ConfigServer+LibConfig: Add way for clients to listen for config changes

This patch adds a Config::Listener abstract class that anyone can
inherit from and receive notifications when configuration values change.

We don't yet monitor file system changes, so these only work for changes
made by ConfigServer itself.

In order to receive these notifications, clients must monitor the domain
by calling monitor_domain(). Only pledged domains can be monitored.

Note that the client initiating the change does not get notified.

Userland/Libraries/LibConfig/CMakeLists.txt

@@ -1,5 +1,6 @@
 set(SOURCES
     Client.cpp
+    Listener.cpp
 )
 
 set(GENERATED_SOURCES

Userland/Libraries/LibConfig/Client.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <LibConfig/Client.h>
+#include <LibConfig/Listener.h>
 
 namespace Config {
 
@@ -24,6 +25,11 @@ void Client::pledge_domains(Vector<String> const& domains)
     async_pledge_domains(domains);
 }
 
+void Client::monitor_domain(String const& domain)
+{
+    async_monitor_domain(domain);
+}
+
 String Client::read_string(StringView domain, StringView group, StringView key, StringView fallback)
 {
     return read_string_value(domain, group, key).value_or(fallback);
@@ -54,4 +60,25 @@ void Client::write_bool(StringView domain, StringView group, StringView key, boo
     async_write_bool_value(domain, group, key, value);
 }
 
+void Client::notify_changed_string_value(String const& domain, String const& group, String const& key, String const& value)
+{
+    Listener::for_each([&](auto& listener) {
+        listener.config_string_did_change(domain, group, key, value);
+    });
+}
+
+void Client::notify_changed_i32_value(String const& domain, String const& group, String const& key, i32 value)
+{
+    Listener::for_each([&](auto& listener) {
+        listener.config_i32_did_change(domain, group, key, value);
+    });
+}
+
+void Client::notify_changed_bool_value(String const& domain, String const& group, String const& key, bool value)
+{
+    Listener::for_each([&](auto& listener) {
+        listener.config_bool_did_change(domain, group, key, value);
+    });
+}
+
 }

Userland/Libraries/LibConfig/Client.h

@@ -22,6 +22,7 @@ class Client final
 
 public:
     void pledge_domains(Vector<String> const&);
+    void monitor_domain(String const&);
 
     String read_string(StringView domain, StringView group, StringView key, StringView fallback);
     i32 read_i32(StringView domain, StringView group, StringView key, i32 fallback);
@@ -38,6 +39,10 @@ private:
         : IPC::ServerConnection<ConfigClientEndpoint, ConfigServerEndpoint>(*this, "/tmp/portal/config")
     {
     }
+
+    void notify_changed_string_value(String const& domain, String const& group, String const& key, String const& value) override;
+    void notify_changed_i32_value(String const& domain, String const& group, String const& key, i32 value) override;
+    void notify_changed_bool_value(String const& domain, String const& group, String const& key, bool value) override;
 };
 
 inline String read_string(StringView domain, StringView group, StringView key, StringView fallback = {})
@@ -80,4 +85,9 @@ inline void pledge_domains(String const& domains)
     Client::the().pledge_domains({ domains });
 }
 
+inline void monitor_domain(String const& domain)
+{
+    Client::the().monitor_domain(domain);
+}
+
 }

Userland/Libraries/LibConfig/Listener.cpp

@@ -0,0 +1,44 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Function.h>
+#include <AK/HashTable.h>
+#include <AK/String.h>
+#include <LibConfig/Listener.h>
+
+namespace Config {
+
+static HashTable<Listener*> s_listeners;
+
+Listener::Listener()
+{
+    s_listeners.set(this);
+}
+
+Listener::~Listener()
+{
+    s_listeners.remove(this);
+}
+
+void Listener::for_each(Function<void(Listener&)> callback)
+{
+    for (auto* listener : s_listeners)
+        callback(*listener);
+}
+
+void Listener::config_string_did_change(String const&, String const&, String const&, String const&)
+{
+}
+
+void Listener::config_i32_did_change(String const&, String const&, String const&, i32)
+{
+}
+
+void Listener::config_bool_did_change(String const&, String const&, String const&, bool)
+{
+}
+
+}

Userland/Libraries/LibConfig/Listener.h

@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Forward.h>
+
+namespace Config {
+
+class Listener {
+public:
+    virtual ~Listener();
+
+    static void for_each(Function<void(Listener&)>);
+
+    virtual void config_string_did_change(String const& domain, String const& group, String const& key, String const& value);
+    virtual void config_i32_did_change(String const& domain, String const& group, String const& key, i32 value);
+    virtual void config_bool_did_change(String const& domain, String const& group, String const& key, bool value);
+
+protected:
+    Listener();
+};
+
+}