MINOR: mux-quic: remove dedicated function to handle standalone FIN

Remove QUIC MUX function qcs_http_handle_standalone_fin(). The purpose
of this function was only used when receiving an empty STREAM frame with
FIN bit. Besides, it was called by each application protocol which could
have different approach and render the function purpose unclear.

Invocation of qcs_http_handle_standalone_fin() have been replaced by
explicit code in both H3 and HTTP/0.9 module. In the process, use
htx_set_eom() to reliably put EOM on the HTX message.

This should be backported up to 2.7, along with the previous patch which
introduced htx_set_eom().
diff --git a/include/haproxy/qmux_http.h b/include/haproxy/qmux_http.h
index 98151db..a7dbe7c 100644
--- a/include/haproxy/qmux_http.h
+++ b/include/haproxy/qmux_http.h
@@ -12,8 +12,6 @@
                         char *fin);
 size_t qcs_http_reset_buf(struct qcs *qcs, struct buffer *buf, size_t count);
 
-void qcs_http_handle_standalone_fin(struct qcs *qcs);
-
 #endif /* USE_QUIC */
 
 #endif /* _HAPROXY_MUX_QUIC_HTTP_H */
diff --git a/src/h3.c b/src/h3.c
index e686f0d..d41a20c 100644
--- a/src/h3.c
+++ b/src/h3.c
@@ -1036,8 +1036,19 @@
 	}
 
 	if (!b_data(b) && fin && quic_stream_is_bidi(qcs->id)) {
+		struct buffer *appbuf;
+		struct htx *htx;
+
 		TRACE_PROTO("received FIN without data", H3_EV_RX_FRAME, qcs->qcc->conn, qcs);
-		qcs_http_handle_standalone_fin(qcs);
+		appbuf = qc_get_buf(qcs, &qcs->rx.app_buf);
+		BUG_ON(!appbuf);
+
+		htx = htx_from_buf(appbuf);
+		if (!htx_set_eom(htx)) {
+			TRACE_ERROR("cannot set EOM", H3_EV_RX_FRAME, qcs->qcc->conn, qcs);
+			h3c->err = H3_INTERNAL_ERROR;
+		}
+		htx_to_buf(htx, appbuf);
 		goto done;
 	}
 
diff --git a/src/hq_interop.c b/src/hq_interop.c
index 175b92d..cbbf472 100644
--- a/src/hq_interop.c
+++ b/src/hq_interop.c
@@ -21,9 +21,20 @@
 	size_t data = b_data(b);
 
 	if (!data && fin) {
+		struct buffer *appbuf;
+		struct htx *htx;
+
 		/* FIN is notified with an empty STREAM frame. */
 		BUG_ON(!qcs->sd); /* sd must already be attached here */
-		qcs_http_handle_standalone_fin(qcs);
+
+		if (!(appbuf = qc_get_buf(qcs, &qcs->rx.app_buf)))
+			return -1;
+
+		htx = htx_from_buf(appbuf);
+		if (!htx_set_eom(htx))
+			return -1;
+		htx_to_buf(htx, appbuf);
+
 		return 0;
 	}
 
diff --git a/src/qmux_http.c b/src/qmux_http.c
index 73b0038..fc891dc 100644
--- a/src/qmux_http.c
+++ b/src/qmux_http.c
@@ -110,25 +110,3 @@
 
 	return count;
 }
-
-/* Utility function which can be used by app layer an empty STREAM frame is
- * received with FIN bit set for <qcs> stream. It will ensure that HTX EOM is
- * properly inserted in <qcs> app_buf.
- */
-void qcs_http_handle_standalone_fin(struct qcs *qcs)
-{
-	struct buffer *appbuf;
-	struct htx *htx = NULL;
-
-	appbuf = qc_get_buf(qcs, &qcs->rx.app_buf);
-	BUG_ON(!appbuf);
-
-	htx = htx_from_buf(appbuf);
-	if (htx_is_empty(htx)) {
-		if (!htx_add_endof(htx, HTX_BLK_EOT)) {
-			ABORT_NOW(); /* cannot happen for empty HTX message. */
-		}
-	}
-	htx->flags |= HTX_FL_EOM;
-	htx_to_buf(htx, appbuf);
-}