Implement barriers for Threaded Fork

This commit introduces a concept of a barrier, where all threads must
synchronise before continuing. Threaded Fork uses this to ensure that all
input is consumed by each sink exactly once.

Fixes #695.

3 files changed, 77 insertions, 0 deletions

diff --git a/src/lib/utils/barrier.cpp b/src/lib/utils/barrier.cpp
new file mode 100644
index 000000000..eb2ab978c
--- /dev/null
+++ b/src/lib/utils/barrier.cpp
@@ -0,0 +1,35 @@
+/*
+* Barrier
+* (C) 2016 Joel Low
+*
+* Botan is released under the Simplified BSD License (see license.txt)
+*/
+
+#include <botan/internal/barrier.h>
+
+#if defined(BOTAN_TARGET_OS_HAS_THREADS)
+
+namespace Botan {
+
+void Barrier::wait(unsigned delta)
+    {
+    lock_guard_type<mutex_type> lock(m_mutex);
+    m_value += delta;
+    }
+
+void Barrier::sync()
+    {
+    std::unique_lock<mutex_type> lock(m_mutex);
+    --m_value;
+    if(m_value > 0)
+        m_cond.wait(lock, [this] { return m_value <= 0; });
+    else
+        {
+        m_value = 0;
+        m_cond.notify_all();
+        }
+    }
+
+}
+
+#endif
diff --git a/src/lib/utils/barrier.h b/src/lib/utils/barrier.h
new file mode 100644
index 000000000..96b9c0c7b
--- /dev/null
+++ b/src/lib/utils/barrier.h
@@ -0,0 +1,41 @@
+/*
+* Barrier
+* (C) 2016 Joel Low
+*
+* Botan is released under the Simplified BSD License (see license.txt)
+*/
+
+#ifndef BOTAN_UTIL_BARRIER_H__
+#define BOTAN_UTIL_BARRIER_H__
+
+#include <botan/mutex.h>
+
+#if defined(BOTAN_TARGET_OS_HAS_THREADS)
+#include <condition_variable>
+#endif
+
+namespace Botan {
+
+#if defined(BOTAN_TARGET_OS_HAS_THREADS)
+// Barrier implements a barrier synchronization primitive. wait() will indicate
+// how many threads to synchronize; each thread needing synchronization should
+// call sync(). When sync() returns, the barrier is reset to zero.
+class Barrier
+    {
+    public:
+        explicit Barrier(int value = 0) : m_value(value) {}
+
+        void wait(unsigned delta);
+
+        void sync();
+
+    private:
+        int m_value;
+        mutex_type m_mutex;
+        std::condition_variable m_cond;
+    };
+#endif
+
+}
+
+#endif
diff --git a/src/lib/utils/info.txt b/src/lib/utils/info.txt
index 189b2da1f..75a428a83 100644
--- a/src/lib/utils/info.txt
+++ b/src/lib/utils/info.txt
@@ -22,6 +22,7 @@ version.h
 </header:public>
 
 <header:internal>
+barrier.h
 bit_ops.h
 ct_utils.h
 donna128.h