CDE/cde/lib/tt

/* $TOG: README /main/4 1999/08/30 10:44:07 mgreess $ */
1.8 10/27/92
Copyright (c) 1992 by Sun Microsystems, Inc.


TOOLTALK IMPLEMENTATION OVERVIEW


This document presents a roadmap to understanding the implementation
of the main message-passing features of ToolTalk 1.0.1. It does not
aim to describe the details of the algorithms used. In order to find
these out it is necessary to read the source code as well as the
comments. This document should give the reader an overall picture of
how all the various objects that implement message-passing fit
together as well as familiarity with where to find the code that
implements a particular feature. Finally some naming conventions used
throughout the code are given.


********** Please update this document as the code is changed **********



MEMORY MANAGEMENT

The only memory management policy used througout the code is
reference-counting. This is an automatic sort of memory management
since objects are "magically" deleted when there are no longer
references to them. This functionality is implemented by way of
pointer classes that behave just like normal pointers except that they
maintain a reference count in the object they point to. It is
important *not* to mix normal pointers with counted pointers.

MIXING POINTERS

You should never use a C pointer with refcounted objects (i.e., those
inheriting from _Tt_object (actually, from _Tt_new; I have no idea why
_Tt_new and _Tt_object are different)). You should always use
refcounting pointers ("ptrs") with _Tt_objects. The danger is as
follows:

_Tt_object *evilRef;
{
evilRef = new _Tt_object;
//
// refCount is now zero, but evilRef is pointing at our
// object!
//
...
_Tt_object_ptr goodRef = evilRef;
//
// refCount is now 1
//
...
//
// goodRef goes out of scope.
// goodRef::~_Tt_object_ptr() decrements the refCount
// to zero, and deletes our object because the
// refCount is now zero.
//
}
int foo = evilRef->dataMember; // core dump

If you need a static pointer to a _Tt_object, you cannot have a static
_Tt_object_ptr, because static C++ instances aren't allowed. But you
don't want to have a static _Tt_object *, because then you have to
eternally worry about never showing it to a refcounting pointer (which
would try to delete it when it was done with it).

The best thing to do is have a static _Tt_object_ptr *:

static _Tt_object_ptr *ppGlobalObj;
ppGlobalObj = new _Tt_object_ptr;
*ppGlobalObj = new _Tt_object;

Now you can use *ppGlobalObj wherever you would have used your static
_Tt_object_ptr. The _Tt_object you allocated is safely refcounted, so
you can let refcounting pointers do whatever they want with it.
When it's time to clean up, you just

delete ppGlobalObj;

This is a call to ppGlobalObj->~_Tt_object_ptr(), which decrements the
refcount of your object, and *optionally* deletes it if the refcount
is zero. This way, you don't delete your object out from under anyone
who still has a reference to it.

REFCOUNTING INSIDE CONSTRUCTORS

Inside a constructor, never assign the 'this' pointer of a refcounted
object to an automatic refcounting pointer. Inside the constructor,
the refcount is still zero. Assigning 'this' into a refcounting
pointer will increment the refcount to 1. But before the constructor
returns, the automatic refcounting pointer on the stack will be
destructed. The destructor will decrement the object's refcount back
to zero, and therefore delete the object -- before the constructor
even returns!

_Tt_foo::_Tt_foo()
{
// On entry, _Tt_foo::_refcount_ == 0
_Tt_foo_ptr evilRef = this;
// Now, _Tt_foo::_refcount_ == 1
...
// On exit, evilRef->~_Tt_foo_ptr() will be called.
// It will decrement the refcount to zero, and
// then delete this object under construction!
}

Also, be sure not to pass the 'this' pointer to any routine that might
hold a temporary reference to the object while the routine runs.
(Here, "hold a temporary reference to" means "increment the refcount
and then decrement it".) If you must pass the 'this' pointer anywhere
from within the constructor, cast it to const first so that the
compiler will protect you from any routine with an intention of
modifying its refcount. If you absolutely must pass the 'this'
pointer to a routine that might only temporarily reference it, then
manually increment the refcount before doing so, and then manually
decrement the refcount before returning from the constructor

CYCLICAL REFERENCES

Avoid creating data structures that permit cyclical references.
A single reference cycle can cause a massive memory leak.

COMMENT CONVENTIONS

Throughout the code, any comments preceded by XXX: denote notes to the
reader of problems or suggestions for the code they describe.



DESCRIPTION OF OBJECT CLASSES

This section presents all the object classes used to implement the
message-passing portion of ToolTalk along with a brief description of
what each class represents and in which files it can be found. Before
describing the classes we need to mention some important naming
conventions as well as some general comments about client/server
conventions.

Many of the object classes described below have client-side and
server-side implementations. For example, a client-side version of a
ToolTalk message has methods to send the message to the server. On the
server-side there is a corresponding _Tt_message that processes the
new message. In the 1.0 version of ToolTalk, these methods were
roughly coded as:

void _Tt_some_object::
sample_method(...)
{
if (_tt_mp->in_server()) {
... do server-specific actions ...
} else {
... do client-specific actions ...
}
}

where the "in_server" method for the _Tt_mp class returns 1 if the
environment is in server mode (which is true if the method is
executing on behalf of ttsession). The problem with this scheme is
that the library was excessively big because the code to implement the
server, ttsession, was contained in the library even though it wasn't
needed for the clients.

Because of this, the 1.0.1 version split up this method in one of two
ways. The first method is to create two subclasses of the object. One
of the subclasses, named with a "_c_" infix, represents the
client-side state and methods for the object. The other subclass,
named with a "_s_" infix, represents the server-side state and methods
for the object. Each of these subclasses then contains the
corresponding method which now doesn't need the conditional test. The
example above then becomes:

void _Tt_c_some_object::
sample_method(...)
{
... do client-specific actions ...
}


void _Tt_s_some_object::
sample_method(...)
{
... do server-specific actions ...
}

The other mechanism is to still have one object class but name the
object with a "c_" prefix for the client-specific method, and an "s_"
prefix for the server-specific method.

In the 4/93 version of ToolTalk, more classes were split up so that
no references to server-only classes would appear in client code.
As part of this, the client-to-server XDR methods were slightly
changed. You will see (in mp_rpc_implement.C) that apparently
the server uses a XDR function to decode the arguments of some
RPCs which is different from the encoding XDR function. This
seemingly violates the central dogma of XDR, but it really isn't
as bad as it looks. What is happpening is that we have class
hierarchies like:

_Tt_pattern
|
------------------
| |
_Tt_s_pattern _Tt_c_pattern

Only _Tt_pattern has an XDR method -- that is, the data that appears
on the wire is only the common data. A client would create a _Tt_c_pattern
and XDR it out; the server then XDR's it in as a _Tt_s_pattern, but
the _Tt_pattern::XDR routine does all the work and the only difference
is the virtual function table (and any private _Tt_s_pattern data,
which would be initialized by the _Tt_s_pattern constructor.)


The following classes are related to the main message-passing
functionality in ToolTalk. Details on what methods they implement and
precise algorithms are in the source code along with the comments
throughout the code.


Class: _Tt_arg
File: tt/lib/mp/mp_arg.C (client/server methods)
tt/lib/mp/mp_s_arg.C (server-only methods)

This class represents an argument to a message or pattern. Its methods
implement matching arguments with other arguments and constructing and
updating arguments.


Class: _Tt_auth
File: tt/lib/mp/mp_auth.C (client/server methods)
tt/lib/mp/mp_auth_functions.C (utility functions copied from libICE)

This class maintains the authorization level being used by the server
(cookie, des, unix, none). In the case of the cookie authorization
scheme, this class also is responsible for generating the cookie and
writing it to and reading it from the authority file.

Class: _Tt_desktop
File: tt/lib/mp/mp_desktop.C

This class represents "desktop" sessions. Currently this means X11
server sessions but the methods reflect an interface that is largely
independent of X11. As a result, all X11-dependent code is isolated in
this one module.


Class: _Tt_c_message (subclass of _Tt_message)
File: tt/lib/mp/mp_c_message.C

This class represents client-only view of ToolTalk messages. It
contains methods for sending messages to the ttsession server.


Class: _Tt_c_procid (subclass of _Tt_procid)
File: tt/lib/mp/mp_c_procid.C

This class implements client-only methods needed to reply to messages
and to retrieve new messages for the ToolTalk client represented by
this class.


Class: _Tt_file
File: tt/lib/mp/mp_file.C (server/client methods)
tt/lib/mp/mp_s_file.C (server-only methods)

This class represents file objects. The functionality implemented is
for querying for specs contained in the file, joining and quitting
file scopes, and queuing and de-queueing messages on file objects.


Class: _Tt_message
File: tt/lib/mp/mp_message.C

This class represents a ToolTalk message. This class represents
methods that are common to both clients and ttsession. Its subclasses,
_Tt_s_message and _Tt_c_message described below implement methods
specific to client manipulation and server manipulation respectively.
The code is structured so that _Tt_message objects contain the methods
to do delivery to other clients.


Class: _Tt_mp
File: tt/lib/mp/mp_mp.C

This class represents the global state needed by the message-passing
objects. In particular, this object contains many object caches that
are needed by the system as well as holds references to some important
objects such as the default session object.


Class: _Tt_node
File: tt/lib/mp/mp_node.C

This class represents a ToolTalk spec. Its methods create/destroy
specs in a database and add properties to an existing spec.


Class: _Tt_observer
File: tt/lib/mp/mp_observer.C

This class is a very simple class that basically just stores some
message fields that can change for static observers of a message. It
is used only by the _Tt_s_message class. This class is mainly used as
to hold the information necessary to honor static observer "promises"
for message disposition processing.


Class: _Tt_otype
File: tt/lib/mp/mp_otype.C

This class implements methods needed to deal with otype definitions.
In general the operations needed are for setting and returning the
observer, inherited, and handler signatures for this otype definition
as well as printing out the otype definition which, in addition to
debugging, is used to print out the definition for storage under the
Classing Engine and also for dumping out compiled type databases in
source format.


Class: _Tt_pattern
File: tt/lib/mp/mp_pattern.C (server/client methods)
tt/lib/mp/mp_s_pattern.C (server-only methods)

This class represents a ToolTalk pattern. Client-side methods
implement adding/deleting fields to the pattern and
registering/unregistering the pattern by invoking the proper rpc
routines on the server. Server-side methods implement the appropiate
actions for registering/unregistering patterns as well as matching
patterns to messages.


Class: _Tt_procid
File: tt/lib/mp/mp_procid.C

This class represents a ToolTalk client. It represents all of the
methods and data necessary for sending a message to a ToolTalk client.
Its subclasses, _Tt_s_procid and _Tt_c_procid, implement the needed
server-only and client-only methods.


Class: _Tt_ptype
File: tt/lib/mp/mp_ptype.C

This class implements methods needed to deal with ptype definitions
and with starting new instances of ptypes which is functionality
needed to implement the start disposition options for ToolTalk
messages. Also, as with _Tt_otype objects, printing methods are also
defined that are used for storage in type databases and dumping out
ptype definitions in source format.


Class: _Tt_rpc_client
File: tt/lib/mp/mp_rpc_client.C

This class represents a client rpc connection. The methods implemented
are basically wrappers around the rpc interfaces with the important
difference that the api to this object is the same regardless of
whether tli rpc or non-tli rpc is used. The methods for this object
are ifdefed to use tli rpc or not.


Class: _Tt_rpc_server
File: tt/lib/mp/mp_rpc_server.C

This class represents an rpc server. The methods implemented are
wrappers around the rpc server interfaces with the difference that
both tli and non-tli rpc is supported by ifdefing the implementation
code for the methods to select which rpc interface to use.


Class: _Tt_s_message (subclass of _Tt_message)
File: tt/lib/mp/mp_s_message.C

This class represents a ToolTalk message with some specialized methods
that are specific to server-side manipulation of messages. In
particular this class implements methods to dispatch a message (ie.
match the message against static patterns), and deliver the message to
ToolTalk clients with patterns that match it. There are also methods
to implement the reliability for messages that don't match any active
client's patterns.


Class: _Tt_s_mp
File: tt/lib/mp/mp_s_mp.C

This class represents the global server-only state needed by the
message-passing objects. It contains object caches that are relevant
to the server module as well as the ptypes and otypes that were read
in from the type database.


Class: _Tt_s_procid (subclass of _Tt_procid)
File: tt/lib/mp/mp_s_procid.C

This class implements server-only methods needed to send messages to
ToolTalk clients and also methods which process replies to previously
sent messages.


Class: _Tt_session
File: tt/lib/mp/mp_session.C (server/client methods)
tt/lib/mp/mp_s_session.C (server-only methods)

This class represents a ToolTalk session. It has methods that apply in
client-side and server-side environments. The main functionality
required is establishing ToolTalk sessions, advertising the addressing
information to contact the session, and methods for connecting to a
session given its addressing information and auto-starting a session
if necessary.


Class: _Tt_signature
File: tt/lib/mp/mp_signature.C

This class represents a ToolTalk signature which can be either an
otype or ptype signature. The main methods for this object allow one
to create a signature, match the signature against an operation name
and a list of arguments, and match the signature against relevant
message fields.


Class: _Tt_stream_socket
File: tt/lib/mp/mp_stream_socket.C

This class represents a stream connection. Its methods are ifdefed
depending on whether TCP sockets are to be used or TLI streams.


Class: _Tt_typedb
File: tt/lib/mp/mp_typedb.C

This class represents the ToolTalk type database. It has methods for
reading and writing the database in either a native xdr format or to
the Classing Engine. Important Note: since the typedb on disk is
a table_ptr and not just a table, it's possible to have a null
table and a non-null _ptr, which if xdr-decoded will crash the
_Tt_object_table xdr method, because the default constructor
for that class tries to decode a null table.


Class: _Tt_global
File: tt/lib/util/tt_global_env.C

This class is where any global information that is not specific to
message-passing is kept. In particular any global flags or caches are
kept in this object. This class also keeps the number representing the
current xdr version to be used by all xdr methods.


Class: _Tt_host
File: tt/lib/util/tt_host.C

This class packages up methods for mapping host addresses to host
names and viceversa.


Class: _Tt_xdr_version
File: tt/lib/util/tt_xdr_version.C

This class provides a mechanism for temporarily setting the default
xdr version. It sets the version to a given number in the constructor
and then resets the default to the previous value in the destructor.


Class: _Tt_new_ptr
File: tt/lib/util/tt_new_ptr.C

Generic version of a pointer class. This implements the
reference-counting machinery. See declare_ptr_to and implement_ptr_to
in tt/lib/util/tt_ptr.h to see how a specialized pointer class is
derived from this generic class.


Class: _Tt_object_list
File: tt/lib/util/tt_object_list.C

Generic version of a list class. See declare_list_of and
implement_list_of in tt/lib/util/tt_list.h to see how a specialized
list class is derived from the generic one.


Class: _Tt_object_table
File: tt/lib/util/tt_object_table.C

Generic version of a table class. See declare_table_of and
implement_table_of in tt/lib/util/tt_table.h to see how a specialized
list class is derived from the generic one. Important note: unfortunately
you can crash this method by trying to de-xdr on online db because
the default constructor for that class tries to decode a null table.


FINDING ONE'S WAY AROUND

The rich functionality offered by ToolTalk is implemented partly in
the client library, partly in the server (which itself is implemented
by objects contained in a server library). Thus it isn't always
trivial to find which code implements a certain feature. A good
heuristic to use is to decide whether the particular feature is likely
to be implemented on the client side or server side. If it is likely a
client-side feature then it is accessible through one of the tooltalk
api calls. These are all implemented in the tt/lib/api/c directory
where the files are named api_ where is the name of the
entity being operated on by the api call. For example,
tt_pattern_create can be found in "api_pattern.C". The api calls that
don't operate on a particular entity are found in "api_mp.C". For
example, tt_open and tt_fd can be found there.

Once the code for the api is located, the way the feature is
implemented can be traced through the code. Occasionally the api call
may interact with the server-side code. This is usually in the form of
an api call such as:

...
rstatus = call(TT_RPC_JOIN_SESSION,
(xdrproc_t)tt_xdr_procid,
(char *)&procid,
(xdrproc_t)xdr_int,
(char *)&status);
...


This will cause an rpc stub to be invoked which then invokes the
appropiate methods on the server side. All the rpc stubs are
implemented in tt/lib/mp/mp_rpc_implement.C. Furthermore, the naming
scheme for these stubs is to add an underscore to the front of the
enum used to make the call on the client side and then turn all the
letters into lower case. In the example above, one would then look for
the "_tt_rpc_join_session" function in tt/lib/mp/mp_rpc_implement.C.

The details of how the call is mapped to this rpc stub and what
processing goes on before the rpc stub is invoked can be found in the
_tt_service_rpc function which does the actual rpc dispatching.

Since most of the functionality is initiated by an api call (the
exception being the initializiation of the server itself), this
strategy usually succeeds in finding the code one is looking for.


We now list some guides into the code implementing some of the major
features in ToolTalk. For each piece of functionality some important
methods and/or files are listed which represent the main code that
implements the given functionality. Looking at the code for the given
methods and their associated comments should give a good idea as to
how the functionality is implemented.


********** Please add to this list **********

- RPC REQUEST HANDLING

(client) _Tt_session::call - single method that all other methods use
when they want to invoke an rpc call.

(server) tt/lib/mp/mp_rpc_implement.C contains all of the rpc stubs
that get invoked on the server side in response to an rpc request from
a client. The dispatch function in this file "_tt_service_rpc" does
the actual dispatching of the rpc request (_tt_service_rpc is actually
invoked by the rpc function svc_getreqset which is called from
_Tt_s_mp::main_loop by calling _Tt_rpc_server::run).

Looking at the various rpc stubs in mp_rpc_implement.C is an
excellent way of navigating the code to find out how each particular
rpc call is used. In particular, a good strategy for finding out how
an api call is implemented is by looking at the definition of the api
call and tracing through the code until _Tt_session::call invocations
are found. Using the rpc procedure number passed into the
_Tt_session::call method one can then look in mp_rpc_implement.C to
see what happens on the server side.


- NULL MESSAGE SENDING/RECEIVING LOOP

This loop consists of a client sending a request message to ttsession
when no patterns match the message. The message gets failed and
returned to the sender. Understanding the code involved in this loop
means that one understands the basic low-level mechanism by which the
messages sent to and received from ttsession. The major methods
involved here are:

(client) _Tt_c_message::dispatch - send message to ttsession
(server) _Tt_s_message::dispatch - match message with static patterns.
(server) _Tt_s_message::deliver - match message with dynamic patterns.
(server) _Tt_s_procid::add_message - add a message to procid's message
queue.
(server) _Tt_s_procid::signal_new_message - signal a procid that it has
a new message.
(client) _Tt_c_procid::next_message - get next message from ttsession
(server) _Tt_s_procid::next_message - return next message from ttsession


- DISPATCHING AND DELIVERING A MESSAGE

(server) _Tt_s_message::dispatch
(server) _Tt_s_message::deliver



- STARTING A NEW PTYPE

(server) _Tt_s_message::handle_no_recipients
(server) _Tt_s_message::change_state
(server) _Tt_ptype::start


- PROCESSING THE REPLY TO A MESSAGE

(client) _Tt_c_procid::update_msg
(server) _Tt_s_procid::update_msg
(server) _Tt_message::update_msg
(server) _Tt_s_message::change_state


- HONORING START AND QUEUE RELIABILITY OPTIONS

_Tt_s_message::deliver
_Tt_s_message::handle_no_recipients

For file-scope messages, queueing is actually handled on the client
side. For more details look in:

_Tt_c_message::c_dispatch
_Tt_file::q_message
_Tt_file::dq_message
_Tt_file::c_join


- READING IN CLASSING ENGINE DATABASES

_Tt_typedb::init_ce

- ADDING/MERGING NEW TYPES TO A TYPE DATABASE

_Tt_typedb::begin_write
_Tt_typedb::end_write
_Tt_typedb::insert_ptype
_Tt_typedb::insert_otype


- DECLARING A PTYPE

_tt_rpc_declare_ptype
_Tt_s_procid::declare_ptype


- DETECTING PTYPE LAUNCH FAILURE

sig_handler (in tt/bin/ttsession/mp_server.C)
notify_launch_failure (in tt/bin/ttsession/mp_server.C)
_Tt_ptype::launch_failed


- AUTO-STARTING A SESSION

tt_open (tt/lib/api/c/api_mp.C)
_Tt_mp::init
_Tt_session::init
_Tt_session::c_init


- ESTABLISHING RPC CONNECTION TO THE SERVER

_Tt_session::client_session_init - client-side processing
_Tt_s_mp::main_loop - unix and network rpc connection
_Tt_session::u_rpc_init - unix socket rpc connection


- XDR/RPC VERSIONING SCHEME

_tt_service_rpc (tt/lib/mp/mp_rpc_implement.C)
_Tt_xdr_version (tt/lib/util/tt_xdr_version)
_Tt_global::set_xdr_version (tt/lib/util/tt_global_env.C)
_Tt_global::xdr_version (tt/lib/util/tt_global_env.C)


- TRACE/DEBUG/ERROR OUTPUT

For trace output in ttsession and in slib, use the _Tt_trace
class.

For debug output in the old dm code, do it according to the
$TT_DM_DEBUG level. No other module has a similar debug output
facility.

When an error is detected in the client library and a Tt_status
other than TT_ERR_INTERNAL is going to be passed up through
the API to diagnose it, then the client library should not
emit any message anywhere. That's the job of the application using
libtt.

_tt_syslog( 0, LOG_ERR, ) should be used in the client library when:
- a TT_ERR_INTERNAL (bug) arises and we want to diagnose what happened;
- a partial failure of an API call occurs, but the API call is still
considered to have succeeded. For example, when tt_message_send()
for a file-scoped message cannot contact one of the interested
sessions.

The daemons (ttsession and dbserver) should pass either 0 or stderr
to _tt_syslog(), depending on whether they are in daemon mode or not.

In CDE, certain parts (tt_type_comp, ttrm et al.) of ToolTalk are
likely to be run from tools that have stderr aimed at /dev/null.
These parts of ToolTalk should never write to stderr directly, nor
call perror(), nc_perror(), t_error(), etc. Instead, they should pass
stderr to _tt_syslog(), and use strerror() (or %m), nc_strerror(),
t_strerror(), etc. _tt_syslog() will prefix, route, suppress, and
multiplex the output appropriately. For example, in CDE we will
probably multiplex our error output onto ~/.dt/errorlog. For
suppression, _tt_syslog() obeys _tt_global->silent.

No output should include un-internationalized English. The output
should include only ToolTalk-ese and catgets() strings. (ToolTalk-ese
can be C/C++ use of the ToolTalk API, or the ToolTalk types language.)
All modules that call catgets() should include "util/tt_gettext.h", so
that they are really calling _tt_catgets() instead of the one in
/usr/lib. This lets us map catgets() to something else on platforms
where it is not supported.

It is OK to write normal output to stdout.

ASSIGNING NEW CATGETS MESSAGE IDS

When you code up a new call to catgets(), choose your message set out
of util/tt_gettext.h, and use -1 as the message id. Then follow this
procedure to update the message catalog:

0. Make sure your source file(s) is(are) checked in; ttgenmsg will
refuse to work on checked-out files.
0a. Make sure /home/tooltalk/tools/bin is in your PATH
1. Check out SUNW_TOOLTALK.msg and SUNW_TOOLTALK.sets
2. Run "make msgs" from the top of the tree.
2a. Run msgfix which will sort and clean up "msgs" into "msgs.fixed"
3. Check in SUNW_TOOLTALK.sets (which now has been updated).
4. Copy "msgs.fixed" to SUNW_TOOLTALK.msg and check it in.
5. Check in your updated source files.

Note to source customers: ttgenmsg depends on genmsg, an internal
SunSoft tool, and so ttgenmsg is not included in the ToolTalk source
distribution. If you add any new catgets() messages, you will have to
manually assign message ids and update the message catalog.

Note to ToolTalk developers: ttgenmsg and genmsg are in
/home/tooltalk/tools/bin.

THREADING ISSUES

- Locking granularity:
ToolTalk API calls acquire a global lock which they hold as long
as they are doing operations that can't block. The original
intention was to drop the locks around RPC calls and re-acquire
them immediately after returning from such calls. This proved to
be unworkable because it resulted in crashes in the RPC library.
As of the time of this writing, locks are held around RPC calls,
which introduces the danger of a blocking call causing a deadlock
in the entire TT library, since only one thread is actually
manipulating TT structures at any particular time in a process.

- ttsession threading
Currently ttsession only uses a thread for registering as a client
of itself. This is an atomic operation and its success or failure
doesn't have a significant effect on ttsession's ability to serve
client processes. ttsession tracing won't work until the
thread completes successfully.

- ALL thread-specific storage should be from malloc calls, NEVER local
storage. Also, all storage obtained from tt_malloc-type calls is
thread-specific.

- Calling tt_message_receive and receiving the same message simultaneously
in different threads causes the message object to be updated from under
the calling code. This happens, for instance, when one thread has a
handle pattern registered and another has an observe pattern registered,
and they both match the same message. The biggest danger here is
if one thread deletes a message while another thread is accessing
data from the message. It is up to the client program to regulate
access to TT message data.