LibCore: Add peer pid retrieval for LocalSocket

This will allow programs connected over unix sockets to retrieve the
pid of their peers in a nice way.

Userland/Libraries/LibCore/LocalSocket.cpp

@@ -53,6 +53,31 @@ LocalSocket::~LocalSocket()
 {
 }
 
+pid_t LocalSocket::peer_pid() const
+{
+#ifdef AK_OS_MACOS
+    pid_t pid;
+    socklen_t pid_size = sizeof(pid);
+
+    if (getsockopt(fd(), SOL_LOCAL, LOCAL_PEERPID, &pid, &pid_size) < 0) {
+        dbgln("LocalSocket: getsockopt failed, {}", strerror(errno));
+        VERIFY_NOT_REACHED();
+    }
+
+    return pid;
+#else
+    struct ucred creds = {};
+    socklen_t creds_size = sizeof(creds);
+
+    if (getsockopt(fd(), SOL_SOCKET, SO_PEERCRED, &creds, &creds_size) < 0) {
+        dbgln("LocalSocket: getsockopt failed, {}", strerror(errno));
+        VERIFY_NOT_REACHED();
+    }
+
+    return creds.pid;
+#endif
+}
+
 HashMap<String, int> LocalSocket::s_overtaken_sockets {};
 bool LocalSocket::s_overtaken_sockets_parsed { false };
 

Userland/Libraries/LibCore/LocalSocket.h

@@ -16,6 +16,7 @@ public:
     virtual ~LocalSocket() override;
 
     static RefPtr<LocalSocket> take_over_accepted_socket_from_system_server(String const& socket_path = String());
+    pid_t peer_pid() const;
 
 private:
     explicit LocalSocket(Object* parent = nullptr);