From fb192806333ca88d5acb53b849f268611773230d Mon Sep 17 00:00:00 2001
From: David Robillard <d@drobilla.net>
Date: Fri, 15 Nov 2024 20:38:43 -0500
Subject: 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)."
---
 src/backend.h   |  4 ++++
 src/comm.h      | 11 +++++++++++
 src/jack.c      | 27 +++++++++++++++++++++++----
 src/jalv.c      |  2 ++
 src/portaudio.c |  5 +++++
 5 files changed, 45 insertions(+), 4 deletions(-)

(limited to 'src')

diff --git a/src/backend.h b/src/backend.h
index 2965378..f48287c 100644
--- a/src/backend.h
+++ b/src/backend.h
@@ -33,6 +33,10 @@ jalv_backend_close(Jalv* jalv);
 void
 jalv_backend_activate_port(Jalv* jalv, uint32_t port_index);
 
+/// Recompute latencies based on plugin port latencies if necessary
+void
+jalv_backend_recompute_latencies(Jalv* jalv);
+
 JALV_END_DECLS
 
 #endif // JALV_BACKEND_H
diff --git a/src/comm.h b/src/comm.h
index e4579eb..11d7dc3 100644
--- a/src/comm.h
+++ b/src/comm.h
@@ -22,6 +22,7 @@ typedef enum {
   NO_MESSAGE,          ///< Sentinel type for uninitialized messages
   CONTROL_PORT_CHANGE, ///< Value change for a control port (float)
   EVENT_TRANSFER,      ///< Event transfer for a sequence port (atom)
+  LATENCY_CHANGE,      ///< Change to plugin latency
 } JalvMessageType;
 
 /**
@@ -58,6 +59,16 @@ typedef struct {
   LV2_Atom atom;       ///< Event payload header
 } JalvEventTransfer;
 
+/**
+   The payload of a LATENCY_CHANGE message.
+
+   This message has a fixed sized, and is described in its entirety by this
+   struct.
+*/
+typedef struct {
+  float value; ///< Latency in frames at the current sample rate
+} JalvLatencyChange;
+
 /**
    Write a message in two parts to a ring.
 
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)
 {
diff --git a/src/jalv.c b/src/jalv.c
index ea0e13b..beb36ca 100644
--- a/src/jalv.c
+++ b/src/jalv.c
@@ -737,6 +737,8 @@ jalv_update(Jalv* jalv)
                                sizeof(LV2_Atom) + msg->atom.size,
                                jalv->urids.atom_eventTransfer,
                                &msg->atom);
+    } else if (header.type == LATENCY_CHANGE) {
+      jalv_backend_recompute_latencies(jalv);
     } else {
       return ring_error("Unknown message type received from process ring\n");
     }
diff --git a/src/portaudio.c b/src/portaudio.c
index 9e37d29..4760d48 100644
--- a/src/portaudio.c
+++ b/src/portaudio.c
@@ -11,6 +11,7 @@
 
 #include "lilv/lilv.h"
 #include "lv2/atom/atom.h"
+#include "zix/attributes.h"
 
 #include <portaudio.h>
 #include <stdbool.h>
@@ -215,3 +216,7 @@ jalv_backend_activate_port(Jalv* jalv, uint32_t port_index)
     lilv_instance_connect_port(jalv->instance, port_index, &port->control);
   }
 }
+
+void
+jalv_backend_recompute_latencies(Jalv* const ZIX_UNUSED(jalv))
+{}
-- 
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