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.

4 files changed, 40 insertions, 15 deletions

diff --git a/AK/Time.h b/AK/Time.h
new file mode 100644
index 0000000000..3839f18f17
--- /dev/null
+++ b/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;
+    }
+}
+
+}
diff --git a/Kernel/Process.cpp b/Kernel/Process.cpp
index 4063a2263b..ea5cd64935 100644
--- a/Kernel/Process.cpp
+++ b/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);
 
diff --git a/LibCore/CElapsedTimer.cpp b/LibCore/CElapsedTimer.cpp
index b156147a8b..aec3658fc4 100644
--- a/LibCore/CElapsedTimer.cpp
+++ b/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;
 }
diff --git a/LibCore/CEventLoop.cpp b/LibCore/CEventLoop.cpp
index 614e831116..7700a86494 100644
--- a/LibCore/CEventLoop.cpp
+++ b/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;