Strings read from FIFO teleport to other lines - c++

Question

My code reads lines from a fifo, constructs a functor, and queues it with a Boost thread pool. I was having a problem with occasional work items failing, but when I added logging the problem disappeared.
I've worked it down to the location of where a std::ofstream is opened, but I can't figure out why this is true. By moving that line I was able to get the problems to reoccur. Sometimes part of a line goes missing and reappears somewhere else (either preppended to or overwriting another line). This doesn't appear to be random. Given the same input the error always occurs in the same place, but if the input lines are reordered the error will occur in a different location.
As an example
# This line should read
# /dev/shm/test/bluetooth-active-symbolic.symbolic.png.csv.plot.png
# But it is missing the leading "/dev/"
shm/test/bluetooth-active-symbolic.symbolic.png.csv.plot.png
# This line should read
# POOLCMD_CLOSE
# But the missing text reappeared here
/dev/POOLCMD_CLOSE
Code
Where we fork to the background
switch (fork()) {
case -1:
std::cerr << "Error forking\n";
return 1;
case 0:
close(0);
close(1);
close(2);
dup2(stdout_file, fileno(stdout));
dup2(stdout_file, fileno(stderr));
setsid();
went_background = true;
// If this ofstream line is moved elsewhere, things break
log_file = std::ofstream("poolcmd-log.txt");
break;
default:
exit(0);
}
Where we read from the fifo
read_threads.push_back(
decltype(read_threads)::value_type(new std::thread([&]() {
char* line = 0;
size_t n = 0;
while (true) {
auto read = getline(&line, &n, fifo);
locked_print(log_file, "read: ");
locked_print(log_file, line);
if (read >= 0) {
std::string tmp(line, read - 1);
if (tmp == "POOLCMD_CLOSE") {
locked_print(log_file, "Understood POOLCMD_CLOSE\n");
break;
}
tp.add(Executer(cmd, {tmp}, quiet));
}
else {
locked_print(log_file, "read < 0\n");
break;
}
}
free(line);
fclose(fifo);
remove(fifo_name.c_str());
})));