Kernel: Fix timeout support in select

The scheduler expects m_select_timeout to act as a deadline. That is, it
should contain the time that a task should wake at -- but we were
directly copying the time from userspace, which meant that it always
returned virtually immediately.

At the same time, fix CEventLoop to not rely on the broken select behavior
by subtracting the current time from the time of the nearest timer.

AK/Time.h

@@ -0,0 +1,25 @@
+#pragma once
+
+namespace AK {
+
+inline void timeval_sub(const struct timeval* a, const struct timeval* b, struct timeval* result)
+{
+    result->tv_sec = a->tv_sec - b->tv_sec;
+    result->tv_usec = a->tv_usec - b->tv_usec;
+    if (result->tv_usec < 0) {
+        --result->tv_sec;
+        result->tv_usec += 1000000;
+    }
+}
+
+inline void timeval_add(const struct timeval* a, const struct timeval* b, struct timeval* result)
+{
+    result->tv_sec = a->tv_sec + b->tv_sec;
+    result->tv_usec = a->tv_usec + b->tv_usec;
+    if (result->tv_usec > 1000000) {
+        ++result->tv_sec;
+        result->tv_usec -= 1000000;
+    }
+}
+
+}

Kernel/Process.cpp

@@ -21,6 +21,7 @@
 #include <Kernel/TTY/MasterPTY.h>
 #include <Kernel/ELF/exec_elf.h>
 #include <AK/StringBuilder.h>
+#include <AK/Time.h>
 #include <Kernel/SharedMemory.h>
 #include <Kernel/ProcessTracer.h>
 
@@ -1735,7 +1736,9 @@ int Process::sys$select(const Syscall::SC_select_params* params)
     (void)exceptfds;
 
     if (timeout) {
-        current->m_select_timeout = *timeout;
+        struct timeval now;
+        kgettimeofday(now);
+        AK::timeval_add(&now, timeout, &current->m_select_timeout);
         current->m_select_has_timeout = true;
     } else {
         current->m_select_has_timeout = false;
@@ -1829,6 +1832,7 @@ int Process::sys$poll(pollfd* fds, int nfds, int timeout)
             current->m_select_write_fds.append(fds[i].fd);
     }
 
+    // FIXME: this should set m_select_timeout, right?
     if (timeout < 0)
         current->block(Thread::State::BlockedSelect);
 

LibCore/CElapsedTimer.cpp

@@ -1,5 +1,6 @@
 #include <LibCore/CElapsedTimer.h>
 #include <AK/Assertions.h>
+#include <AK/Time.h>
 #include <sys/time.h>
 
 void CElapsedTimer::start()
@@ -8,22 +9,12 @@ void CElapsedTimer::start()
     gettimeofday(&m_start_time, nullptr);
 }
 
-inline void timersub(const struct timeval* a, const struct timeval* b, struct timeval* result)
-{
-    result->tv_sec = a->tv_sec - b->tv_sec;
-    result->tv_usec = a->tv_usec - b->tv_usec;
-    if (result->tv_usec < 0) {
-        --result->tv_sec;
-        result->tv_usec += 1000000;
-    }
-}
-
 int CElapsedTimer::elapsed() const
 {
     ASSERT(is_valid());
     struct timeval now;
     gettimeofday(&now, nullptr);
     struct timeval diff;
-    timersub(&now, &m_start_time, &diff);
+    AK::timeval_sub(&now, &m_start_time, &diff);
     return diff.tv_sec * 1000 + diff.tv_usec / 1000;
 }

LibCore/CEventLoop.cpp

@@ -14,6 +14,7 @@
 #include <LibC/errno.h>
 #include <LibC/string.h>
 #include <LibC/stdlib.h>
+#include <AK/Time.h>
 
 //#define CEVENTLOOP_DEBUG
 //#define DEFERRED_INVOKE_DEBUG
@@ -179,18 +180,22 @@ void CEventLoop::wait_for_event()
         queued_events_is_empty = m_queued_events.is_empty();
     }
 
+    timeval now;
     struct timeval timeout = { 0, 0 };
-    if (!s_timers->is_empty() && queued_events_is_empty)
+    if (!s_timers->is_empty() && queued_events_is_empty) {
+        gettimeofday(&now, nullptr);
         get_next_timer_expiration(timeout);
+        AK::timeval_sub(&timeout, &now, &timeout);
+    }
 
     int rc = select(max_fd + 1, &rfds, &wfds, nullptr, (queued_events_is_empty && s_timers->is_empty()) ? nullptr : &timeout);
     if (rc < 0) {
         ASSERT_NOT_REACHED();
     }
 
-    timeval now;
-    if (!s_timers->is_empty())
+    if (!s_timers->is_empty()) {
         gettimeofday(&now, nullptr);
+    }
 
     for (auto& it : *s_timers) {
         auto& timer = *it.value;