From bd1f07f864a7f1790cec08a306ccc17507f7e5a8 Mon Sep 17 00:00:00 2001
From: Dan Liew <daniel.liew@imperial.ac.uk>
Date: Sun, 11 Dec 2016 23:07:48 +0000
Subject: [PATCH] Fix implementation of `scoped_timer` under Linux where it was
 incorrectly assumed that `pthread_cond_timedwait()` would exit due to a
 condition variable being signaled or a timeout occuring.

According to the documentation `pthread_cond_timedwait()` might
spuriously wake so we introduce a new variable `m_signal_sent` (that is
guarded by an existing mutex) that is used as the variable to check that
the thread wake was not spurious.

This is intended to partially fix #839 .
---
 src/util/scoped_timer.cpp | 19 ++++++++++++++++---
 1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/src/util/scoped_timer.cpp b/src/util/scoped_timer.cpp
index 335b2c3bb..3b87f880f 100644
--- a/src/util/scoped_timer.cpp
+++ b/src/util/scoped_timer.cpp
@@ -73,6 +73,7 @@ struct scoped_timer::imp {
     pthread_cond_t  m_cond;
     unsigned        m_ms;
     bool            m_initialized;
+    bool            m_signal_sent;
 #else
     // Other
 #endif
@@ -119,10 +120,18 @@ struct scoped_timer::imp {
 
         pthread_mutex_lock(&st->m_mutex);
         st->m_initialized = true;
-        int e = pthread_cond_timedwait(&st->m_cond, &st->m_mutex, &end_time);
-        ENSURE(e == 0 || e == ETIMEDOUT);
-
+        int e = 0;
+        // `pthread_cond_timedwait()` may spuriously wake even if the signal
+        // was not sent so we loop until a timeout occurs or the signal was
+        // **really** sent.
+        while (!(e == 0 && st->m_signal_sent)) {
+          e = pthread_cond_timedwait(&st->m_cond, &st->m_mutex, &end_time);
+          ENSURE(e == 0 || e == ETIMEDOUT);
+          if (e == ETIMEDOUT)
+            break;
+        }
         pthread_mutex_unlock(&st->m_mutex);
+
         if (e == ETIMEDOUT)
             st->m_eh->operator()();
         return 0;
@@ -170,6 +179,7 @@ struct scoped_timer::imp {
         // Linux & FreeBSD
         m_ms = ms;
         m_initialized = false;
+        m_signal_sent = false;
         ENSURE(pthread_mutex_init(&m_mutex, NULL) == 0);
         ENSURE(pthread_cond_init(&m_cond, NULL) == 0);
         ENSURE(pthread_create(&m_thread_id, NULL, &thread_func, this) == 0);
@@ -218,7 +228,10 @@ struct scoped_timer::imp {
             if (!init)
                 sched_yield();
         }
+        pthread_mutex_lock(&m_mutex);
+        m_signal_sent = true;
         pthread_cond_signal(&m_cond);
+        pthread_mutex_unlock(&m_mutex);
 
         pthread_join(m_thread_id, NULL);
         pthread_cond_destroy(&m_cond);