pavmon, a simple tool to monitor the volume of a PulseAudio sinkHEADmain

6 files changed, 1040 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
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--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,17 @@
+# Copyright 2022 Benjamin Barenblat
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+.ninja_*
+*.o
+pavmon
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..d645695
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,202 @@
+
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diff --git a/build.ninja b/build.ninja
new file mode 100644
index 0000000..7824f0b
--- /dev/null
+++ b/build.ninja
@@ -0,0 +1,35 @@
+# Copyright 2022 Benjamin Barenblat
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+ninja_required_version = 1.3
+
+rule cxx 
+  command = g++ -MD -MT $out -MF $out.d -pipe -std=c++20 -fno-rtti -Wall $
+    -Wextra -Wno-sign-compare -fdiagnostics-show-template-tree -O3 -flto $
+    -fstack-protector-strong -Wformat -Werror=format-security $
+    -D_FORTIFY_SOURCE=2 -DNDEBUG -D_REENTRANT -ffunction-sections $
+    -fdata-sections -c $in -o $out
+  description = Compiling $out
+  depfile = $out.d
+  deps = gcc
+
+rule link
+  command = g++ -fuse-ld=gold -flto -Wl,-O2 -Wl,--gc-sections -Wl,--as-needed $
+    -Wl,-z,relro -Wl,-z,now -o $out $in $libs && strip $out
+  description = Linking $out
+
+build pavmon.o: cxx pavmon.cc
+build pulse.o: cxx pulse.cc
+build pavmon: link pavmon.o pulse.o
+  libs = -lpulse
diff --git a/pavmon.cc b/pavmon.cc
new file mode 100644
index 0000000..df639e4
--- /dev/null
+++ b/pavmon.cc
@@ -0,0 +1,184 @@
+// Copyright 2022 Benjamin Barenblat
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+#include <getopt.h>
+#include <pulse/def.h>
+#include <pulse/error.h>
+#include <pulse/introspect.h>
+#include <pulse/volume.h>
+#include <signal.h>
+#include <unistd.h>
+
+#include <iomanip>
+#include <ios>
+#include <iostream>
+#include <locale>
+#include <string>
+#include <string_view>
+
+#include "pulse.h"
+
+namespace {
+
+constexpr std::string_view kShortUsage = "Usage: pavmon SINK_NAME\n";
+
+constexpr std::string_view kHelp = R"(
+Monitor the volume level of a PulseAudio sink, printing a new line each time it
+changes.
+
+Options:
+      --help                  display this help and exit
+      --version               display version information and exit
+)";
+
+constexpr std::string_view kAskForHelp =
+    "Try \"pavmon --help\" for more information.\n";
+
+constexpr std::string_view kVersionInfo = R"(pavmon 1.0.0
+Copyright 2022 Benjamin Barenblat
+Licensed under the Apache License, Version 2.0
+)";
+
+enum {
+  kHelpLongOption = 128,
+  kVersionLongOption,
+};
+
+void GetAndPrintVolume(const std::string& sink_name, pulse::PollMainLoop& loop,
+                       pulse::Context& ctx) noexcept {
+  ctx.GetSinkInfo(
+      sink_name.c_str(),
+      [&]() noexcept {
+        std::clog << "pavmon: failed to get volume" << std::endl;
+        loop.Quit(1);
+      },
+      [&](const pa_sink_info& info) noexcept {
+        if (info.mute) {
+          std::cout << "muted (";
+        }
+        std::cout << std::fixed << std::setprecision(1)
+                  << pa_sw_volume_to_dB(pa_cvolume_avg(&info.volume)) << " dB";
+        if (info.mute) {
+          std::cout << ')';
+        }
+        std::cout << std::endl;
+      });
+}
+
+void SubscribeToEvents(const std::string& sink_name, pulse::PollMainLoop& loop,
+                       pulse::Context& ctx) noexcept {
+  ctx.Subscribe(
+      PA_SUBSCRIPTION_MASK_SINK,
+      [&]() noexcept {
+        std::clog << "pavmon: failed to subscribe to events" << std::endl;
+        loop.Quit(1);
+      },
+      [&](pa_subscription_event_type_t) noexcept {
+        GetAndPrintVolume(sink_name, loop, ctx);
+      });
+}
+
+}  // namespace
+
+int main(int argc, char* argv[]) {
+  std::locale loc("");
+  std::locale::global(loc);
+  std::cin.imbue(loc);
+  std::cout.imbue(loc);
+  std::cerr.imbue(loc);
+  std::clog.imbue(loc);
+  std::wcin.imbue(loc);
+  std::wcout.imbue(loc);
+  std::wcerr.imbue(loc);
+  std::wclog.imbue(loc);
+
+  std::ios_base::sync_with_stdio(false);
+
+  static option long_options[] = {
+      {"help", no_argument, nullptr, kHelpLongOption},
+      {"version", no_argument, nullptr, kVersionLongOption},
+      {nullptr, 0, nullptr, 0},
+  };
+  while (true) {
+    int c = getopt_long(argc, argv, "", long_options, /*longindex=*/nullptr);
+    if (c == -1) {
+      break;
+    }
+    switch (c) {
+      case kHelpLongOption:
+        std::cout << kShortUsage << kHelp;
+        return 0;
+      case kVersionLongOption:
+        std::cout << kVersionInfo;
+        return 0;
+      case '?':
+        std::clog << kAskForHelp;
+        return 1;
+      default:
+        std::clog << "Internal error; please report.\n";
+        return 1;
+    }
+  }
+  if (optind != argc - 1) {
+    std::clog << kShortUsage << kAskForHelp;
+    return 1;
+  }
+
+  std::string sink_name = argv[1];
+
+  pulse::PollMainLoop loop;
+
+  loop.HandleSignals();
+  auto quit_on_signal = [&]() noexcept { loop.Quit(0); };
+  pulse::SignalEventSource quit_on_sigterm(SIGTERM, quit_on_signal);
+  pulse::SignalEventSource quit_on_sigint(SIGINT, quit_on_signal);
+
+  pulse::PropertyList proplist;
+  proplist.Set(PA_PROP_APPLICATION_NAME, "PulseAudio volume monitor");
+  proplist.Set(PA_PROP_APPLICATION_ID, "pavmon");
+  proplist.Set(PA_PROP_APPLICATION_VERSION, "1.0.0");
+  proplist.Set(PA_PROP_APPLICATION_PROCESS_ID, std::to_string(getpid()));
+  if (const char* user = getlogin(); user != nullptr) {
+    proplist.Set(PA_PROP_APPLICATION_PROCESS_USER, user);
+  }
+  // TODO(bbarenblat@gmail.com): More of these?
+
+  pulse::Context ctx("PulseAudio volume meter", proplist, loop);
+  ctx.Connect(PA_CONTEXT_NOFAIL, [&](pa_context_state_t state) noexcept {
+    switch (state) {
+      case PA_CONTEXT_UNCONNECTED:
+      case PA_CONTEXT_CONNECTING:
+      case PA_CONTEXT_AUTHORIZING:
+      case PA_CONTEXT_SETTING_NAME:
+        break;
+
+      case PA_CONTEXT_READY:
+        GetAndPrintVolume(sink_name, loop, ctx);
+        SubscribeToEvents(sink_name, loop, ctx);
+        break;
+
+      case PA_CONTEXT_TERMINATED:
+        loop.Quit(0);
+        break;
+
+      case PA_CONTEXT_FAILED:
+      default:
+        std::clog << "pavmon: failed to connect to PulseAudio" << std::endl;
+        loop.Quit(1);
+        break;
+    }
+  });
+
+  return loop.Run();
+}
diff --git a/pulse.cc b/pulse.cc
new file mode 100644
index 0000000..ebe72b9
--- /dev/null
+++ b/pulse.cc
@@ -0,0 +1,360 @@
+// Copyright 2022 Benjamin Barenblat
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+#include "pulse.h"
+
+#include <assert.h>
+#include <pulse/context.h>
+#include <pulse/def.h>
+#include <pulse/error.h>
+#include <pulse/introspect.h>
+#include <pulse/mainloop-api.h>
+#include <pulse/mainloop-signal.h>
+#include <pulse/mainloop.h>
+#include <pulse/operation.h>
+#include <pulse/proplist.h>
+#include <pulse/subscribe.h>
+
+#include <functional>
+#include <memory>
+#include <string_view>
+#include <utility>
+
+namespace pulse {
+
+// Adapters for libpulse callbacks.
+//
+// libpulse presents a callback-driven C API--callbacks must be C function
+// pointers, and each callback can be passed a user-controlled void*. This isn't
+// a great API for a C++ library, though, so this library accepts std::functions
+// and pass them by pointer to statically defined callbacks.
+//
+// Even though these functions are defined in an anonymous namespace, the extern
+// "C" means they need "static" to get the desired internal linkage.
+
+namespace {
+
+using SignalCallback = std::function<void()>;
+
+extern "C" {
+
+static void CallSignalCallback(pa_mainloop_api*, pa_signal_event*,
+                               int /*signal*/, void* real_callback) noexcept {
+  assert(real_callback != nullptr);
+  (*reinterpret_cast<SignalCallback*>(real_callback))();
+}
+
+static void DeleteSignalCallback(pa_mainloop_api*, pa_signal_event*,
+                                 void* real_callback) noexcept {
+  assert(real_callback != nullptr);
+  delete reinterpret_cast<SignalCallback*>(real_callback);
+}
+
+}  // extern "C"
+
+using ContextNotifyCallback = std::function<void(pa_context_state_t)>;
+
+extern "C" {
+
+static void CallContextNotifyCallback(pa_context* ctx,
+                                      void* real_callback) noexcept {
+  assert(real_callback != nullptr);
+  (*reinterpret_cast<ContextNotifyCallback*>(real_callback))(
+      pa_context_get_state(ctx));
+}
+
+}  // extern "C"
+
+struct SinkInfoCallbacks {
+  std::function<void()> on_failure;
+  std::function<void(const pa_sink_info&)> on_data;
+};
+
+extern "C" {
+
+static void CallOrDeleteSinkInfoCallbacks(pa_context*, const pa_sink_info* info,
+                                          int status,
+                                          void* real_callbacks) noexcept {
+  assert(real_callbacks != nullptr);
+  auto* sink_info_callbacks =
+      reinterpret_cast<SinkInfoCallbacks*>(real_callbacks);
+
+  if (status == 0) {
+    // The RPC succeeded, and info contains sink information. Tell the user.
+    assert(info != nullptr);
+    sink_info_callbacks->on_data(*info);
+  } else {
+    // No sink information came through...
+    if (status < 0) {
+      // ...because of an error. Tell the user.
+      sink_info_callbacks->on_failure();
+    } else {
+      // ...because no more data is available from the PulseAudio server. The
+      // user doesn't need to hear about that.
+    }
+
+    // In any case, no more data are coming, so don't hang onto the callbacks
+    // anymore.
+    delete sink_info_callbacks;
+  }
+}
+
+}  // extern "C"
+
+struct RpcResultCallbacks {
+  std::function<void()> on_failure;
+  std::function<void()> on_success;
+};
+
+extern "C" {
+
+static void CallAndDeleteRpcResultCallbacks(pa_context*, int success,
+                                            void* real_callbacks) noexcept {
+  assert(real_callbacks != nullptr);
+  auto* rpc_result_callbacks =
+      reinterpret_cast<RpcResultCallbacks*>(real_callbacks);
+  if (success) {
+    rpc_result_callbacks->on_success();
+  } else {
+    rpc_result_callbacks->on_failure();
+  }
+  delete rpc_result_callbacks;
+}
+
+}  // extern "C"
+
+using SubscribeCallback = std::function<void(pa_subscription_event_type_t)>;
+
+extern "C" {
+
+static void CallSubscribeCallback(pa_context*,
+                                  pa_subscription_event_type_t type,
+                                  uint32_t /*index*/,
+                                  void* real_callback) noexcept {
+  assert(real_callback != nullptr);
+  (*reinterpret_cast<SubscribeCallback*>(real_callback))(type);
+}
+
+}  // extern "C"
+
+}  // namespace
+
+PropertyList::PropertyList() : list_(pa_proplist_new()) {
+  if (list_ == nullptr) {
+    throw Error::InFunction("pa_proplist_new");
+  }
+}
+
+PropertyList::PropertyList(const PropertyList& other)
+    : list_(pa_proplist_copy(other.list_)) {
+  if (list_ == nullptr) {
+    throw Error::InFunction("pa_proplist_copy");
+  }
+}
+
+PropertyList& PropertyList::operator=(const PropertyList& other) {
+  if (this != &other) {
+    PropertyList other2(other);
+    swap(*this, other2);
+  }
+  return *this;
+}
+
+PropertyList::~PropertyList() noexcept {
+  if (list_ != nullptr) {
+    pa_proplist_free(list_);
+  }
+}
+
+void PropertyList::Set(const char key[], std::string_view value) {
+  if (pa_proplist_set(list_, key, value.data(), value.size())) {
+    throw Error::InFunction("pa_proplist_set");
+  }
+}
+
+PollMainLoop::PollMainLoop()
+    : loop_(pa_mainloop_new()), handling_signals_(false) {
+  if (loop_ == nullptr) {
+    throw Error::InFunction("pa_mainloop_new");
+  }
+  api_ = pa_mainloop_get_api(loop_);
+}
+
+PollMainLoop::~PollMainLoop() noexcept {
+  if (handling_signals_) {
+    StopHandlingSignals();
+  }
+  if (loop_ != nullptr) {
+    pa_mainloop_free(loop_);
+  }
+}
+
+void PollMainLoop::HandleSignals() {
+  if (pa_signal_init(api_)) {
+    throw Error::InFunction("pa_signal_init");
+  }
+  handling_signals_ = true;
+}
+
+int PollMainLoop::Run() {
+  int r;
+  if (pa_mainloop_run(loop_, &r) < 0) {
+    throw Error::InFunction("pa_mainloop_run");
+  }
+  return r;
+}
+
+SignalEventSource::SignalEventSource(int signal,
+                                     std::function<void()> real_callback) {
+  // Construct source_ using CallSignalCallback as the C callback and
+  // real_callback as the real callback. real_callback is going to get passed by
+  // pointer, so that address needs to continue to point to real_callback until
+  // pa_signal_free. Happily, libpulse provides a mechanism to run an arbitrary
+  // callback during pa_signal_free, so move the callback onto the heap and
+  // delete it during pa_signal_free. This function is still responsible for
+  // deleting it if pa_signal_new fails, though, so stash it inside a unique_ptr
+  // and release the unique_ptr if pa_signal_new succeeds.
+  auto heap_callback =
+      std::make_unique<SignalCallback>(std::move(real_callback));
+  source_ = pa_signal_new(signal, &CallSignalCallback, heap_callback.get());
+  if (source_ == nullptr) {
+    throw Error::InFunction("pa_signal_new");
+  }
+  pa_signal_set_destroy(source_, &DeleteSignalCallback);
+  heap_callback.release();
+}
+
+SignalEventSource::~SignalEventSource() noexcept {
+  if (source_ != nullptr) {
+    pa_signal_free(source_);
+  }
+}
+
+Context::Context(const char application_name[], PropertyList& proplist,
+                 PollMainLoop& loop)
+    : ctx_(pa_context_new_with_proplist(pa_mainloop_get_api(loop.get()),
+                                        application_name, proplist.get())) {
+  if (ctx_ == nullptr) {
+    throw Error::InFunction("pa_context_new_with_proplist");
+  }
+}
+
+Context::~Context() noexcept {
+  if (state_callback_ != nullptr) {
+    Disconnect();
+  }
+  if (ctx_ != nullptr) {
+    pa_context_unref(ctx_);
+  }
+}
+
+void Context::Connect(
+    pa_context_flags_t flags,
+    std::function<void(pa_context_state_t)> on_state_change) noexcept {
+  // Set Up CallContextNotifyCallback to call on_state_change. on_state_change
+  // is going to get passed by pointer, so its address needs to continue to
+  // point to on_state_change until pa_context_disconnect. Saving the callback
+  // as a member ensures it stays alive long enough, but it doesn't ensure
+  // pointer stability; to get that, save it on the heap and keep a unique_ptr
+  // to it as a member.
+  state_callback_ =
+      std::make_unique<ContextNotifyCallback>(std::move(on_state_change));
+  pa_context_set_state_callback(ctx_, &CallContextNotifyCallback,
+                                state_callback_.get());
+  if (pa_context_connect(ctx_, /*server=*/nullptr, flags,
+                         /*spawn_api=*/nullptr)) {
+    // The RPC never even went out. libpulse won't call the callback in this
+    // case, so do it manually.
+    (*state_callback_)(PA_CONTEXT_FAILED);
+
+    // The callback will never be called again at this point, and hanging onto
+    // it could cause resource leakage (e.g., if it captured a std::shared_ptr).
+    // Get rid of it.
+    state_callback_ = nullptr;
+  }
+}
+
+void Context::GetSinkInfo(
+    const char name[], std::function<void()> on_failure,
+    std::function<void(const pa_sink_info&)> on_data) const noexcept {
+  // Set up CallOrDeleteSinkInfoCallbacks to call on_failure or on_data as
+  // appropriate. These callbacks are going to get passed by pointer, so they
+  // need to live on the heap. If constructing the get_sink_info_by_name RPC
+  // fails, though, libpulse won't call on_failure; to allow calling it
+  // manually, stash the callbacks inside a unique_ptr and release the
+  // unique_ptr if the RPC actually gets dispatched.
+  auto callbacks = std::make_unique<SinkInfoCallbacks>();
+  callbacks->on_failure = std::move(on_failure);
+  callbacks->on_data = std::move(on_data);
+  pa_operation* op = pa_context_get_sink_info_by_name(
+      ctx_, name, &CallOrDeleteSinkInfoCallbacks, callbacks.get());
+  if (op == nullptr) {
+    // The RPC never even went out. libpulse won't call on_failure in this case,
+    // so do it manually.
+    callbacks->on_failure();
+  } else {
+    pa_operation_unref(op);  // The RPC handle is no longer needed.
+
+    // The RPC is running, which means CallOrDeleteSinkInfoCallbacks will be
+    // called. (In fact, it's possible that CallOrDeleteSinkInfoCallbacks has
+    // already been called and the callbacks struct has already been deleted!)
+    // Stop managing the callbacks struct.
+    callbacks.release();
+  }
+}
+
+void Context::Subscribe(
+    pa_subscription_mask_t desired_events, std::function<void()> on_failure,
+    std::function<void(pa_subscription_event_type_t)> on_event) const noexcept {
+  // Set up CallSubscribeCallback to call on_event. This callback is going to
+  // get passed by pointer, so its address needs to continue to point to
+  // on_event until pa_context_disconnect. Saving the callback as a member
+  // ensures it stays alive long enough, but it doesn't ensure pointer
+  // stability; to get that, save it on the heap and keep a unique_ptr to it as
+  // a member.
+  subscribe_callback_ =
+      std::make_unique<SubscribeCallback>(std::move(on_event));
+  pa_context_set_subscribe_callback(ctx_, &CallSubscribeCallback,
+                                    subscribe_callback_.get());
+
+  // Set up CallAndDeleteRpcResultCallbacks to handle the result of the
+  // subscribe RPC. The real callbacks struct is going to get passed by pointer,
+  // so it needs to live on the heap. If constructing the subscribe RPC fails,
+  // though, libpulse won't call on_failure; to allow calling it manually, stash
+  // the callbacks inside a unique_ptr and release the unique_ptr if the RPC
+  // actually gets dispatched.
+  auto subscribe_completion_callbacks = std::make_unique<RpcResultCallbacks>();
+  subscribe_completion_callbacks->on_failure = std::move(on_failure);
+  // We don't need to tell the user the RPC succeded; the event callback can
+  // tell them that.
+  subscribe_completion_callbacks->on_success = []() noexcept {};
+  pa_operation* op = pa_context_subscribe(ctx_, desired_events,
+                                          &CallAndDeleteRpcResultCallbacks,
+                                          subscribe_completion_callbacks.get());
+  if (op == nullptr) {
+    // The RPC never even went out. libpulse won't call
+    // CallAndDeleteRpcResultCallbacks in this case, call on_failure manually.
+    subscribe_completion_callbacks->on_failure();
+  } else {
+    pa_operation_unref(op);  // The RPC handle is no longer needed.
+
+    // The RPC is running, which means CallAndDeleteRpcResultCallbacks will be
+    // called. (In fact, it's possible that CallAndDeleteRpcResultCallbacks has
+    // already been called and the result callbacks struct has already been
+    // deleted!) Stop managing the result callbacks struct.
+    subscribe_completion_callbacks.release();
+  }
+}
+
+}  // namespace pulse
diff --git a/pulse.h b/pulse.h
new file mode 100644
index 0000000..e71e5f2
--- /dev/null
+++ b/pulse.h
@@ -0,0 +1,242 @@
+// Copyright 2022 Benjamin Barenblat
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+// An idiomatic C++ interface to (parts of) PulseAudio.
+
+#ifndef PAVMON_PULSE_H_
+#define PAVMON_PULSE_H_
+
+#include <pulse/context.h>
+#include <pulse/def.h>
+#include <pulse/introspect.h>
+#include <pulse/mainloop-api.h>
+#include <pulse/mainloop-signal.h>
+#include <pulse/mainloop.h>
+#include <pulse/proplist.h>
+
+#include <exception>
+#include <functional>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <utility>
+#include <vector>
+
+namespace pulse {
+
+// The base exception type for all exceptions thrown by this library.
+class Error final : public std::exception {
+ public:
+  static Error InFunction(std::string function_name) noexcept {
+    return Error(function_name + " failed");
+  }
+
+  explicit Error(std::string what) noexcept : what_(std::move(what)) {}
+
+  Error(const Error&) = default;
+  Error& operator=(const Error&) = default;
+  Error(Error&&) noexcept = default;
+  Error& operator=(Error&&) noexcept = default;
+
+  const char* what() const noexcept override { return what_.c_str(); }
+
+ private:
+  std::string what_;
+};
+
+// A simple key-value store.
+//
+// This class is thread-compatible.
+class PropertyList final {
+ public:
+  explicit PropertyList();
+
+  PropertyList(const PropertyList&);
+
+  PropertyList& operator=(const PropertyList& other);
+
+  PropertyList(PropertyList&& other) noexcept : list_(nullptr) {
+    *this = std::move(other);
+  }
+
+  PropertyList& operator=(PropertyList&& other) noexcept {
+    swap(*this, other);
+    return *this;
+  }
+
+  ~PropertyList() noexcept;
+
+  // Adds or overwrites a mapping. Useful keys are #defined in
+  // <pulse/proplist.h>.
+  void Set(const char key[], std::string_view value);
+
+  pa_proplist* get() noexcept { return list_; }
+
+  friend void swap(PropertyList& left, PropertyList& right) noexcept {
+    using ::std::swap;
+    swap(left.list_, right.list_);
+  }
+
+ private:
+  pa_proplist* list_;
+};
+
+// A main event loop implementation based on poll(2).
+//
+// This class is thread-compatible.
+class PollMainLoop final {
+ public:
+  explicit PollMainLoop();
+
+  PollMainLoop(PollMainLoop&& other) noexcept
+      : loop_(nullptr), api_(nullptr), handling_signals_(false) {
+    *this = std::move(other);
+  }
+
+  PollMainLoop& operator=(PollMainLoop&& other) noexcept {
+    swap(*this, other);
+    return *this;
+  }
+
+  ~PollMainLoop() noexcept;
+
+  // Configures process signals to be delivered to the event loop.
+  void HandleSignals();
+
+  // Undoes a HandleSignals call.
+  void StopHandlingSignals() noexcept {
+    pa_signal_done();
+    handling_signals_ = false;
+  }
+
+  // Runs the main loop. This function does not return until somebody invokes
+  // Quit or an error occurs; it returns the argument to Quit.
+  int Run();
+
+  // Instructs the main loop to stop and return the specified code.
+  void Quit(int retval) noexcept { api_->quit(api_, retval); }
+
+  pa_mainloop* get() noexcept { return loop_; }
+
+  friend void swap(PollMainLoop& left, PollMainLoop& right) noexcept {
+    using ::std::swap;
+    swap(left.loop_, right.loop_);
+    swap(left.api_, right.api_);
+    swap(left.handling_signals_, right.handling_signals_);
+  }
+
+ private:
+  pa_mainloop* loop_;
+  pa_mainloop_api* api_;
+
+  bool handling_signals_;
+};
+
+// An RAII object to control signal handling in the event loop. Call
+// HandleSignals() on the event loop and then create one of these for each
+// signal you'd like to handle.
+//
+// This class is thread-compatible.
+class SignalEventSource final {
+ public:
+  // Starts handling a particular signal. Calls the specified callback whenever
+  // that signal is received.
+  explicit SignalEventSource(int signal, std::function<void()>);
+
+  SignalEventSource(SignalEventSource&& other) noexcept : source_(nullptr) {
+    *this = std::move(other);
+  }
+
+  SignalEventSource& operator=(SignalEventSource&& other) noexcept {
+    swap(*this, other);
+    return *this;
+  }
+
+  ~SignalEventSource() noexcept;
+
+  friend void swap(SignalEventSource& left, SignalEventSource& right) noexcept {
+    using ::std::swap;
+    swap(left.source_, right.source_);
+  }
+
+ private:
+  pa_signal_event* source_;
+};
+
+// A connection to a PulseAudio server.
+//
+// This class is thread-compatible.
+class Context final {
+ public:
+  explicit Context(const char application_name[], PropertyList&, PollMainLoop&);
+
+  Context(Context&& other) noexcept : ctx_(nullptr) {
+    *this = std::move(other);
+  }
+
+  Context& operator=(Context&& other) noexcept {
+    swap(*this, other);
+    return *this;
+  }
+
+  ~Context() noexcept;
+
+  // Connects to the default PulseAudio server. Set options by bitwise-ORing
+  // flags defined in <pulse/def.h>. Calls the specified callback each time the
+  // state of the connection changes.
+  void Connect(
+      pa_context_flags_t,
+      std::function<void(pa_context_state_t)> on_state_change) noexcept;
+
+  // Disconnects from the server.
+  void Disconnect() noexcept {
+    pa_context_disconnect(ctx_);
+    state_callback_ = nullptr;
+  }
+
+  // Requests information about a sink specified by name. Calls on_failure if
+  // the request fails; calls on_data if it succeeds. Each callback will only be
+  // called once.
+  void GetSinkInfo(
+      const char name[], std::function<void()> on_failure,
+      std::function<void(const pa_sink_info&)> on_data) const noexcept;
+
+  // Subscribes to some subset of server event notifications (see <pulse/def.h>
+  // for allowable mask values and event types). Calls on_failure once if the
+  // request fails. If the request succeeds, calls on_data once every time an
+  // event occurs.
+  void Subscribe(pa_subscription_mask_t, std::function<void()> on_failure,
+                 std::function<void(pa_subscription_event_type_t)> on_event)
+      const noexcept;
+
+  friend void swap(Context& left, Context& right) noexcept {
+    using ::std::swap;
+    swap(left.ctx_, right.ctx_);
+    swap(left.state_callback_, right.state_callback_);
+    swap(left.subscribe_callback_, right.subscribe_callback_);
+  }
+
+ private:
+  pa_context* ctx_;
+
+  // These are pointers so the underlying std::function is always
+  // at the same address, even if this object gets moved.
+  std::unique_ptr<std::function<void(pa_context_state_t)>> state_callback_;
+  mutable std::unique_ptr<std::function<void(pa_subscription_event_type_t)>>
+      subscribe_callback_;
+};
+
+}  // namespace pulse
+
+#endif  // PAVMON_PULSE_H_