Userland: Add pager option to man and use less by default

Userland/Utilities/less.cpp

@@ -288,6 +288,8 @@ static String get_key_sequence()
 
 int main(int argc, char** argv)
 {
+    VERIFY(isatty(STDOUT_FILENO));
+
     char const* filename = "-";
     char const* prompt = "?f%f :.(line %l)?e (END):.";
     bool dont_switch_buffer = false;

Userland/Utilities/man.cpp

@@ -10,10 +10,39 @@
 #include <LibCore/ArgsParser.h>
 #include <LibCore/File.h>
 #include <LibMarkdown/Document.h>
+#include <fcntl.h>
+#include <spawn.h>
 #include <stdio.h>
 #include <sys/ioctl.h>
+#include <sys/wait.h>
 #include <unistd.h>
 
+static pid_t pipe_to_pager(String const& command)
+{
+    char const* argv[] = { "sh", "-c", command.characters(), nullptr };
+
+    int stdout_pipe[2] = {};
+    if (pipe2(stdout_pipe, O_CLOEXEC)) {
+        perror("pipe2");
+        exit(1);
+    }
+    posix_spawn_file_actions_t action;
+    posix_spawn_file_actions_init(&action);
+    posix_spawn_file_actions_adddup2(&action, stdout_pipe[0], STDIN_FILENO);
+
+    pid_t pid;
+    if ((errno = posix_spawnp(&pid, argv[0], &action, nullptr, const_cast<char**>(argv), environ))) {
+        perror("posix_spawn");
+        exit(1);
+    }
+    posix_spawn_file_actions_destroy(&action);
+
+    dup2(stdout_pipe[1], STDOUT_FILENO);
+    close(stdout_pipe[1]);
+    close(stdout_pipe[0]);
+    return pid;
+}
+
 int main(int argc, char* argv[])
 {
     int view_width = 0;
@@ -26,7 +55,7 @@ int main(int argc, char* argv[])
     if (view_width == 0)
         view_width = 80;
 
-    if (pledge("stdio rpath", nullptr) < 0) {
+    if (pledge("stdio rpath exec proc", nullptr) < 0) {
         perror("pledge");
         return 1;
     }
@@ -36,15 +65,22 @@ int main(int argc, char* argv[])
         return 1;
     }
 
+    if (unveil("/bin", "x") < 0) {
+        perror("unveil");
+        return 1;
+    }
+
     unveil(nullptr, nullptr);
 
     const char* section = nullptr;
     const char* name = nullptr;
+    const char* pager = nullptr;
 
     Core::ArgsParser args_parser;
     args_parser.set_general_help("Read manual pages. Try 'man man' to get started.");
     args_parser.add_positional_argument(section, "Section of the man page", "section", Core::ArgsParser::Required::No);
     args_parser.add_positional_argument(name, "Name of the man page", "name");
+    args_parser.add_option(pager, "Pager to pipe the man page to", "pager", 'P', "pager");
 
     args_parser.parse(argc, argv);
 
@@ -78,12 +114,23 @@ int main(int argc, char* argv[])
     auto file = Core::File::construct();
     file->set_filename(make_path(section));
 
+    String pager_command = pager;
+    if (!pager) {
+        String clean_name(name);
+        String clean_section(section);
+
+        clean_name.replace("'", "'\\''");
+        clean_section.replace("'", "'\\''");
+        pager_command = String::formatted("less -P 'Manual Page {}({}) line %l?e (END):.'", clean_name, clean_section);
+    }
+    pid_t pager_pid = pipe_to_pager(pager_command);
+
     if (!file->open(Core::OpenMode::ReadOnly)) {
         perror("Failed to open man page file");
         exit(1);
     }
 
-    if (pledge("stdio", nullptr) < 0) {
+    if (pledge("stdio proc", nullptr) < 0) {
         perror("pledge");
         return 1;
     }
@@ -99,4 +146,10 @@ int main(int argc, char* argv[])
 
     String rendered = document->render_for_terminal(view_width);
     out("{}", rendered);
+
+    // FIXME: Remove this wait, it shouldn't be necessary but Shell does not
+    //        resume properly without it. This wait also breaks <C-z> backgrounding
+    fclose(stdout);
+    int wstatus;
+    waitpid(pager_pid, &wstatus, 0);
 }