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, 45 insertions, 1 deletions

diff --git a/channels/chan_sip.c b/channels/chan_sip.c
index 941a1e96d..dcfadf4a8 100644
--- a/channels/chan_sip.c
+++ b/channels/chan_sip.c
@@ -377,7 +377,20 @@
 			<para>Since there are several headers (such as Via) which can occur multiple
 			times, SIP_HEADER takes an optional second argument to specify which header with
 			that name to retrieve. Headers start at offset <literal>1</literal>.</para>
-			<para>Please observe that contents of the SDP (an attachment to the
+			<para>This function does not access headers from the REFER message if the call
+			was transferred. To obtain the REFER headers, set the dialplan variable
+			<variable>GET_TRANSFERRER_DATA</variable> to the prefix of the headers of the
+			REFER message that you need to access; for example, <literal>X-</literal> to
+			get all headers starting with <literal>X-</literal>. The variable must be set
+			before a call to the application that starts the channel that may eventually
+			transfer back into the dialplan, and must be inherited by that channel, so prefix
+			it with the <literal>_</literal> or <literal>__</literal> when setting (or
+			set it in the pre-dial handler executed on the new channel). To get all headers
+			of the REFER message, set the value to <literal>*</literal>. Headers
+			are returned in the form of a dialplan hash TRANSFER_DATA, and can be accessed
+			with the functions <variable>HASHKEYS(TRANSFER_DATA)</variable> and, e. g.,
+			<variable>HASH(TRANSFER_DATA,X-That-Special-Header)</variable>.</para>
+			<para>Please also note that contents of the SDP (an attachment to the
 			SIP request) can't be accessed with this function.</para>
 		</description>
 		<see-also>
@@ -18662,6 +18675,29 @@ static int get_sip_pvt_from_replaces(const char *callid, const char *totag,
 	return 0;
 }
 
+static void extract_transferrer_headers(const char *prefix, struct ast_channel *peer, const struct sip_request *req)
+{
+	struct ast_str *pbxvar = ast_str_alloca(120);
+	int i;
+
+	/* The '*' alone matches all headers. */
+	if (strcmp(prefix, "*") == 0) {
+		prefix = "";
+	}
+
+	for (i = 0; i < req->headers; i++) {
+		const char *header = REQ_OFFSET_TO_STR(req, header[i]);
+		if (ast_begins_with(header, prefix)) {
+			int hdrlen = strcspn(header, " \t:");
+			const char *val = ast_skip_blanks(header + hdrlen);
+			if (hdrlen > 0 && *val == ':') {
+				ast_str_set(&pbxvar, -1, "~HASH~TRANSFER_DATA~%.*s~", hdrlen, header);
+				pbx_builtin_setvar_helper(peer, ast_str_buffer(pbxvar), ast_skip_blanks(val + 1));
+			}
+		}
+	}
+}
+
 /*! \brief Call transfer support (the REFER method)
  * 	Extracts Refer headers into pvt dialog structure
  *
@@ -18724,10 +18760,18 @@ static int get_refer_info(struct sip_pvt *transferer, struct sip_request *outgoi
 
 		peer = ast_channel_bridge_peer(owner_ref);
 		if (peer) {
+			const char *get_xfrdata;
+
 			pbx_builtin_setvar_helper(peer, "SIPREFERRINGCONTEXT",
 				S_OR(transferer->context, NULL));
 			pbx_builtin_setvar_helper(peer, "__SIPREFERREDBYHDR",
 				S_OR(p_referred_by, NULL));
+
+			ast_channel_lock(peer);
+			get_xfrdata = pbx_builtin_getvar_helper(peer, "GET_TRANSFERRER_DATA");
+			if (!ast_strlen_zero(get_xfrdata)) {
+				extract_transferrer_headers(get_xfrdata, peer, req);
+			}
 			ast_channel_unlock(peer);
 		}