chan_sip: Access incoming REFER headers in dialplan

This adds a way to access information passed along with SIP headers in
a REFER message that initiates a transfer. Headers matching a dialplan
variable GET_TRANSFERRER_DATA in the transferrer channel are added to
a HASH object TRANSFER_DATA to be accessed with functions HASHKEY and HASH.

The variable GET_TRANSFERRER_DATA is interpreted to be a prefix for
headers that should be put into the hash. If not set, no headers are
included. If set to a string (perhaps 'X-' in a typical case), all headers
starting this string are added. Empty string matches all headers.

If there are multiple of the same header, only the latest occurrence in
the REFER message is available in the hash.

Obviously, the variable GET_TRANSFERRER_DATA must be inherited by the
referrer channel, and should be set with the '_' or '__' prefix.

I avoided a specific reference to SIP or REFER, as in my mind the mechanism
can be generalized to other channel techs.

ASTERISK-27162

Change-Id: I73d7a1e95981693bc59aa0d5093c074b555f708e

1 files changed, 4 insertions, 0 deletions

diff --git a/CHANGES b/CHANGES
index 4f5225ac1..67d33c95b 100644
--- a/CHANGES
+++ b/CHANGES
@@ -16,6 +16,10 @@ chan_sip
 ------------------
  * New function SIP_HEADERS() enumerates all headers in the incoming INVITE.
 
+ * The variable GET_TRANSFERRER_DATA set in the peer channel causes matching
+   headers be retrieved from the REFER message and made accessible to the
+   dialplan in the hash TRANSFER_DATA.
+
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 --- Functionality changes from Asterisk 14 to Asterisk 15 --------------------
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