From b56e29cbdf58c1baa75e3340ff25c493329f2b9e Mon Sep 17 00:00:00 2001
From: Scott Worley <scottworley@scottworley.com>
Date: Wed, 13 Sep 2023 14:14:22 -0700
Subject: [PATCH] Hold a lock while appending

---
 tl-append.c | 46 ++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 40 insertions(+), 6 deletions(-)

diff --git a/tl-append.c b/tl-append.c
index 3e60a24..0650721 100644
--- a/tl-append.c
+++ b/tl-append.c
@@ -1,6 +1,7 @@
 #define _POSIX_C_SOURCE 199309L
 
 #include <errno.h>
+#include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -15,6 +16,7 @@ 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 {
@@ -45,9 +47,7 @@ static void read_line(conf_t conf, char *buf) {
   }
 }
 
-static void write_line(conf_t conf, const char *line) {
-  const char *now = encode_time(time(NULL));
-  FILE *f = fopen(FILENAME, "a");
+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)
@@ -56,8 +56,29 @@ static void write_line(conf_t conf, const char *line) {
     die("Error writing to output file");
   if (fputs(line, f) == EOF)
     die("Error writing to output file");
-  if (fclose(f) != 0)
-    die_err("Error closing 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");
@@ -66,11 +87,24 @@ static void write_line(conf_t conf, const char *line) {
   }
 }
 
+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)) {
-    write_line(conf, buf);
+    lock_and_write_line(conf, buf);
   }
   return 0;
 }
-- 
2.44.1