SystemServer: Implement keepalive

When reaping a child, SystemServer will now match up child's pid with its own
record of the services, and respawn the service if keepalive is enabled for it.

For example, we want to restart the WindowServer if it crashes, but we wouldn't
want to restart the Terminal if it gets closed.

Base/etc/SystemServer.ini

@@ -6,22 +6,27 @@ Priority=high
 
 [ProtocolServer]
 Priority=low
+KeepAlive=1
 User=anon
 
 [LookupServer]
 Priority=low
+KeepAlive=1
 User=anon
 
 [WindowServer]
 Priority=high
+KeepAlive=1
 User=anon
 
 [AudioServer]
 Priority=high
+KeepAlive=1
 User=anon
 
 [Taskbar]
 Priority=high
+KeepAlive=1
 User=anon
 
 [Terminal]

Servers/SystemServer/Service.cpp

@@ -16,6 +16,7 @@ struct UidAndGid {
 };
 
 static HashMap<String, UidAndGid>* s_user_map;
+static HashMap<pid_t, Service*> s_service_map;
 
 void Service::resolve_user()
 {
@@ -35,6 +36,14 @@ void Service::resolve_user()
     m_gid = user.value().gid;
 }
 
+Service* Service::find_by_pid(pid_t pid)
+{
+    auto it = s_service_map.find(pid);
+    if (it == s_service_map.end())
+        return nullptr;
+    return (*it).value;
+}
+
 void Service::spawn()
 {
     dbg() << "Spawning " << name();
@@ -81,9 +90,26 @@ void Service::spawn()
         rc = execv(argv[0], argv);
         perror("exec");
         ASSERT_NOT_REACHED();
+    } else {
+        // We are the parent.
+        s_service_map.set(m_pid, this);
     }
 }
 
+void Service::did_exit(int exit_code)
+{
+    ASSERT(m_pid > 0);
+    (void)exit_code;
+
+    dbg() << "Service " << name() << " has exited";
+
+    s_service_map.remove(m_pid);
+    m_pid = -1;
+
+    if (m_keep_alive)
+        spawn();
+}
+
 Service::Service(const CConfigFile& config, const StringView& name)
     : CObject(nullptr)
 {
@@ -106,6 +132,8 @@ Service::Service(const CConfigFile& config, const StringView& name)
     else
         ASSERT_NOT_REACHED();
 
+    m_keep_alive = config.read_bool_entry(name, "KeepAlive");
+
     m_user = config.read_entry(name, "User");
     if (!m_user.is_null())
         resolve_user();
@@ -127,6 +155,7 @@ void Service::save_to(JsonObject& json)
 
     json.set("stdio_file_path", m_stdio_file_path);
     json.set("priority", m_priority);
+    json.set("keep_alive", m_keep_alive);
     json.set("user", m_user);
     json.set("uid", m_uid);
     json.set("gid", m_gid);

Servers/SystemServer/Service.h

@@ -15,6 +15,9 @@ class Service final : public CObject {
 
 public:
     void spawn();
+    void did_exit(int exit_code);
+
+    static Service* find_by_pid(pid_t);
 
     void save_to(AK::JsonObject&) override;
 
@@ -28,6 +31,8 @@ private:
     // File path to open as stdio fds.
     String m_stdio_file_path;
     int m_priority { 1 };
+    // Whether we should re-launch it if it exits.
+    bool m_keep_alive { false };
     // The name of the user we should run this service as.
     String m_user;
     uid_t m_uid { 0 };

Servers/SystemServer/main.cpp

@@ -10,12 +10,28 @@
 #include <sys/wait.h>
 #include <unistd.h>
 
-void sigchld_handler(int)
+static void sigchld_handler(int)
 {
     int status = 0;
     pid_t pid = waitpid(-1, &status, WNOHANG);
-    if (pid)
-        dbg() << "reaped pid " << pid;
+    if (!pid)
+        return;
+
+    dbg() << "Reaped child with pid " << pid;
+    Service* service = Service::find_by_pid(pid);
+    if (service == nullptr) {
+        dbg() << "There was no service with this pid, what is going on?";
+        return;
+    }
+
+    // Call service->did_exit(status) some time soon.
+    // We wouldn't want to run the complex logic, such
+    // as possibly spawning the service again, from the
+    // signal handler, so defer it.
+    CEventLoop::main().post_event(*service, make<CDeferredInvocationEvent>([=](CObject&) {
+        service->did_exit(status);
+    }));
+    CEventLoop::wake();
 }
 
 static void check_for_test_mode()
@@ -63,7 +79,12 @@ int main(int, char**)
 {
     mount_all_filesystems();
 
-    signal(SIGCHLD, sigchld_handler);
+    struct sigaction sa = {
+        .sa_handler = sigchld_handler,
+        .sa_mask = 0,
+        .sa_flags = SA_RESTART
+    };
+    sigaction(SIGCHLD, &sa, nullptr);
 
     CEventLoop event_loop;