tcp: support variable-length tcp_dissect_pdus

Originally suggested by Bill Meier for the MQTT protocol[1], but the
Websocket protocol can also benefit from this. Since
DESEGMENT_ONE_MORE_SEGMENT is a valid packet length, use the zero length
instead as an indicator that the length is not yet known.

Updated documentation too and remove the function documentation from
packet-tcp.c since it is duplicated in packet-tcp.h.

A noteworthy WSDG change is that the get_pdu_len parameter of
tcp_dissect_pdus gained another void pointer since
v1.99.2rc0-890-gceb8d95 ("Lua: Expose tcp_dissect_pdus() to Lua").

 [1]: https://www.wireshark.org/lists/wireshark-dev/201405/msg00044.html

Change-Id: I4eba380e00cd757635eb5639c2857356dae3171e
Reviewed-on: https://code.wireshark.org/review/7279
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/doc/README.dissector b/doc/README.dissector
index 10ba4e6b3f..8eefc8b06f 100644
--- a/doc/README.dissector
+++ b/doc/README.dissector
@@ -2972,16 +2972,17 @@ The arguments to tcp_dissect_pdus are:
     a routine that takes as arguments a packet_info pointer, a tvbuff
     pointer and an offset value representing the offset into the tvbuff
     at which a PDU begins, and a void pointer for user data, and should
-    return - *without* throwing an exception (it is guaranteed that the
+    return the total length of the PDU in bytes (or 0 if more bytes are
+    needed to determine the message length).
+    The routine must not throw exceptions (it is guaranteed that the
     number of bytes specified by the previous argument to
     tcp_dissect_pdus is available, but more data might not be available,
-    so don't refer to any data past that) - the total length of the PDU,
-    in bytes;
+    so don't refer to any data past that);
 
     a new_dissector_t routine to dissect the pdu that's passed a tvbuff
     pointer, packet_info pointer, proto_tree pointer and a void pointer for
     user data, with the tvbuff containing a possibly-reassembled PDU. (The
-    "reported_length" of the tvbuff will be the length of the PDU).
+    "reported_length" of the tvbuff will be the length of the PDU);
 
     a void pointer to user data that is passed to the length-determining
     routine, and the dissector routine referenced in the previous parameter.