/*
 * State diagram and cleanup
 * -------------------------
 *
 * If the program exits while a temporary file is active, we want to
 * make sure that we remove it. This is done by remembering the active
 * temporary files in a linked list, `tempfile_list`. An `atexit(3)`
 * handler and a signal handler are registered, to clean up any active
 * temporary files.
 *
 * Because the signal handler can run at any time, `tempfile_list` and
 * the `tempfile` objects that comprise it must be kept in
 * self-consistent states at all times.
 *
 * The possible states of a `tempfile` object are as follows:
 *
 * - Uninitialized. In this state the object's `on_list` field must be
 *   zero but the rest of its contents need not be initialized. As
 *   soon as the object is used in any way, it is irrevocably
 *   registered in `tempfile_list`, and `on_list` is set.
 *
 * - Active, file open (after `create_tempfile()` or
 *   `reopen_tempfile()`). In this state:
 *
 *   - the temporary file exists
 *   - `active` is set
 *   - `filename` holds the filename of the temporary file
 *   - `fd` holds a file descriptor open for writing to it
 *   - `fp` holds a pointer to an open `FILE` object if and only if
 *     `fdopen_tempfile()` has been called on the object
 *   - `owner` holds the PID of the process that created the file
 *
 * - Active, file closed (after successful `close_tempfile()`). Same
 *   as the previous state, except that the temporary file is closed,
 *   `fd` is -1, and `fp` is `NULL`.
 *
 * - Inactive (after `delete_tempfile()`, `rename_tempfile()`, a
 *   failed attempt to create a temporary file, or a failed
 *   `close_tempfile()`). In this state:
 *
 *   - `active` is unset
 *   - `filename` is empty (usually, though there are transitory
 *     states in which this condition doesn't hold). Client code should
 *     *not* rely on the filename being empty in this state.
 *   - `fd` is -1 and `fp` is `NULL`
 *   - the object is left registered in the `tempfile_list`, and
 *     `on_list` is set.
 *
 * A temporary file is owned by the process that created it. The
 * `tempfile` has an `owner` field that records the owner's PID. This
 * field is used to prevent a forked process from deleting a temporary
 * file created by its parent.
 */

#include "cache.h"
#include "tempfile.h"
#include "sigchain.h"

static struct tempfile *volatile tempfile_list;

static void remove_tempfiles(int skip_fclose)
{
        pid_t me = getpid();

        while (tempfile_list) {
                if (tempfile_list->owner == me) {
                        /* fclose() is not safe to call in a signal handler */
                        if (skip_fclose)
                                tempfile_list->fp = NULL;
                        delete_tempfile(tempfile_list);
                }
                tempfile_list = tempfile_list->next;
        }
}

static void remove_tempfiles_on_exit(void)
{
        remove_tempfiles(0);
}

static void remove_tempfiles_on_signal(int signo)
{
        remove_tempfiles(1);
        sigchain_pop(signo);
        raise(signo);
}

/* Make sure errno contains a meaningful value on error */
int create_tempfile(struct tempfile *tempfile, const char *path)
{
        size_t pathlen = strlen(path);

        if (!tempfile_list) {
                /* One-time initialization */
                sigchain_push_common(remove_tempfiles_on_signal);
                atexit(remove_tempfiles_on_exit);
        }

        if (tempfile->active)
                die("BUG: create_tempfile called for active object");
        if (!tempfile->on_list) {
                /* Initialize *tempfile and add it to tempfile_list: */
                tempfile->fd = -1;
                tempfile->fp = NULL;
                tempfile->active = 0;
                tempfile->owner = 0;
                strbuf_init(&tempfile->filename, pathlen);
                tempfile->next = tempfile_list;
                tempfile_list = tempfile;
                tempfile->on_list = 1;
        } else if (tempfile->filename.len) {
                /* This shouldn't happen, but better safe than sorry. */
                die("BUG: create_tempfile called for improperly-reset object");
        }

        strbuf_add_absolute_path(&tempfile->filename, path);
        tempfile->fd = open(tempfile->filename.buf, O_RDWR | O_CREAT | O_EXCL, 0666);
        if (tempfile->fd < 0) {
                strbuf_reset(&tempfile->filename);
                return -1;
        }
        tempfile->owner = getpid();
        tempfile->active = 1;
        if (adjust_shared_perm(tempfile->filename.buf)) {
                int save_errno = errno;
                error("cannot fix permission bits on %s", tempfile->filename.buf);
                delete_tempfile(tempfile);
                errno = save_errno;
                return -1;
        }
        return tempfile->fd;
}

FILE *fdopen_tempfile(struct tempfile *tempfile, const char *mode)
{
        if (!tempfile->active)
                die("BUG: fdopen_tempfile() called for inactive object");
        if (tempfile->fp)
                die("BUG: fdopen_tempfile() called for open object");

        tempfile->fp = fdopen(tempfile->fd, mode);
        return tempfile->fp;
}

const char *get_tempfile_path(struct tempfile *tempfile)
{
        if (!tempfile->active)
                die("BUG: get_tempfile_path() called for inactive object");
        return tempfile->filename.buf;
}

int get_tempfile_fd(struct tempfile *tempfile)
{
        if (!tempfile->active)
                die("BUG: get_tempfile_fd() called for inactive object");
        return tempfile->fd;
}

FILE *get_tempfile_fp(struct tempfile *tempfile)
{
        if (!tempfile->active)
                die("BUG: get_tempfile_fp() called for inactive object");
        return tempfile->fp;
}

int close_tempfile(struct tempfile *tempfile)
{
        int fd = tempfile->fd;
        FILE *fp = tempfile->fp;
        int err;

        if (fd < 0)
                return 0;

        tempfile->fd = -1;
        if (fp) {
                tempfile->fp = NULL;

                /*
                 * Note: no short-circuiting here; we want to fclose()
                 * in any case!
                 */
                err = ferror(fp) | fclose(fp);
        } else {
                err = close(fd);
        }

        if (err) {
                int save_errno = errno;
                delete_tempfile(tempfile);
                errno = save_errno;
                return -1;
        }

        return 0;
}

int reopen_tempfile(struct tempfile *tempfile)
{
        if (0 <= tempfile->fd)
                die("BUG: reopen_tempfile called for an open object");
        if (!tempfile->active)
                die("BUG: reopen_tempfile called for an inactive object");
        tempfile->fd = open(tempfile->filename.buf, O_WRONLY);
        return tempfile->fd;
}

int rename_tempfile(struct tempfile *tempfile, const char *path)
{
        if (!tempfile->active)
                die("BUG: rename_tempfile called for inactive object");

        if (close_tempfile(tempfile))
                return -1;

        if (rename(tempfile->filename.buf, path)) {
                int save_errno = errno;
                delete_tempfile(tempfile);
                errno = save_errno;
                return -1;
        }

        tempfile->active = 0;
        strbuf_reset(&tempfile->filename);
        return 0;
}

void delete_tempfile(struct tempfile *tempfile)
{
        if (!tempfile->active)
                return;

        if (!close_tempfile(tempfile)) {
                unlink_or_warn(tempfile->filename.buf);
                tempfile->active = 0;
                strbuf_reset(&tempfile->filename);
        }
}