mgcp: Document transcoding semantic and follow it
Transcoding from GSM to PCMA can lead to the MGCP MGW sending
two PCMA packages with the same sequence number and timestamp.
Once with the encoded audio and once completely empty.
This is because "state->dst_packet_duration" is 0 in most cases
(unless a ptime is forced) and we attempt to encode audio even
if there are not enough samples. The encode_audio return will
return 0 in that case which is not trated as an error by the
mgcp network code.
Handle rc == 0 specially and document the semantic.
diff --git a/openbsc/tests/mgcp/mgcp_transcoding_test.ok b/openbsc/tests/mgcp/mgcp_transcoding_test.ok
index e06b0e1..7c1c8ce 100644
--- a/openbsc/tests/mgcp/mgcp_transcoding_test.ok
+++ b/openbsc/tests/mgcp/mgcp_transcoding_test.ok
@@ -144,19 +144,11 @@
Invalid size:
== Transcoding test ==
converting gsm -> pcma
-encoded:
- 80 03 00 01 00 00 00 a0 11 22 33 44 d4 7c e3 e9
- 62 50 39 f0 f8 b4 68 ea 6c 0e 81 1b 56 2a d5 bc
- 69 9c d1 f0 66 7a ec 49 7a
-counted: 0
+Nothing encoded due: No message of desired type
Invalid data:
== Transcoding test ==
converting gsm -> pcma
-encoded:
- 80 03 00 01 00 00 00 a0 11 22 33 44 ee ee ee ee
- ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
- ee ee ee ee ee ee ee ee ee ee ee ee ee
-counted: 0
+Nothing encoded due: No message of desired type
Invalid payload type:
== Transcoding test ==
converting gsm -> pcma