Add a CLAP chainloader

The functions it's supposed to load however don't exist yet.

meson.build

@@ -314,16 +314,15 @@ if with_clap
     dependencies : clap_plugin_deps,
     cpp_args : compiler_options,
   )
-  # TODO: Chainloader
-  # shared_library(
-  #   'yabridge-chainloader-clap',
-  #   clap_chainloader_sources,
-  #   native : true,
-  #   dependencies : chainloader_deps,
-  #   cpp_args : compiler_options + chainloader_compiler_options,
-  #   # See above
-  #   override_options : ['b_lto=true'],
-  # )
+  shared_library(
+    'yabridge-chainloader-clap',
+    clap_chainloader_sources,
+    native : true,
+    dependencies : clap_chainloader_deps,
+    cpp_args : compiler_options + chainloader_compiler_options,
+    # See above
+    override_options : ['b_lto=true'],
+  )
 endif
 
 if with_vst3

src/chainloader/clap-chainloader.cpp

@@ -0,0 +1,159 @@
+// yabridge: a Wine plugin bridge
+// Copyright (C) 2020-2022 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#include <atomic>
+#include <cassert>
+#include <mutex>
+
+#include <clap/entry.h>
+#include <dlfcn.h>
+
+// Generated inside of the build directory
+#include <config.h>
+
+#include "../common/linking.h"
+#include "../common/utils.h"
+#include "utils.h"
+
+// These chainloader libraries are tiny, mostly dependencyless libraries that
+// `dlopen()` the actual `libyabridge-{vst2,vst3}.so` files and forward the
+// entry point function calls from this library to those. Or technically, these
+// libraries use dedicated entry point functions because multiple chainloader
+// libraries may all dynamically link to the exact same plugin library, so we
+// can't store any bridge information in a global there. This approach avoids
+// wasting disk space on copies on file systems that don't support reflinking,
+// but more importantly it also avoids the need to rerun `yabridgectl sync`
+// whenever yabridge is updated. This is even more important when considering
+// distro packaging, because updates to Boost might require the package to be
+// rebuilt, which in turn would also require a resync.
+
+namespace fs = ghc::filesystem;
+
+// These functions are loaded from `libyabridge-clap.so` the first time
+// `clap_entry.init` gets called
+using ClapPluginBridge = void;
+
+ClapPluginBridge* (*yabridge_bridge_init)(const char* plugin_path) = nullptr;
+void (*yabridge_bridge_free)(ClapPluginBridge* instance) = nullptr;
+const void* (*yabridge_module_get_plugin_factory)(ClapPluginBridge* instance,
+                                                  const char* factory_id) =
+    nullptr;
+
+/**
+ * The bridge instance for this chainloader. This is initialized when
+ * `clap_entry.init` first gets called.
+ */
+std::unique_ptr<ClapPluginBridge, decltype(yabridge_bridge_free)> bridge(
+    nullptr,
+    nullptr);
+/**
+ * The number of active instances. Incremented when `clap_entry_init()` is
+ * called, decremented when `clap_entry_exit()` is called. We'll initialize the
+ * bridge when this is first incremented from 0, and we'll free the bridge again
+ * when a `clap_entry_exit()` call causes this to return back to 0.
+ */
+std::atomic_size_t active_instances = 0;
+
+/**
+ * The first time one of the exported functions from this library gets called,
+ * we'll need to load the corresponding `libyabridge-*.so` file and fetch the
+ * the entry point functions from that file.
+ */
+bool initialize_library() {
+    static void* library_handle = nullptr;
+    static std::mutex library_handle_mutex;
+
+    std::lock_guard lock(library_handle_mutex);
+
+    // There should be no situation where this library gets loaded and then two
+    // threads immediately start calling functions, but we'll handle that
+    // situation just in case it does happen
+    if (library_handle) {
+        return true;
+    }
+
+    library_handle = find_plugin_library(yabridge_clap_plugin_name);
+    if (!library_handle) {
+        return false;
+    }
+
+#define LOAD_FUNCTION(name)                                                 \
+    do {                                                                    \
+        (name) =                                                            \
+            reinterpret_cast<decltype(name)>(dlsym(library_handle, #name)); \
+        if (!(name)) {                                                      \
+            log_failing_dlsym(yabridge_clap_plugin_name, #name);            \
+            return false;                                                   \
+        }                                                                   \
+    } while (false)
+
+    LOAD_FUNCTION(yabridge_bridge_init);
+    LOAD_FUNCTION(yabridge_bridge_free);
+    LOAD_FUNCTION(yabridge_module_get_plugin_factory);
+
+#undef LOAD_FUNCTION
+
+    return true;
+}
+
+bool clap_entry_init(const char* /*plugin_path*/) {
+    // This function can be called multiple times, so we should make sure to
+    // only initialize the bridge on the first call
+    if (active_instances.fetch_add(1, std::memory_order_seq_cst) == 0) {
+        if (!initialize_library()) {
+            return false;
+        }
+
+        // You can't change the deleter function with `.reset()` so we'll need
+        // this abomination instead
+        // XXX: The host also provides us with the plugin path which we could
+        //      just use instead. Should we? The advantage of doing it this way
+        //      instead is that we'll have consistent behavior between all
+        //      plugin formats.
+        const fs::path this_plugin_path = get_this_file_location();
+        bridge =
+            decltype(bridge)(yabridge_bridge_init(this_plugin_path.c_str()),
+                             yabridge_bridge_free);
+        if (!bridge) {
+            return false;
+        }
+    }
+
+    return true;
+}
+
+void clap_entry_deinit() {
+    // We'll free the bridge when this exits brings the reference count back to
+    // zero
+    if (active_instances.fetch_sub(1, std::memory_order_seq_cst) == 1) {
+        bridge.reset();
+    }
+}
+
+const void* clap_entry_get_factory(const char* factory_id) {
+    // The host should have called `clap_entry.init` first
+    assert(bridge);
+    assert(factory_id);
+
+    return yabridge_module_get_plugin_factory(bridge.get(), factory_id);
+}
+
+CLAP_EXPORT const clap_plugin_entry_t clap_entry = {
+    .clap_version = CLAP_VERSION_INIT,
+    .init = clap_entry_init,
+    .deinit = clap_entry_deinit,
+    .get_factory = clap_entry_get_factory,
+};

src/chainloader/meson.build

@@ -9,6 +9,10 @@ chainloader_deps = [
   rt_dep,
 ]
 
+if with_clap
+  clap_chainloader_deps = chainloader_deps + [clap_dep]
+endif
+
 vst2_chainloader_sources = files(
   '../common/logging/common.cpp',
   '../common/linking.cpp',
@@ -18,6 +22,17 @@ vst2_chainloader_sources = files(
   'vst2-chainloader.cpp',
 )
 
+if with_clap
+  clap_chainloader_sources = files(
+    '../common/logging/common.cpp',
+    '../common/linking.cpp',
+    '../common/notifications.cpp',
+    '../common/process.cpp',
+    'utils.cpp',
+    'clap-chainloader.cpp',
+  )
+endif
+
 if with_vst3
   vst3_chainloader_sources = files(
     '../common/logging/common.cpp',

src/common/config/config.h.in

@@ -16,6 +16,11 @@
 
 #pragma once
 
+/**
+ * The name of yabridge's CLAP library, e.g. `libyabridge-clap.so`.
+ */
+constexpr char yabridge_clap_plugin_name[] = "@clap_plugin_name@";
+
 /**
  * The name of yabridge's VST2 library, e.g. `libyabridge-vst2.so`.
  */