Scheduler: Allow reentry into block()

With the presence of signal handlers, it is possible that a thread might
be blocked multiple times. Picture for instance a signal handler using
read(), or wait() while the thread is already blocked elsewhere before
the handler is invoked.

To fix this, we turn m_blocker into a chain of handlers. Each block()
call now prepends to the list, and unblocking will only consider the
most recent (first) blocker in the chain.

Fixes #309

Kernel/Scheduler.cpp

@@ -221,8 +221,8 @@ void Thread::consider_unblock(time_t now_sec, long now_usec)
         /* don't know, don't care */
         return;
     case Thread::Blocked:
-        ASSERT(m_blocker);
-        if (m_blocker->should_unblock(*this, now_sec, now_usec))
+        ASSERT(!m_blockers.is_empty());
+        if (m_blockers.first()->should_unblock(*this, now_sec, now_usec))
             unblock();
         return;
     case Thread::Skip1SchedulerPass:
@@ -307,8 +307,8 @@ bool Scheduler::pick_next()
             return IterationDecision::Continue;
         if (was_blocked) {
             dbgprintf("Unblock %s(%u) due to signal\n", thread.process().name().characters(), thread.pid());
-            ASSERT(thread.m_blocker);
-            thread.m_blocker->set_interrupted_by_signal();
+            ASSERT(!thread.m_blockers.is_empty());
+            thread.m_blockers.first()->set_interrupted_by_signal();
             thread.unblock();
         }
         return IterationDecision::Continue;

Kernel/Thread.cpp

@@ -148,8 +148,8 @@ const char* Thread::state_string() const
     case Thread::Skip0SchedulerPasses:
         return "Skip0";
     case Thread::Blocked:
-        ASSERT(m_blocker);
-        return m_blocker->state_string();
+        ASSERT(!m_blockers.is_empty());
+        return m_blockers.first()->state_string();
     }
     kprintf("to_string(Thread::State): Invalid state: %u\n", state());
     ASSERT_NOT_REACHED();
@@ -539,8 +539,8 @@ void Thread::set_state(State new_state)
 {
     InterruptDisabler disabler;
     if (new_state == Blocked) {
-        // we should always have an m_blocker while blocked
-        ASSERT(m_blocker != nullptr);
+        // we should always have a Blocker while blocked
+        ASSERT(!m_blockers.is_empty());
     }
 
     m_state = new_state;

Kernel/Thread.h

@@ -72,6 +72,8 @@ public:
         bool was_interrupted_by_signal() const { return m_was_interrupted_while_blocked; }
     private:
         bool m_was_interrupted_while_blocked { false };
+        friend class Thread;
+        IntrusiveListNode m_blocker_list_node;
     };
 
     class FileDescriptionBlocker : public Blocker {
@@ -217,21 +219,39 @@ public:
     template <typename T, class... Args>
     [[nodiscard]] BlockResult block(Args&& ... args)
     {
-        // If this is triggered, state has gotten messed: we should never have a
-        // blocker already set. That means we're re-entering somehow, which is
-        // bad.
-        ASSERT(m_blocker == nullptr);
-
         // We should never be blocking a blocked (or otherwise non-active) thread.
         ASSERT(state() == Thread::Running);
+
         T t(AK::forward<Args>(args)...);
-        m_blocker = &t;
+        m_blockers.prepend(t);
+
+        // Enter blocked state.
         set_state(Thread::Blocked);
-        block_helper(); // this will unlock, yield, and eventually unblock us to return here.
+
+        // Yield to the scheduler, and wait for us to resume unblocked.
+        block_helper();
+
+        // We should no longer be blocked once we woke up
         ASSERT(state() != Thread::Blocked);
-        m_blocker = nullptr;
+
+        // We should be the first blocker, otherwise we're waking up in a
+        // different order than we are waiting: scheduler bug?
+        ASSERT(m_blockers.first() == &t);
+
+        // Remove ourselves...
+        m_blockers.remove(t);
+
+        // If there are still pending blockers that need to be waited for, then
+        // set state back to Blocked, for the next one to be handled.
+        // Otherwise, we're good now, and done with blocking state.
+        if (!m_blockers.is_empty()) {
+            if (!m_blockers.first()->was_interrupted_by_signal())
+                set_state(Thread::Blocked);
+        }
+
         if (t.was_interrupted_by_signal())
             return BlockResult::InterruptedBySignal;
+
         return BlockResult::WokeNormally;
     };
 
@@ -321,7 +341,7 @@ private:
     RefPtr<Region> m_kernel_stack_for_signal_handler_region;
     SignalActionData m_signal_action_data[32];
     Region* m_signal_stack_user_region { nullptr };
-    Blocker* m_blocker { nullptr };
+    IntrusiveList<Blocker, &Blocker::m_blocker_list_node> m_blockers;
     FPUState* m_fpu_state { nullptr };
     State m_state { Invalid };
     bool m_has_used_fpu { false };