kern/59578: open() hangs indefinitely on FIFO with frequent signal interruption

To: kern-bug-people%netbsd.org@localhost,gnats-admin%netbsd.org@localhost,netbsd-bugs%netbsd.org@localhost
Subject: kern/59578: open() hangs indefinitely on FIFO with frequent signal interruption
From: furkanonder%protonmail.com@localhost
Date: Wed, 6 Aug 2025 19:00:00 +0000 (UTC)
>Number:         59578
>Category:       kern
>Synopsis:       open() hangs indefinitely on FIFO with frequent signal interruption
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Aug 06 19:00:00 +0000 2025
>Originator:     Furkan Onder
>Release:        NetBSD 10.0
>Organization:
Cpython
>Environment:
NetBSD home.localhost 10.0 NetBSD 10.0 (GENERIC) #0: Thu Mar 28 08:33:33 UTC 2024  mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64
>Description:
This issue was exposed through the CPython test suite, specifically the `test_eintr` module's `SocketEINTRTest.test_os_open` test,
which hangs indefinitely on NetBSD 10.0 (see https://github.com/python/cpython/issues/137397).

The underlying problem is that when attempting to open a FIFO for writing (O_WRONLY) while the process receives frequent signals,
the open() system call gets repeatedly interrupted with EINTR and never completes successfully, even when a reader process is
available on the other end.

This creates an infinite loop where the process hangs indefinitely, continuously retrying the open() call which always returns EINTR.
>How-To-Repeat:
Save the following C program as `reproducer.c`

```
#include <errno.h>
#include <fcntl.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/wait.h>
#include <time.h>
#include <unistd.h>

volatile sig_atomic_t signal_count = 0;

/**
 * Signal handler for SIGALRM.
 */
void handle_signal(int sig) {
    (void)sig;   // Suppress unused parameter warning
    signal_count++;
}

/**
 * Retry open() if it's interrupted by a signal (EINTR).
 */
int safe_open(const char *path, int flags) {
    int fd;
    while ((fd = open(path, flags)) < 0) {
        if (errno != EINTR) {
            perror("open");
            exit(EXIT_FAILURE);
        }
        write(STDOUT_FILENO, ".", 1);
    }
    return fd;
}

/**
 * Retry close() if it's interrupted by a signal (EINTR).
 */
int safe_close(int fd) {
    int ret;
    while ((ret = close(fd)) < 0) {
        if (errno != EINTR) {
            perror("close");
            return ret;
        }
        write(STDOUT_FILENO, "C", 1);
    }
    return ret;
}

/**
 * Sleep for a specified number of milliseconds.
 */
void sleep_ms(long ms) {
    struct timespec ts;
    ts.tv_sec = ms / 1000;
    ts.tv_nsec = (ms % 1000) * 1000000L;
    nanosleep(&ts, NULL);
}

/**
 * Set up a timer to send SIGALRM every 10 milliseconds.
 */
void setup_timer(void) {
    struct sigaction sa = {0};
    sa.sa_handler = handle_signal;
    sigaction(SIGALRM, &sa, NULL);

    struct itimerval timer = {
        .it_value = {0, 10000},      // Start after 10ms
        .it_interval = {0, 10000}    // Repeat every 10ms
    };

    if (setitimer(ITIMER_REAL, &timer, NULL) < 0) {
        perror("setitimer");
        exit(EXIT_FAILURE);
    }
}

int main() {
    printf("EINTR test - Ctrl+C to stop\n");
    setup_timer();

    for (int i = 1; i <= 50; ++i) {
        char fifo[64];
        snprintf(fifo, sizeof(fifo), "/tmp/test_fifo_%d", i);
        unlink(fifo);
        if (mkfifo(fifo, 0666) < 0) {
            perror("mkfifo");
            exit(EXIT_FAILURE);
        }

        pid_t pid = fork();
        if (pid < 0) {
            perror("fork");
            exit(EXIT_FAILURE);
        }
        if (pid == 0) {
            // Child opens FIFO for reading
            sleep_ms(50);  // 50ms delay to let parent open writer
            int fd = safe_open(fifo, O_RDONLY);
            safe_close(fd);
            exit(EXIT_SUCCESS);
        }
        else {
            // Parent opens FIFO for writing
            int fd = safe_open(fifo, O_WRONLY);
            safe_close(fd);
            wait(NULL);
            unlink(fifo);
            printf("Loop %d OK (signals: %d)\n", i, signal_count);
            signal_count = 0;
        }

        sleep_ms(1);  // Small pause before next iteration
    }

    printf("Test complete.\n");
    return EXIT_SUCCESS;
}
```

Compile and run:
```
gcc -o reproducer reproducer.c
./reproducer
```

The program should complete successfully with output like:
```
EINTR test - Ctrl+C to stop
.....Loop 1 OK (signals: 5)
.....Loop 2 OK (signals: 5)
.....Loop 3 OK (signals: 5)
.....Loop 4 OK (signals: 5)
.....Loop 5 OK (signals: 5)
.....Loop 6 OK (signals: 5)
.....Loop 7 OK (signals: 6)
.....Loop 8 OK (signals: 5)
.....Loop 9 OK (signals: 5)
.....Loop 10 OK (signals: 5)
.....Loop 11 OK (signals: 5)
.....Loop 12 OK (signals: 5)
.....Loop 13 OK (signals: 5)
.....Loop 14 OK (signals: 6)
.....Loop 15 OK (signals: 5)
.....Loop 16 OK (signals: 5)
.....Loop 17 OK (signals: 5)
.....Loop 18 OK (signals: 5)
.....Loop 19 OK (signals: 5)
.....Loop 20 OK (signals: 5)
.....Loop 21 OK (signals: 6)
.....Loop 22 OK (signals: 5)
.....Loop 23 OK (signals: 5)
.....Loop 24 OK (signals: 5)
.....Loop 25 OK (signals: 5)
.....Loop 26 OK (signals: 5)
.....Loop 27 OK (signals: 5)
.....Loop 28 OK (signals: 6)
.....Loop 29 OK (signals: 5)
.....Loop 30 OK (signals: 5)
.....Loop 31 OK (signals: 5)
.....Loop 32 OK (signals: 5)
.....Loop 33 OK (signals: 5)
.....Loop 34 OK (signals: 5)
.....Loop 35 OK (signals: 6)
.....Loop 36 OK (signals: 5)
.....Loop 37 OK (signals: 5)
.....Loop 38 OK (signals: 5)
.....Loop 39 OK (signals: 5)
.....Loop 40 OK (signals: 5)
.....Loop 41 OK (signals: 5)
.....Loop 42 OK (signals: 6)
.....Loop 43 OK (signals: 5)
.....Loop 44 OK (signals: 5)
.....Loop 45 OK (signals: 5)
.....Loop 46 OK (signals: 5)
.....Loop 47 OK (signals: 5)
.....Loop 48 OK (signals: 5)
.....Loop 49 OK (signals: 6)
.....Loop 50 OK (signals: 5)
Test complete.
```

The program hangs indefinitely:
```
EINTR test - Ctrl+C to stop
...................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................^C
```

```
ktrace -f trace.out ./reproducer
# After hanging, press Ctrl+C and examine trace
kdump -f trace.out | tail -50
```

Output:
```
 17368  17368 reproducer CALL  setcontext(0x7f7fff444000)
 17368  17368 reproducer RET   setcontext JUSTRETURN
 17368  17368 reproducer CALL  write(1,0x401173,1)
 17368  17368 reproducer GIO   fd 1 wrote 1 bytes
       "."
 17368  17368 reproducer RET   write 1
 17368  17368 reproducer CALL  open(0x7f7fff4443d0,1,1)
 17368  17368 reproducer NAMI  "/tmp/test_fifo_1"
 17368  17368 reproducer RET   open -1 errno 4 Interrupted system call
 17368  17368 reproducer PSIG  SIGALRM caught handler=0x400dba mask=(): code=SI_TIMER sent by pid=0, uid=0 with sigval 0x0)
 17368  17368 reproducer CALL  setcontext(0x7f7fff444000)
 17368  17368 reproducer RET   setcontext JUSTRETURN
 17368  17368 reproducer CALL  write(1,0x401173,1)
 17368  17368 reproducer GIO   fd 1 wrote 1 bytes
       "."
 17368  17368 reproducer RET   write 1
 17368  17368 reproducer CALL  open(0x7f7fff4443d0,1,1)
 17368  17368 reproducer NAMI  "/tmp/test_fifo_1"
 17368  17368 reproducer RET   open -1 errno 4 Interrupted system call
 17368  17368 reproducer PSIG  SIGALRM caught handler=0x400dba mask=(): code=SI_TIMER sent by pid=0, uid=0 with sigval 0x0)
 17368  17368 reproducer CALL  setcontext(0x7f7fff444000)
 17368  17368 reproducer RET   setcontext JUSTRETURN
 17368  17368 reproducer CALL  write(1,0x401173,1)
 17368  17368 reproducer GIO   fd 1 wrote 1 bytes
       "."
 17368  17368 reproducer RET   write 1
 17368  17368 reproducer CALL  open(0x7f7fff4443d0,1,1)
 17368  17368 reproducer NAMI  "/tmp/test_fifo_1"
 17368  17368 reproducer RET   open -1 errno 4 Interrupted system call
 17368  17368 reproducer PSIG  SIGALRM caught handler=0x400dba mask=(): code=SI_TIMER sent by pid=0, uid=0 with sigval 0x0)
 17368  17368 reproducer CALL  setcontext(0x7f7fff444000)
 17368  17368 reproducer RET   setcontext JUSTRETURN
 17368  17368 reproducer CALL  write(1,0x401173,1)
 17368  17368 reproducer GIO   fd 1 wrote 1 bytes
       "."
 17368  17368 reproducer RET   write 1
 17368  17368 reproducer CALL  open(0x7f7fff4443d0,1,1)
 17368  17368 reproducer NAMI  "/tmp/test_fifo_1"
 17368  17368 reproducer RET   open -1 errno 4 Interrupted system call
 17368  17368 reproducer PSIG  SIGALRM caught handler=0x400dba mask=(): code=SI_TIMER sent by pid=0, uid=0 with sigval 0x0)
 17368  17368 reproducer CALL  setcontext(0x7f7fff444000)
 17368  17368 reproducer RET   setcontext JUSTRETURN
 17368  17368 reproducer CALL  write(1,0x401173,1)
 17368  17368 reproducer GIO   fd 1 wrote 1 bytes
       "."
 17368  17368 reproducer RET   write 1
 17368  17368 reproducer CALL  open(0x7f7fff4443d0,1,1)
 17368  17368 reproducer NAMI  "/tmp/test_fifo_1"
 17368  17368 reproducer RET   open RESTART
 17368  17368 reproducer PSIG  SIGINT SIG_DFL: code=SI_NOINFO
```
>Fix:
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