MINOR: Add sample fetches to get the frontend and backend stream ID

"fc.id" and "bc.id" sample fetches can now be used to get, respectively, the
frontend or the backend stream ID. They rely on ->sctl() callback function
on the mux attached to the corresponding SC.

It means these sample fetches work only for connection, not applets, and
from the time a multiplexer is installed.

doc/configuration.txt

@@ -21213,7 +21213,7 @@ ssl_bc_client_random : binary
 
 ssl_bc_curve : string
   Returns the name of the curve used in the key agreement when the outgoing
-  connection was made over an SSL/TLS transport layer. This requires 
+  connection was made over an SSL/TLS transport layer. This requires
   OpenSSL >= 3.0.0.
 
 ssl_bc_err : integer
@@ -21913,6 +21913,10 @@ Warning : Following sample fetches are ignored if used from HTTP proxies. They
           all invalid usage (for instance inside a log-format string or a
           sample expression). So be careful.
 
+bs.id : integer
+  Returns the multiplexer's stream ID on the server side. It is the
+  multiplexer's responsibility to return the appropriate information.
+
 distcc_body(<token>[,<occ>]) : binary
   Parses a distcc message and returns the body associated to occurrence #<occ>
   of the token <token>. Occurrences start at 1, and when unspecified, any may
@@ -21939,6 +21943,11 @@ distcc_param(<token>[,<occ>]) : integer
       # send large files to the big farm
       use_backend big_farm if { distcc_param(DOTI) gt 1000000 }
 
+fs.id : integer
+  Returns the multiplexer's stream ID on the client side. It is the
+  multiplexer's responsibility to return the appropriate information. For
+  instance, on a raw TCP, 0 is always returned because there is no stream.
+
 payload(<offset>,<length>) : binary (deprecated)
   This is an alias for "req.payload" when used in the context of a request (e.g.
   "stick on", "stick match"), and for "res.payload" when used in the context of

src/stconn.c

@@ -17,6 +17,7 @@
 #include <haproxy/http_ana.h>
 #include <haproxy/pipe.h>
 #include <haproxy/pool.h>
+#include <haproxy/sample.h>
 #include <haproxy/sc_strm.h>
 #include <haproxy/stconn.h>
 #include <haproxy/xref.h>
@@ -1990,3 +1991,54 @@ void sc_conn_commit_endp_upgrade(struct stconn *sc)
 	BUG_ON(!sc);
 	BUG_ON(!sc->sedesc);
 }
+
+/* return the frontend or backend mux stream ID.
+ */
+static int
+smp_fetch_sid(const struct arg *args, struct sample *smp, const char *kw, void *private)
+{
+	struct connection *conn;
+	struct stconn *sc;
+	int64_t sid = 0;
+
+	if (!smp->strm)
+		return 0;
+
+	sc = (kw[0] == 'f' ? smp->strm->scf : smp->strm->scb);
+	conn = sc_conn(sc);
+
+	/* No connection */
+	if (!conn)
+		return 0;
+
+	/* No mux install, this may change */
+	if (!conn->mux) {
+		smp->flags |= SMP_F_MAY_CHANGE;
+		return 0;
+	}
+
+	/* No sctl, report sid=0 in this case */
+	if (conn->mux->sctl) {
+		if (conn->mux->sctl(sc, MUX_SCTL_SID, &sid) == -1)
+			return 0;
+	}
+
+	smp->flags = SMP_F_VOL_TXN;
+	smp->data.type = SMP_T_SINT;
+	smp->data.u.sint = sid;
+
+	return 1;
+}
+
+/* Note: must not be declared <const> as its list will be overwritten.
+ * Note: fetches that may return multiple types should be declared using the
+ * appropriate pseudo-type. If not available it must be declared as the lowest
+ * common denominator, the type that can be casted into all other ones.
+ */
+static struct sample_fetch_kw_list sample_fetch_keywords = {ILH, {
+	{ "bs.id", smp_fetch_sid, 0, NULL, SMP_T_SINT, SMP_USE_L6REQ },
+	{ "fs.id", smp_fetch_sid, 0, NULL, SMP_T_STR, SMP_USE_L6RES },
+	{ /* END */ },
+}};
+
+INITCALL1(STG_REGISTER, sample_register_fetches, &sample_fetch_keywords);