Adding libglim as an external library

libglim is an Apache-licensed C++ wrapper for lmdb, and rather than
rolling our own it seems prudent to use it.

Note: lmdb is not included in it, and unless something happens as did
with libunbound, should be installed via each OS' package manager or
equivalent.

1 files changed, 40 insertions, 0 deletions

diff --git a/external/glim/channel.hpp b/external/glim/channel.hpp
new file mode 100644
index 000000000..e5ca23bdf
--- /dev/null
+++ b/external/glim/channel.hpp
@@ -0,0 +1,40 @@
+#ifndef _GLIM_CHANNEL_INCLUDED
+#define _GLIM_CHANNEL_INCLUDED
+
+#include <atomic>
+#include <mutex>
+#include <thread>
+
+namespace glim {
+
+/// Unbuffered channel.
+/// Optimized for a single value (busy-waits on a second one).
+template <typename V>
+struct Channel {
+  V _v;
+  std::mutex _mutex;  // Locked when there is no value.
+  std::atomic_int_fast8_t _state; enum State {EMPTY = 0, WRITING = 1, FULL = 2};
+  Channel(): _state (EMPTY) {_mutex.lock();}
+  // Waits until the Channel is empty then stores the value.
+  template <typename VA> void send (VA&& v) {
+    for (;;) {
+      int_fast8_t expectEmpty = EMPTY; if (_state.compare_exchange_weak (expectEmpty, WRITING)) break;
+      std::this_thread::sleep_for (std::chrono::milliseconds (20));
+    }
+    try {_v = std::forward<V> (v);} catch (...) {_state = EMPTY; throw;}
+    _state = FULL;
+    _mutex.unlock();  // Allows the reader to proceed.
+  }
+  // Waits untill there is a value to receive.
+  V receive() {
+    _mutex.lock();  // Wait.
+    V tmp = std::move (_v);
+    assert (_state == FULL);
+    _state = EMPTY;
+    return tmp;
+  }
+};
+
+} // namespace glim
+
+#endif