Hold a lock while appending

[tl-append] / tl-append.c

#define _POSIX_C_SOURCE 199309L

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>

#include "common.h"

const int BUF_SIZE = 1024;

const char PROMPT[] = "\33[H" /* Move cursor 'home' */
                      "\33[J" /* Clear screen */
                      "> ";
const char ACKNOWLEDGE[] = "[OK]";
const char WAITING[] = "[Waiting on lock...]\n";
const struct timespec ACKNOWLEDGE_DELAY = {0, 300000000};

typedef struct {
  int interactive;
} conf_t;

conf_t parse_command_line(int argc, char *argv[]) {
  conf_t conf = {0};

  if (argc == 2 && strcmp(argv[1], "-i") == 0 && isatty(2))
    conf.interactive = 1;

  return conf;
}

static void read_line(conf_t conf, char *buf) {
  if (conf.interactive)
    if (fputs(PROMPT, stderr) == EOF)
      die("I/O error writing prompt");
  if (fgets(buf, BUF_SIZE, stdin) == NULL) {
    if (ferror(stdin))
      die("I/O error reading line");
    if (feof(stdin)) {
      buf[0] = '\0'; /* Unclear if fgets does this already */
      return;
    }
    die("Unexpected error reading line");
  }
}

static void write_line(const char *now, FILE *f, const char *line) {
  if (f == NULL)
    die_err("Error opening output file");
  if (fputs(now, f) == EOF)
    die("Error writing to output file");
  if (fputc(' ', f) == EOF)
    die("Error writing to output file");
  if (fputs(line, f) == EOF)
    die("Error writing to output file");
}

static void take_lock(FILE *f) {
  struct flock lock;
  lock.l_type = F_WRLCK;
  lock.l_whence = SEEK_SET;
  lock.l_start = 0;
  lock.l_len = 0;
  int fd = fileno(f);
  if (fd == -1)
    die_err("Couldn't get file descriptor for locking");
  if (fcntl(fd, F_SETLK, &lock) == 0)
    return;
  if (errno != EACCES && errno != EAGAIN)
    die_err("Couldn't take lock");
  if (fputs(WAITING, stderr) == EOF)
    die("Error writing waiting message");
  if (fcntl(fd, F_SETLKW, &lock) == 0)
    return;
  die_err("Couldn't take lock");
}

static void write_acknowledgment(conf_t conf) {
  if (conf.interactive) {
    if (fputs(ACKNOWLEDGE, stderr) == EOF)
      die("Error writing acknowledgment");
    if (nanosleep(&ACKNOWLEDGE_DELAY, NULL) == -1 && errno != EINTR)
      die_err("Error sleeping");
  }
}

static void lock_and_write_line(conf_t conf, const char *line) {
  const char *now = encode_time(time(NULL));
  FILE *f = fopen(FILENAME, "a");

  take_lock(f);
  write_line(now, f, line);

  if (fclose(f) != 0)
    die_err("Error closing output file");

  write_acknowledgment(conf);
}

int main(int argc, char *argv[]) {
  conf_t conf = parse_command_line(argc, argv);
  char buf[BUF_SIZE];
  for (read_line(conf, buf); buf[0]; read_line(conf, buf)) {
    lock_and_write_line(conf, buf);
  }
  return 0;
}