standards/189353: POSIX sem_unlink does not actually unlink the semaphore in the process context

[Joris Giovannangeli]

The following reply was made to PR standards/189353; it has been noted by GNATS.

From: Joris Giovannangeli <joris at giovannangeli.fr>
To: Konstantin Belousov <kostikbel at gmail.com>
Cc: freebsd-gnats-submit at FreeBSD.org
Subject: Re: standards/189353: POSIX sem_unlink does not actually unlink the
 semaphore in the process context
Date: Sun, 04 May 2014 14:46:43 +0200

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 I've found this issue while implementing semaphores for dragonflyBSD,
 and at first i came with the same solution than yours. But it won't
 work when you do the same in two processes : unlink will only remove
 the caching on the process calling unlink, not all of them. See
 attached test case.
 
 It looks like glibc is using inodes numbers and dev number for that,
 which, while not being strictly correct (inodes can be reused), seems
 to work fine on linux.
 
 For dragonfly, i'm currently trying to keep filedescriptors opened in
 the nameinfo structure. This adds quite a bit of overhead, but this
 way you can check link count with fstat during a sem_open to see if
 the semaphore file still exist, and return the cached mapping only if
 st_nlink > 0.
 
 This solution at least should be strictly correct, but the inode
 solution could be fine in practice.
 
 Regards,
 joris
 
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  filename="sem_test.c"
 
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
 #include <sys/types.h>
 #include <semaphore.h>
 
 #include <stdio.h>
 #include <unistd.h>
 
 #include <err.h>
 #include <errno.h>
 
 
 int main() {
 	pid_t pid;
 	sem_t *sem1, *sem2;
 	int error, saved_errno, status;
 	int v1, v2;
 	int pd[2];
 	char er[1];
 	char ok[1];
 	char buf[1];
 	size_t size;
 
 	*er = 'X';
 	*ok = 'O';
 
 	if (pipe(pd) == -1)
 		err(1, "Cannot create sync pipe");
 
 	sem1 = sem_open("/test-sem", O_CREAT | O_EXCL, S_IRWXU, 1);
 	if (sem1 == SEM_FAILED)
 		err(1,"Cannot create test /test-sem semaphore");
 
 	char c = getchar();
 	pid = fork();
 
 	if ( pid == -1)
 		err(1, "Unable to fork test process");
 
 	if (pid == 0) {
 
 		close(pd[0]);
 
 		error = sem_unlink("/test-sem");
 
 		if (error) {
 			perror("Cannot unlink semaphore /test-sem");
 			goto error;
 		}
 
 		sem2 = sem_open("/test-sem", O_CREAT | O_EXCL, S_IRWXU, 2);
 
 		if (sem2 == SEM_FAILED) {
 			perror("Cannot re-create semaphore /test-sem");
 			goto error;
 		}
 
 		write(pd[1], ok, 1);
 
 		error = wait(&status);
 		if (error && errno == EINTR)
 			goto error;
 
 		return(status);
 error:
 		write(pd[1], er, 1);
 		close(pd[1]);
 		error = wait(&status);
 		if (error && errno == EINTR)
 			goto error;
 
 		errx(1, "TEST UNRESOLVED");
 
 		return(1);
 
 	} else {
 
 		close(pd[1]);
 		size = read(pd[0], buf, 1);
 
 		if (size != 1)
 			err(2, "process 2 cannot wait for process 1");
 
 		if (*buf == *er)
 			errx(2, "Test error");
 		else {
 
 			sem2 = sem_open("/test-sem", 0);
 
 			if (sem2 == SEM_FAILED)
 				err(2, "process 2 failed reopeing semaphore");
 
 			// sem_unlink("/test-sem");
 
 			sem_getvalue(sem1, &v1);
 			sem_getvalue(sem2, &v2);
 			printf("v1 = %d and v2 = %d\n", v1, v2);
 			if (v1 == v2) {
 				errx(2, "*** TEST FAILED. NOT POSIX ***");
 			} else {
 				errx(0, "*** TEST SUCCESS. POSIX CONFORMANT ***");
 			}
 		}
 
 	}
 }
 
 
 
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