thread_pthread.c: Use a `fork_gen` to protect against fork instead of getpid()

[Feature #19443]

Until recently most libc would cache `getpid()` so this was a cheap check to make.

However as of glibc version 2.25 the PID cache is removed and calls to getpid() always
invoke the actual system call which significantly degrades the performance of existing applications.

The reason glibc removed the cache is that some libraries were bypassing fork(2)
by issuing system calls themselves, causing stale cache issues.

That isn't a concern for Ruby as bypassing MRI's primitive for forking would
render the VM unusable, so we can safely cache the PID.

process.c

@@ -511,13 +511,6 @@ clear_pid_cache(void)
     cached_pid = Qnil;
 }
 
-static inline void
-rb_process_atfork(void)
-{
-    clear_pid_cache();
-    rb_thread_atfork(); /* calls mjit_resume() */
-}
-
 /*
  *  call-seq:
  *     Process.pid   -> integer
@@ -1564,9 +1557,13 @@ before_fork_ruby(void)
 }
 
 static void
-after_fork_ruby(void)
+after_fork_ruby(rb_pid_t pid)
 {
     rb_threadptr_pending_interrupt_clear(GET_THREAD());
+    if (pid == 0) {
+        clear_pid_cache();
+        rb_thread_atfork();
+    }
     after_exec();
 }
 #endif
@@ -4073,11 +4070,10 @@ rb_fork_ruby2(struct rb_process_status *status)
             status->pid = pid;
             status->error = err;
         }
-        after_fork_ruby();
+        after_fork_ruby(pid);
         disable_child_handler_fork_parent(&old); /* yes, bad name */
 
         if (pid >= 0) { /* fork succeed */
-            if (pid == 0) rb_process_atfork();
             return pid;
         }
 
@@ -6849,8 +6845,7 @@ rb_daemon(int nochdir, int noclose)
 #ifdef HAVE_DAEMON
     before_fork_ruby();
     err = daemon(nochdir, noclose);
-    after_fork_ruby();
-    rb_process_atfork();
+    after_fork_ruby(0);
 #else
     int n;
 

thread_pthread.c

@@ -109,6 +109,8 @@ struct rb_internal_thread_event_hook {
 static rb_internal_thread_event_hook_t *rb_internal_thread_event_hooks = NULL;
 static pthread_rwlock_t rb_internal_thread_event_hooks_rw_lock = PTHREAD_RWLOCK_INITIALIZER;
 
+static rb_serial_t current_fork_gen = 1; /* We can't use GET_VM()->fork_gen */
+
 #define RB_INTERNAL_THREAD_HOOK(event) if (rb_internal_thread_event_hooks) { rb_thread_execute_hooks(event); }
 
 rb_internal_thread_event_hook_t *
@@ -203,7 +205,7 @@ enum rtimer_state {
 static const struct itimerspec zero;
 static struct {
     rb_atomic_t state_; /* rtimer_state */
-    rb_pid_t owner;
+    rb_serial_t fork_gen;
     timer_t timerid;
 } timer_posix = {
     /* .state = */ RTIMER_DEAD,
@@ -253,7 +255,7 @@ static void *timer_pthread_fn(void *);
 static struct {
     int low[2];
     rb_atomic_t armed; /* boolean */
-    rb_pid_t owner;
+    rb_serial_t fork_gen;
     pthread_t thid;
 } timer_pthread = {
     { -1, -1 },
@@ -268,7 +270,7 @@ static void threadptr_trap_interrupt(rb_thread_t *);
 static void ubf_wakeup_all_threads(void);
 static int ubf_threads_empty(void);
 
-#define TIMER_THREAD_CREATED_P() (signal_self_pipe.owner_process == getpid())
+#define TIMER_THREAD_CREATED_P() (signal_self_pipe.fork_gen == current_fork_gen)
 
 /* for testing, and in case we come across a platform w/o pipes: */
 #define BUSY_WAIT_SIGNALS (0)
@@ -533,6 +535,7 @@ static void thread_cache_reset(void);
 static void
 thread_sched_atfork(struct rb_thread_sched *sched)
 {
+    current_fork_gen++;
     thread_cache_reset();
     rb_thread_sched_init(sched);
     thread_sched_to_running(sched, GET_THREAD());
@@ -1560,7 +1563,7 @@ static struct {
     int ub_main[2]; /* unblock main thread from native_ppoll_sleep */
 
     /* volatile for signal handler use: */
-    volatile rb_pid_t owner_process;
+    volatile rb_serial_t fork_gen;
 } signal_self_pipe = {
     {-1, -1},
     {-1, -1},
@@ -1605,10 +1608,10 @@ rb_thread_wakeup_timer_thread_fd(int fd)
  * process could not react to the original signal in time.
  */
 static void
-ubf_timer_arm(rb_pid_t current) /* async signal safe */
+ubf_timer_arm(rb_serial_t fork_gen) /* async signal safe */
 {
 #if UBF_TIMER == UBF_TIMER_POSIX
-    if ((!current || timer_posix.owner == current) &&
+    if ((!fork_gen || timer_posix.fork_gen == fork_gen) &&
         timer_state_cas(RTIMER_DISARM, RTIMER_ARMING) == RTIMER_DISARM) {
         struct itimerspec it;
 
@@ -1642,7 +1645,7 @@ ubf_timer_arm(rb_pid_t current) /* async signal safe */
         }
     }
 #elif UBF_TIMER == UBF_TIMER_PTHREAD
-    if (!current || current == timer_pthread.owner) {
+    if (!fork_gen || fork_gen == timer_pthread.fork_gen) {
         if (ATOMIC_EXCHANGE(timer_pthread.armed, 1) == 0)
             rb_thread_wakeup_timer_thread_fd(timer_pthread.low[1]);
     }
@@ -1652,8 +1655,6 @@ ubf_timer_arm(rb_pid_t current) /* async signal safe */
 void
 rb_thread_wakeup_timer_thread(int sig)
 {
-    rb_pid_t current;
-
     /* non-sighandler path */
     if (sig <= 0) {
         rb_thread_wakeup_timer_thread_fd(signal_self_pipe.normal[1]);
@@ -1664,8 +1665,7 @@ rb_thread_wakeup_timer_thread(int sig)
     }
 
     /* must be safe inside sighandler, so no mutex */
-    current = getpid();
-    if (signal_self_pipe.owner_process == current) {
+    if (signal_self_pipe.fork_gen == current_fork_gen) {
         rb_thread_wakeup_timer_thread_fd(signal_self_pipe.normal[1]);
 
         /*
@@ -1690,7 +1690,7 @@ rb_thread_wakeup_timer_thread(int sig)
 
             if (ec) {
                 RUBY_VM_SET_TRAP_INTERRUPT(ec);
-                ubf_timer_arm(current);
+                ubf_timer_arm(current_fork_gen);
 
                 /* some ubfs can interrupt single-threaded process directly */
                 if (vm->ubf_async_safe && mth->unblock.func) {
@@ -1904,11 +1904,11 @@ ubf_timer_invalidate(void)
 }
 
 static void
-ubf_timer_pthread_create(rb_pid_t current)
+ubf_timer_pthread_create(rb_serial_t fork_gen)
 {
 #if UBF_TIMER == UBF_TIMER_PTHREAD
     int err;
-    if (timer_pthread.owner == current)
+    if (timer_pthread.fork_gen == fork_gen)
         return;
 
     if (setup_communication_pipe_internal(timer_pthread.low) < 0)
@@ -1916,7 +1916,7 @@ ubf_timer_pthread_create(rb_pid_t current)
 
     err = pthread_create(&timer_pthread.thid, 0, timer_pthread_fn, GET_VM());
     if (!err)
-        timer_pthread.owner = current;
+        timer_pthread.fork_gen = fork_gen;
     else
         rb_warn("pthread_create failed for timer: %s, signals racy",
                 strerror(err));
@@ -1924,7 +1924,7 @@ ubf_timer_pthread_create(rb_pid_t current)
 }
 
 static void
-ubf_timer_create(rb_pid_t current)
+ubf_timer_create(rb_serial_t fork_gen)
 {
 #if UBF_TIMER == UBF_TIMER_POSIX
 #  if defined(__sun)
@@ -1945,24 +1945,23 @@ ubf_timer_create(rb_pid_t current)
         if (prev != RTIMER_DEAD) {
             rb_bug("timer_posix was not dead: %u\n", (unsigned)prev);
         }
-        timer_posix.owner = current;
+        timer_posix.fork_gen = fork_gen;
     }
     else {
         rb_warn("timer_create failed: %s, signals racy", strerror(errno));
     }
 #endif
     if (UBF_TIMER == UBF_TIMER_PTHREAD)
-        ubf_timer_pthread_create(current);
+        ubf_timer_pthread_create(fork_gen);
 }
 
 static void
 rb_thread_create_timer_thread(void)
 {
     /* we only create the pipe, and lazy-spawn */
-    rb_pid_t current = getpid();
-    rb_pid_t owner = signal_self_pipe.owner_process;
+    rb_serial_t fork_gen = signal_self_pipe.fork_gen;
 
-    if (owner && owner != current) {
+    if (fork_gen && fork_gen != current_fork_gen) {
         CLOSE_INVALIDATE_PAIR(signal_self_pipe.normal);
         CLOSE_INVALIDATE_PAIR(signal_self_pipe.ub_main);
         ubf_timer_invalidate();
@@ -1971,11 +1970,11 @@ rb_thread_create_timer_thread(void)
     if (setup_communication_pipe_internal(signal_self_pipe.normal) < 0) return;
     if (setup_communication_pipe_internal(signal_self_pipe.ub_main) < 0) return;
 
-    ubf_timer_create(current);
-    if (owner != current) {
+    ubf_timer_create(current_fork_gen);
+    if (fork_gen != current_fork_gen) {
         /* validate pipe on this process */
         sigwait_th = THREAD_INVALID;
-        signal_self_pipe.owner_process = current;
+        signal_self_pipe.fork_gen = current_fork_gen;
     }
 }
 
@@ -1985,7 +1984,7 @@ ubf_timer_disarm(void)
 #if UBF_TIMER == UBF_TIMER_POSIX
     rb_atomic_t prev;
 
-    if (timer_posix.owner && timer_posix.owner != getpid()) return;
+    if (timer_posix.fork_gen && timer_posix.fork_gen != current_fork_gen) return;
     prev = timer_state_cas(RTIMER_ARMED, RTIMER_DISARM);
     switch (prev) {
       case RTIMER_DISARM: return; /* likely */
@@ -2018,7 +2017,7 @@ static void
 ubf_timer_destroy(void)
 {
 #if UBF_TIMER == UBF_TIMER_POSIX
-    if (timer_posix.owner == getpid()) {
+    if (timer_posix.fork_gen == current_fork_gen) {
         rb_atomic_t expect = RTIMER_DISARM;
         size_t i, max = 10000000;
 
@@ -2055,7 +2054,7 @@ done:
 #elif UBF_TIMER == UBF_TIMER_PTHREAD
     int err;
 
-    timer_pthread.owner = 0;
+    timer_pthread.fork_gen = 0;
     ubf_timer_disarm();
     rb_thread_wakeup_timer_thread_fd(timer_pthread.low[1]);
     err = pthread_join(timer_pthread.thid, 0);
@@ -2136,15 +2135,15 @@ rb_reserved_fd_p(int fd)
 
 #if UBF_TIMER == UBF_TIMER_PTHREAD
     if (fd == timer_pthread.low[0] || fd == timer_pthread.low[1])
-        goto check_pid;
+        goto check_fork_gen;
 #endif
     if (fd == signal_self_pipe.normal[0] || fd == signal_self_pipe.normal[1])
-        goto check_pid;
+        goto check_fork_gen;
     if (fd == signal_self_pipe.ub_main[0] || fd == signal_self_pipe.ub_main[1])
-        goto check_pid;
+        goto check_fork_gen;
     return 0;
-check_pid:
-    if (signal_self_pipe.owner_process == getpid()) /* async-signal-safe */
+check_fork_gen:
+    if (signal_self_pipe.fork_gen == current_fork_gen) /* async-signal-safe */
         return 1;
     return 0;
 }
@@ -2159,7 +2158,7 @@ int
 rb_sigwait_fd_get(const rb_thread_t *th)
 {
     if (signal_self_pipe.normal[0] >= 0) {
-        VM_ASSERT(signal_self_pipe.owner_process == getpid());
+        VM_ASSERT(signal_self_pipe.fork_gen == current_fork_gen);
         /*
          * no need to keep firing the timer if any thread is sleeping
          * on the signal self-pipe