Fix Jack latency recomputation when plugin latency changes

Paul Davis says "jack_recompute_total_latencies() is a server call. It is not
legal to make server calls from within a server callback (like the process
callback)."

1 files changed, 23 insertions, 4 deletions

diff --git a/src/jack.c b/src/jack.c
index cf633f9..ea37ac5 100644
--- a/src/jack.c
+++ b/src/jack.c
@@ -1,4 +1,4 @@
-// Copyright 2007-2022 David Robillard <d@drobilla.net>
+// Copyright 2007-2024 David Robillard <d@drobilla.net>
 // SPDX-License-Identifier: ISC
 
 #include "backend.h"
@@ -46,6 +46,9 @@ struct JalvBackendImpl {
   bool           is_internal_client; ///< Running inside jackd
 };
 
+/// Maximum supported latency in frames (at most 2^24 so all integers work)
+static const float max_latency = 16777216.0f;
+
 /// Internal Jack client initialization entry point
 int
 jack_initialize(jack_client_t* client, const char* load_init);
@@ -201,9 +204,19 @@ jack_process_cb(jack_nframes_t nframes, void* data)
     struct Port* const port = &jalv->ports[p];
     if (port->flow == FLOW_OUTPUT && port->type == TYPE_CONTROL &&
         port->reports_latency) {
-      if (jalv->plugin_latency != port->control) {
-        jalv->plugin_latency = port->control;
-        jack_recompute_total_latencies(client);
+      // Get the latency in frames from the control output truncated to integer
+      const float    value = port->control;
+      const uint32_t frames =
+        (value >= 0.0f && value <= max_latency) ? (uint32_t)value : 0U;
+
+      if (jalv->plugin_latency != frames) {
+        // Update the cached value and notify the UI if the latency changed
+        jalv->plugin_latency = frames;
+
+        const JalvLatencyChange body   = {frames};
+        const JalvMessageHeader header = {LATENCY_CHANGE, sizeof(body)};
+        jalv_write_split_message(
+          jalv->plugin_to_ui, &header, sizeof(header), &body, sizeof(body));
       }
     } else if (port->flow == FLOW_OUTPUT && port->type == TYPE_EVENT) {
       void* buf = NULL;
@@ -461,6 +474,12 @@ jalv_backend_activate_port(Jalv* jalv, uint32_t port_index)
 #endif
 }
 
+void
+jalv_backend_recompute_latencies(Jalv* const jalv)
+{
+  jack_recompute_total_latencies(jalv->backend->client);
+}
+
 int
 jack_initialize(jack_client_t* const client, const char* const load_init)
 {