LibC+LibPthread: Implement pthread_atfork()

This required a bit of rearchitecture, as pthread_atfork() required a
mutex, and duplicating a mutex impl for it was silly.
As such, this patch moves some standalone bits of pthread into LibC and
uses those to implement atfork().
It should be noted that for programs that don't use atfork(), this
mechanism only costs two atomic loads (as opposed to the normal mutex
lock+unlock) :^)

Userland/Libraries/LibC/CMakeLists.txt

@@ -17,6 +17,7 @@ set(LIBC_SOURCES
     mntent.cpp
     netdb.cpp
     poll.cpp
+    pthread_integration.cpp
     pwd.cpp
     qsort.cpp
     scanf.cpp

Userland/Libraries/LibC/bits/pthread_integration.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+void __pthread_fork_prepare(void);
+void __pthread_fork_child(void);
+void __pthread_fork_parent(void);
+void __pthread_fork_atfork_register_prepare(void (*)(void));
+void __pthread_fork_atfork_register_parent(void (*)(void));
+void __pthread_fork_atfork_register_child(void (*)(void));
+
+int __pthread_mutex_lock(void*);
+int __pthread_mutex_unlock(void*);
+int __pthread_mutex_init(void*, const void*);
+
+int __pthread_self();
+
+#define __PTHREAD_MUTEX_NORMAL 0
+#define __PTHREAD_MUTEX_RECURSIVE 1
+#define __PTHREAD_MUTEX_INITIALIZER     \
+    {                                   \
+        0, 0, 0, __PTHREAD_MUTEX_NORMAL \
+    }
+
+__END_DECLS

Userland/Libraries/LibC/pthread_integration.cpp

@@ -0,0 +1,156 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Atomic.h>
+#include <AK/NeverDestroyed.h>
+#include <AK/Types.h>
+#include <AK/Vector.h>
+#include <bits/pthread_integration.h>
+#include <sched.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+namespace {
+
+// Most programs don't need this, no need to incur an extra mutex lock/unlock on them
+static Atomic<bool> g_did_touch_atfork { false };
+static pthread_mutex_t g_atfork_list_mutex __PTHREAD_MUTEX_INITIALIZER;
+static NeverDestroyed<Vector<void (*)(void), 4>> g_atfork_prepare_list;
+static NeverDestroyed<Vector<void (*)(void), 4>> g_atfork_child_list;
+static NeverDestroyed<Vector<void (*)(void), 4>> g_atfork_parent_list;
+
+}
+
+extern "C" {
+void __pthread_fork_prepare(void)
+{
+    if (!g_did_touch_atfork.load())
+        return;
+
+    __pthread_mutex_lock(&g_atfork_list_mutex);
+    for (auto entry : g_atfork_prepare_list.get())
+        entry();
+    __pthread_mutex_unlock(&g_atfork_list_mutex);
+}
+
+void __pthread_fork_child(void)
+{
+    if (!g_did_touch_atfork.load())
+        return;
+
+    __pthread_mutex_lock(&g_atfork_list_mutex);
+    for (auto entry : g_atfork_child_list.get())
+        entry();
+    __pthread_mutex_unlock(&g_atfork_list_mutex);
+}
+
+void __pthread_fork_parent(void)
+{
+    if (!g_did_touch_atfork.load())
+        return;
+
+    __pthread_mutex_lock(&g_atfork_list_mutex);
+    for (auto entry : g_atfork_parent_list.get())
+        entry();
+    __pthread_mutex_unlock(&g_atfork_list_mutex);
+}
+
+void __pthread_fork_atfork_register_prepare(void (*func)(void))
+{
+    g_did_touch_atfork.store(true);
+
+    __pthread_mutex_lock(&g_atfork_list_mutex);
+    g_atfork_prepare_list->append(func);
+    __pthread_mutex_unlock(&g_atfork_list_mutex);
+}
+
+void __pthread_fork_atfork_register_parent(void (*func)(void))
+{
+    g_did_touch_atfork.store(true);
+
+    __pthread_mutex_lock(&g_atfork_list_mutex);
+    g_atfork_parent_list->append(func);
+    __pthread_mutex_unlock(&g_atfork_list_mutex);
+}
+
+void __pthread_fork_atfork_register_child(void (*func)(void))
+{
+    g_did_touch_atfork.store(true);
+
+    __pthread_mutex_lock(&g_atfork_list_mutex);
+    g_atfork_child_list->append(func);
+    __pthread_mutex_unlock(&g_atfork_list_mutex);
+}
+
+int __pthread_self()
+{
+    return gettid();
+}
+
+int __pthread_mutex_lock(void* mutexp)
+{
+    auto* mutex = reinterpret_cast<pthread_mutex_t*>(mutexp);
+    auto& atomic = reinterpret_cast<Atomic<u32>&>(mutex->lock);
+    pthread_t this_thread = __pthread_self();
+    for (;;) {
+        u32 expected = false;
+        if (!atomic.compare_exchange_strong(expected, true, AK::memory_order_acq_rel)) {
+            if (mutex->type == __PTHREAD_MUTEX_RECURSIVE && mutex->owner == this_thread) {
+                mutex->level++;
+                return 0;
+            }
+            sched_yield();
+            continue;
+        }
+        mutex->owner = this_thread;
+        mutex->level = 0;
+        return 0;
+    }
+}
+
+int __pthread_mutex_unlock(void* mutexp)
+{
+    auto* mutex = reinterpret_cast<pthread_mutex_t*>(mutexp);
+    if (mutex->type == __PTHREAD_MUTEX_RECURSIVE && mutex->level > 0) {
+        mutex->level--;
+        return 0;
+    }
+    mutex->owner = 0;
+    mutex->lock = 0;
+    return 0;
+}
+
+int __pthread_mutex_init(void* mutexp, const void* attrp)
+{
+    auto* mutex = reinterpret_cast<pthread_mutex_t*>(mutexp);
+    auto* attributes = reinterpret_cast<const pthread_mutexattr_t*>(attrp);
+    mutex->lock = 0;
+    mutex->owner = 0;
+    mutex->level = 0;
+    mutex->type = attributes ? attributes->type : __PTHREAD_MUTEX_NORMAL;
+    return 0;
+}
+}

Userland/Libraries/LibC/unistd.cpp

@@ -29,6 +29,7 @@
 #include <AK/Vector.h>
 #include <alloca.h>
 #include <assert.h>
+#include <bits/pthread_integration.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <getopt.h>
@@ -75,10 +76,15 @@ int fchown(int fd, uid_t uid, gid_t gid)
 
 pid_t fork()
 {
+    __pthread_fork_prepare();
+
     int rc = syscall(SC_fork);
     if (rc == 0) {
         s_cached_tid = 0;
         s_cached_pid = 0;
+        __pthread_fork_child();
+    } else if (rc != -1) {
+        __pthread_fork_parent();
     }
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }

Userland/Libraries/LibPthread/pthread.cpp

@@ -30,6 +30,7 @@
 #include <AK/StdLibExtras.h>
 #include <Kernel/API/Syscall.h>
 #include <LibSystem/syscall.h>
+#include <bits/pthread_integration.h>
 #include <limits.h>
 #include <pthread.h>
 #include <serenity.h>
@@ -110,7 +111,7 @@ static int create_thread(pthread_t* thread, void* (*entry)(void*), void* argumen
 
 int pthread_self()
 {
-    return gettid();
+    return __pthread_self();
 }
 
 int pthread_create(pthread_t* thread, pthread_attr_t* attributes, void* (*start_routine)(void*), void* argument_to_start_routine)
@@ -172,11 +173,7 @@ int pthread_sigmask(int how, const sigset_t* set, sigset_t* old_set)
 
 int pthread_mutex_init(pthread_mutex_t* mutex, const pthread_mutexattr_t* attributes)
 {
-    mutex->lock = 0;
-    mutex->owner = 0;
-    mutex->level = 0;
-    mutex->type = attributes ? attributes->type : PTHREAD_MUTEX_NORMAL;
-    return 0;
+    return __pthread_mutex_init(mutex, attributes);
 }
 
 int pthread_mutex_destroy(pthread_mutex_t*)
@@ -186,22 +183,7 @@ int pthread_mutex_destroy(pthread_mutex_t*)
 
 int pthread_mutex_lock(pthread_mutex_t* mutex)
 {
-    auto& atomic = reinterpret_cast<Atomic<u32>&>(mutex->lock);
-    pthread_t this_thread = pthread_self();
-    for (;;) {
-        u32 expected = false;
-        if (!atomic.compare_exchange_strong(expected, true, AK::memory_order_acq_rel)) {
-            if (mutex->type == PTHREAD_MUTEX_RECURSIVE && mutex->owner == this_thread) {
-                mutex->level++;
-                return 0;
-            }
-            sched_yield();
-            continue;
-        }
-        mutex->owner = this_thread;
-        mutex->level = 0;
-        return 0;
-    }
+    return __pthread_mutex_lock(mutex);
 }
 
 int pthread_mutex_trylock(pthread_mutex_t* mutex)
@@ -222,13 +204,7 @@ int pthread_mutex_trylock(pthread_mutex_t* mutex)
 
 int pthread_mutex_unlock(pthread_mutex_t* mutex)
 {
-    if (mutex->type == PTHREAD_MUTEX_RECURSIVE && mutex->level > 0) {
-        mutex->level--;
-        return 0;
-    }
-    mutex->owner = 0;
-    mutex->lock = 0;
-    return 0;
+    return __pthread_mutex_unlock(mutex);
 }
 
 int pthread_mutexattr_init(pthread_mutexattr_t* attr)
@@ -922,9 +898,12 @@ int pthread_rwlockattr_setpshared(pthread_rwlockattr_t*, int)
     ASSERT_NOT_REACHED();
 }
 
-int pthread_atfork(void (*)(void), void (*)(void), void (*)(void))
+int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void))
 {
-    ASSERT_NOT_REACHED();
+    __pthread_fork_atfork_register_prepare(prepare);
+    __pthread_fork_atfork_register_parent(parent);
+    __pthread_fork_atfork_register_child(child);
+    return 0;
 }
 
 } // extern "C"

Userland/Libraries/LibPthread/pthread.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <bits/pthread_integration.h>
 #include <sched.h>
 #include <stdint.h>
 #include <sys/cdefs.h>
@@ -75,13 +76,11 @@ int pthread_setspecific(pthread_key_t key, const void* value);
 int pthread_getschedparam(pthread_t thread, int* policy, struct sched_param* param);
 int pthread_setschedparam(pthread_t thread, int policy, const struct sched_param* param);
 
-#define PTHREAD_MUTEX_NORMAL 0
-#define PTHREAD_MUTEX_RECURSIVE 1
+#define PTHREAD_MUTEX_NORMAL __PTHREAD_MUTEX_NORMAL
+#define PTHREAD_MUTEX_RECURSIVE __PTHREAD_MUTEX_RECURSIVE
 #define PTHREAD_MUTEX_DEFAULT PTHREAD_MUTEX_NORMAL
-#define PTHREAD_MUTEX_INITIALIZER      \
-    {                                  \
-        0, 0, 0, PTHREAD_MUTEX_DEFAULT \
-    }
+#define PTHREAD_MUTEX_INITIALIZER __PTHREAD_MUTEX_INITIALIZER
+
 #define PTHREAD_COND_INITIALIZER     \
     {                                \
         0, 0, CLOCK_MONOTONIC_COARSE \