of the remote ``get_record`` method's
API: it either returns the data, or it raises KeyError, and anything else
indicates a bug. The caller explicitly catches KeyError and responds by
either falling back to the second server (the first time) or announcing a
servers-have-no-record error (if the fallback failed too). But if something
else goes wrong, the client indicates a different error, along with the
exception that triggered it, so that a programmer can investigate.

The remote error-handling code is slightly simpler, relative to the identical
behavior expressed in Example 4, since ``maybe_try_server2`` only needs to
use ``f.trap(KeyError)`` , instead of needing to unwrap a ``RemoteException``
first. But when this error-handling code is at the end of a larger block
(such as the ``f.trap(IndexError)`` in ``ignore_unknown_names()`` , or the
``f.check(KeyError)`` in ``failed()`` ), it is vulnerable to confusion: if
``local_db.getIDNumber`` raised ``KeyError`` (instead of the expected
``IndexError`` ), or if the remote server raised ``IndexError`` (instead of
``KeyError`` ), then the error-handling logic would follow the wrong path.

Default Mode
~~~~~~~~~~~~

Exception modes were introduced in Foolscap-0.4.0 . Releases before that only
offered "E mode". The default in 0.4.0 is "E mode"
(expose-remote-exception-types=True), to retain compatibility with the
exception-handling code in existing applications. A future release of
Foolscap may change the default mode to expose-remote-exception-types=False,
since it seems likely that apps written in this style are less likely to be
confused by remote exceptions of unexpected types.

CopiedFailures
--------------

Twisted uses the ``twisted.python.failure.Failure`` class to encapsulate
Python exceptions in an instance which can be passed around, tested, and
examined in an asynchronous fashion. It does this by copying much of the
information out of the original exception context (including a stack trace
and the exception instance itself) into the ``Failure`` instance. When an
exception is raised during a Deferred callback function, it is converted into
a Failure instance and passed to the next errback handler in the chain.

When ``RemoteReference.callRemote`` needs to transport information about a
remote exception over the wire, it uses the same convention. However, Failure
objects cannot be cleanly serialized and sent over the wire, because they
contain references to local state which cannot be precisely replicated on a
different system (stack frames and exception classes). So, when an exception
happens on the remote side of a ``callRemote`` invocation, and the
exception-handling mode passes the remote exception back to the calling code
somehow, that code will receive a ``CopiedFailure`` instance instead.

In "E mode", the ``callRemote`` 's errback function will receive a
``CopiedFailure`` in response to a remote exception, and will receive a
regular ``Failure`` in response to locally-generated exceptions. In "Distrust
Outsiders" mode, the errback will always receive a regular ``Failure`` , but
if ``f.check(foolscap.api.RemoteException)`` is True, then the
``CopiedFailure`` can be obtained with ``f.value.failure`` and examined
further.

``CopiedFailure`` is designed to behave very much like a regular ``Failure``
object. The ``check`` and ``trap`` methods work on ``CopiedFailure`` s just
like they do on ``Failure`` s.

However, all of the Failure's attributes must be converted into strings for
serialization. As a result, the original ``.value`` attribute (which contains
the exception instance, which might contain additional information about the
problem) is replaced by a stringified representation, which tends to lose
information. The frames of the original stack trace are also replaced with a
string, so they can be printed but not examined. The exception class is also
passed as a string (using Twisted's ``reflect.qual`` fully-qualified-name
utility), but ``check`` and ``trap`` both compare by string name instead of
object equality, so most applications won't notice the difference.

The default behavior of CopiedFailure is to include a string copy of the
stack trace, generated with ``printTraceback()`` , which will include lines
of source code when available. To reduce the amount of information sent over
the wire, stack trace strings larger than about 2000 bytes are truncated in a
fashion that tries to preserve the top and bottom of the stack.

unsafeTracebacks
~~~~~~~~~~~~~~~~

Applications which consider their lines of source code or their exceptions'
list of (filename, line number) tuples to be sensitive information can set
the "unsafeTracebacks" flag in their Tub to False; the server will then
remove stack information from the CopiedFailure objects it sends to other
systems.

.. code-block:: python

    
    t = Tub()
    t.unsafeTracebacks = False

When unsafeTracebacks is False, the ``CopiedFailure`` will only contain the
stringified exception type, value, and parent class names.
