/* SPDX-License-Identifier: LGPL-2.1+ */

#include <errno.h>
#include <signal.h>
#include <stddef.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/prctl.h>
#include <unistd.h>

#include "copy.h"
#include "fd-util.h"
#include "locale-util.h"
#include "log.h"
#include "macro.h"
#include "pager.h"
#include "process-util.h"
#include "signal-util.h"
#include "string-util.h"
#include "strv.h"
#include "terminal-util.h"

static pid_t pager_pid = 0;

static int stored_stdout = -1;
static int stored_stderr = -1;
static bool stdout_redirected = false;
static bool stderr_redirected = false;

_noreturn_ static void pager_fallback(void) {
        int r;

        r = copy_bytes(STDIN_FILENO, STDOUT_FILENO, (uint64_t) -1, 0);
        if (r < 0) {
                log_error_errno(r, "Internal pager failed: %m");
                _exit(EXIT_FAILURE);
        }

        _exit(EXIT_SUCCESS);
}

int pager_open(bool no_pager, bool jump_to_end) {
        _cleanup_close_pair_ int fd[2] = { -1, -1 };
        const char *pager;
        int r;

        if (no_pager)
                return 0;

        if (pager_pid > 0)
                return 1;

        if (terminal_is_dumb())
                return 0;

        if (!is_main_thread())
                return -EPERM;

        pager = getenv("SYSTEMD_PAGER");
        if (!pager)
                pager = getenv("PAGER");

        /* If the pager is explicitly turned off, honour it */
        if (pager && STR_IN_SET(pager, "", "cat"))
                return 0;

        /* Determine and cache number of columns/lines before we spawn the pager so that we get the value from the
         * actual tty */
        (void) columns();
        (void) lines();

        if (pipe2(fd, O_CLOEXEC) < 0)
                return log_error_errno(errno, "Failed to create pager pipe: %m");

        r = safe_fork("(pager)", FORK_RESET_SIGNALS|FORK_DEATHSIG|FORK_LOG, &pager_pid);
        if (r < 0)
                return r;
        if (r == 0) {
                const char* less_opts, *less_charset;

                /* In the child start the pager */

                (void) dup2(fd[0], STDIN_FILENO);
                safe_close_pair(fd);

                /* Initialize a good set of less options */
                less_opts = getenv("SYSTEMD_LESS");
                if (!less_opts)
                        less_opts = "FRSXMK";
                if (jump_to_end)
                        less_opts = strjoina(less_opts, " +G");
                if (setenv("LESS", less_opts, 1) < 0)
                        _exit(EXIT_FAILURE);

                /* Initialize a good charset for less. This is
                 * particularly important if we output UTF-8
                 * characters. */
                less_charset = getenv("SYSTEMD_LESSCHARSET");
                if (!less_charset && is_locale_utf8())
                        less_charset = "utf-8";
                if (less_charset &&
                    setenv("LESSCHARSET", less_charset, 1) < 0)
                        _exit(EXIT_FAILURE);

                if (pager) {
                        execlp(pager, pager, NULL);
                        execl("/bin/sh", "sh", "-c", pager, NULL);
                }

                /* Debian's alternatives command for pagers is
                 * called 'pager'. Note that we do not call
                 * sensible-pagers here, since that is just a
                 * shell script that implements a logic that
                 * is similar to this one anyway, but is
                 * Debian-specific. */
                execlp("pager", "pager", NULL);

                execlp("less", "less", NULL);
                execlp("more", "more", NULL);

                pager_fallback();
                /* not reached */
        }

        /* Return in the parent */
        stored_stdout = fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 3);
        if (dup2(fd[1], STDOUT_FILENO) < 0) {
                stored_stdout = safe_close(stored_stdout);
                return log_error_errno(errno, "Failed to duplicate pager pipe: %m");
        }
        stdout_redirected = true;

        stored_stderr = fcntl(STDERR_FILENO, F_DUPFD_CLOEXEC, 3);
        if (dup2(fd[1], STDERR_FILENO) < 0) {
                stored_stderr = safe_close(stored_stderr);
                return log_error_errno(errno, "Failed to duplicate pager pipe: %m");
        }
        stderr_redirected = true;

        return 1;
}

void pager_close(void) {

        if (pager_pid <= 0)
                return;

        /* Inform pager that we are done */
        (void) fflush(stdout);
        if (stdout_redirected)
                if (stored_stdout < 0 || dup2(stored_stdout, STDOUT_FILENO) < 0)
                        (void) close(STDOUT_FILENO);
        stored_stdout = safe_close(stored_stdout);
        (void) fflush(stderr);
        if (stderr_redirected)
                if (stored_stderr < 0 || dup2(stored_stderr, STDERR_FILENO) < 0)
                        (void) close(STDERR_FILENO);
        stored_stderr = safe_close(stored_stderr);
        stdout_redirected = stderr_redirected = false;

        (void) kill(pager_pid, SIGCONT);
        (void) wait_for_terminate(pager_pid, NULL);
        pager_pid = 0;
}

bool pager_have(void) {
        return pager_pid > 0;
}

#if 0 /// UNNEEDED by elogind
int show_man_page(const char *desc, bool null_stdio) {
        const char *args[4] = { "man", NULL, NULL, NULL };
        char *e = NULL;
        pid_t pid;
        size_t k;
        int r;

        k = strlen(desc);

        if (desc[k-1] == ')')
                e = strrchr(desc, '(');

        if (e) {
                char *page = NULL, *section = NULL;

                page = strndupa(desc, e - desc);
                section = strndupa(e + 1, desc + k - e - 2);

                args[1] = section;
                args[2] = page;
        } else
                args[1] = desc;

        r = safe_fork("(man)", FORK_RESET_SIGNALS|FORK_DEATHSIG|(null_stdio ? FORK_NULL_STDIO : 0)|FORK_LOG, &pid);
        if (r < 0)
                return r;
        if (r == 0) {
                /* Child */
                execvp(args[0], (char**) args);
                log_error_errno(errno, "Failed to execute man: %m");
                _exit(EXIT_FAILURE);
        }

        return wait_for_terminate_and_check(NULL, pid, 0);
}
#endif // 0
