LibC: Implement pthread_cancel

Kernel/Thread.cpp

@@ -871,6 +871,7 @@ static DefaultSignalAction default_signal_action(u8 signal)
     case SIGIO:
     case SIGPROF:
     case SIGTERM:
+    case SIGCANCEL:
         return DefaultSignalAction::Terminate;
     case SIGCHLD:
     case SIGURG:

Userland/Libraries/LibC/bits/pthread_cancel.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <sys/cdefs.h>
+#include <sys/types.h>
+
+__BEGIN_DECLS
+
+// This is our hook for cancellation points.
+#ifdef _DYNAMIC_LOADER
+inline void __pthread_maybe_cancel(void)
+{
+}
+#else
+void __pthread_maybe_cancel(void);
+#endif
+
+__END_DECLS

Userland/Libraries/LibC/pthread.cpp

@@ -12,6 +12,7 @@
 #include <AK/StdLibExtras.h>
 #include <Kernel/API/Syscall.h>
 #include <LibSystem/syscall.h>
+#include <bits/pthread_cancel.h>
 #include <bits/pthread_integration.h>
 #include <errno.h>
 #include <limits.h>
@@ -51,6 +52,8 @@ struct CleanupHandler {
 
 static thread_local SinglyLinkedList<CleanupHandler> cleanup_handlers;
 
+static __thread bool pending_cancellation = false;
+
 extern "C" {
 
 [[noreturn]] static void exit_thread(void* code, void* stack_location, size_t stack_size)
@@ -154,6 +157,21 @@ void pthread_exit(void* value_ptr)
     pthread_exit_without_cleanup_handlers(value_ptr);
 }
 
+void __pthread_maybe_cancel()
+{
+    // Check if we have cancellations enabled.
+    if (s_thread_cancel_state != PTHREAD_CANCEL_ENABLE)
+        return;
+
+    // Check if a cancellation request is pending.
+    if (!pending_cancellation)
+        return;
+
+    // Exit the thread via `pthread_exit`. This handles passing the
+    // return value and calling the cleanup handlers for us.
+    pthread_exit(PTHREAD_CANCELED);
+}
+
 // https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_cleanup_push.html
 void pthread_cleanup_push(void (*routine)(void*), void* arg)
 {
@@ -481,11 +499,35 @@ int pthread_setschedparam([[maybe_unused]] pthread_t thread, [[maybe_unused]] in
     return 0;
 }
 
+static void pthread_cancel_signal_handler(int signal)
+{
+    // SIGCANCEL is a custom signal that is beyond the usual range of signal numbers.
+    // Let's make sure we know about it in case we still end up in here, but the signal
+    // number is being mangled.
+    VERIFY(signal == SIGCANCEL);
+
+    // Note: We don't handle PTHREAD_CANCEL_ASYNCHRONOUS any different from PTHREAD_CANCEL_DEFERRED,
+    // since ASYNCHRONOUS just means that the thread can be cancelled at any time (instead of just
+    // at the next cancellation point) and it seems to be generally discouraged to use it at all.
+    pending_cancellation = true;
+}
+
 // https://pubs.opengroup.org/onlinepubs/009695399/functions/pthread_cancel.html
 // NOTE: libgcc expects this function to exist in libpthread, even if it is not implemented.
-int pthread_cancel(pthread_t)
+int pthread_cancel(pthread_t thread)
 {
-    TODO();
+    // Set up our signal handler, which listens on SIGCANCEL and flips the cancellation indicator.
+    // Note that signal handlers are shared across the whole process, so we can just set that up at any time.
+    static bool set_up_cancel_handler = false;
+
+    if (!set_up_cancel_handler) {
+        struct sigaction act = {};
+        act.sa_handler = pthread_cancel_signal_handler;
+        sigaction(SIGCANCEL, &act, nullptr);
+        set_up_cancel_handler = true;
+    }
+
+    return pthread_kill(thread, SIGCANCEL);
 }
 
 int pthread_setname_np(pthread_t thread, char const* name)

Userland/Libraries/LibC/pthread.h

@@ -33,7 +33,7 @@ int pthread_attr_destroy(pthread_attr_t*);
 #define PTHREAD_CREATE_JOINABLE 0
 #define PTHREAD_CREATE_DETACHED 1
 
-#define PTHREAD_CANCELED (-1)
+#define PTHREAD_CANCELED ((void*)-1)
 
 int pthread_attr_getdetachstate(pthread_attr_t const*, int*);
 int pthread_attr_setdetachstate(pthread_attr_t*, int);

Userland/Libraries/LibC/signal_numbers.h

@@ -38,4 +38,5 @@
 #define SIGIO 29
 #define SIGINFO 30
 #define SIGSYS 31
-#define NSIG 32
+#define SIGCANCEL 32
+#define NSIG 33