SystemServer: Read service list from a config file

This replaces the hardcoded services list with a very simple config
file in /etc/SystemServer.ini :^)

Closes https://github.com/SerenityOS/serenity/issues/610

Servers/SystemServer/Service.cpp

@@ -0,0 +1,138 @@
+#include "Service.h"
+#include <AK/HashMap.h>
+#include <AK/JsonArray.h>
+#include <AK/JsonObject.h>
+#include <LibCore/CConfigFile.h>
+#include <fcntl.h>
+#include <pwd.h>
+#include <sched.h>
+#include <stdio.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+struct UidAndGid {
+    uid_t uid;
+    gid_t gid;
+};
+
+static HashMap<String, UidAndGid>* s_user_map;
+
+void Service::resolve_user()
+{
+    if (s_user_map == nullptr) {
+        s_user_map = new HashMap<String, UidAndGid>;
+        for (struct passwd* passwd = getpwent(); passwd; passwd = getpwent())
+            s_user_map->set(passwd->pw_name, { passwd->pw_uid, passwd->pw_gid });
+        endpwent();
+    }
+
+    auto user = s_user_map->get(m_user);
+    if (!user.has_value()) {
+        dbg() << "Failed to resolve user name " << m_user;
+        ASSERT_NOT_REACHED();
+    }
+    m_uid = user.value().uid;
+    m_gid = user.value().gid;
+}
+
+void Service::spawn()
+{
+    dbg() << "Spawning " << name();
+
+    m_pid = fork();
+
+    if (m_pid < 0) {
+        perror("fork");
+        ASSERT_NOT_REACHED();
+    } else if (m_pid == 0) {
+        // We are the child.
+
+        struct sched_param p;
+        p.sched_priority = m_priority;
+        int rc = sched_setparam(0, &p);
+        if (rc < 0) {
+            perror("sched_setparam");
+            ASSERT_NOT_REACHED();
+        }
+
+        if (!m_stdio_file_path.is_null()) {
+            close(0);
+            int fd = open_with_path_length(m_stdio_file_path.characters(), m_stdio_file_path.length(), O_RDWR, 0);
+            ASSERT(fd <= 0);
+            if (fd < 0) {
+                perror("open");
+                ASSERT_NOT_REACHED();
+            }
+            dup2(0, 1);
+            dup2(0, 2);
+        }
+
+        if (!m_user.is_null()) {
+            setuid(m_uid);
+            setgid(m_gid);
+        }
+
+        char* argv[m_extra_arguments.size() + 2];
+        argv[0] = const_cast<char*>(m_executable_path.characters());
+        for (int i = 0; i < m_extra_arguments.size(); i++)
+            argv[i + 1] = const_cast<char*>(m_extra_arguments[i].characters());
+        argv[m_extra_arguments.size() + 1] = nullptr;
+
+        rc = execv(argv[0], argv);
+        perror("exec");
+        ASSERT_NOT_REACHED();
+    }
+}
+
+Service::Service(const CConfigFile& config, const StringView& name)
+    : CObject(nullptr)
+{
+    ASSERT(config.has_group(name));
+
+    set_name(name);
+    m_executable_path = config.read_entry(name, "Executable", String::format("/bin/%s", this->name().characters()));
+    m_extra_arguments = config.read_entry(name, "Arguments", "").split(' ');
+    m_stdio_file_path = config.read_entry(name, "StdIO");
+
+    String prio = config.read_entry(name, "Priority");
+    if (prio == "idle")
+        m_priority = 0;
+    else if (prio == "low")
+        m_priority = 1;
+    else if (prio == "normal" || prio.is_null())
+        m_priority = 2;
+    else if (prio == "high")
+        m_priority = 3;
+    else
+        ASSERT_NOT_REACHED();
+
+    m_user = config.read_entry(name, "User");
+    if (!m_user.is_null())
+        resolve_user();
+}
+
+void Service::save_to(JsonObject& json)
+{
+    CObject::save_to(json);
+
+    json.set("executable_path", m_executable_path);
+
+    // FIXME: This crashes Inspector.
+    /*
+    JsonArray extra_args;
+    for (String& arg : m_extra_arguments)
+        extra_args.append(arg);
+    json.set("extra_arguments", move(extra_args));
+    */
+
+    json.set("stdio_file_path", m_stdio_file_path);
+    json.set("priority", m_priority);
+    json.set("user", m_user);
+    json.set("uid", m_uid);
+    json.set("gid", m_gid);
+
+    if (m_pid > 0)
+        json.set("pid", m_pid);
+    else
+        json.set("pid", nullptr);
+}