SERVER-41131 Add StrongWeakFinishLine

Add support for a new type which coordinates a number of parallel
participants in determining the first matching participant or the last
non-matching participant.

3 files changed, 197 insertions, 0 deletions

diff --git a/src/mongo/util/SConscript b/src/mongo/util/SConscript
index 016a799b15c..6c9c2b5bc2b 100644
--- a/src/mongo/util/SConscript
+++ b/src/mongo/util/SConscript
@@ -747,6 +747,16 @@ env.CppUnitTest(
 )
 
 env.CppUnitTest(
+    target='strong_weak_finish_line_test',
+    source=[
+        'strong_weak_finish_line_test.cpp'
+    ],
+    LIBDEPS=[
+        '$BUILD_DIR/mongo/base',
+    ],
+)
+
+env.CppUnitTest(
     target="concepts_test",
     source=[
         "concepts_test.cpp",
diff --git a/src/mongo/util/strong_weak_finish_line.h b/src/mongo/util/strong_weak_finish_line.h
new file mode 100644
index 00000000000..2f7cc395e10
--- /dev/null
+++ b/src/mongo/util/strong_weak_finish_line.h
@@ -0,0 +1,100 @@
+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include "mongo/platform/atomic_word.h"
+
+#include <utility>
+
+namespace mongo {
+
+/**
+ * A finish line that coordinates the winer of a race amongst a series of participants.
+ *
+ * The winner of the race is the first to arriveStrongly or the last to arriveWeakly (think the last
+ * to get disqualified).
+ *
+ * Every caller of arriveStrong/Weak can depend on the fact that only one caller will be returned a
+ * true value.  That means that consumers of this latch may use it to coordinate access to mutable
+ * shared state as long as they only mutate that state when receiving a true response.
+ */
+class StrongWeakFinishLine {
+    constexpr static inline uint64_t kHighBit = 1ull << 63;
+
+public:
+    explicit StrongWeakFinishLine(uint64_t n) : _state(n) {}
+
+    /**
+     * Returns true if this is the first call to arriveStrongly
+     *
+     * If the return value is true, a participant will know that it won the race.  Note that there
+     * may still be other participants executing however (though none of them will return true from
+     * a call to arrive).
+     */
+    bool arriveStrongly() {
+        if ((_state.load() & kHighBit) || (_state.fetchAndBitOr(kHighBit) & kHighBit)) {
+            return false;
+        }
+
+        return true;
+    }
+
+    /**
+     * Returns true if it is the last call to arriveWeakly
+     *
+     * If the return value is true, a participant will know that it is the last caller and that all
+     * other participants also called arriveWeakly.
+     */
+    bool arriveWeakly() {
+        return _state.subtractAndFetch(1) == 0;
+    }
+
+    /**
+     * Returns true if someone has arrivedStrongly, or all participants have arrived weakly.
+     */
+    bool isReady() const {
+        auto value = _state.load();
+        return (value & kHighBit) || (value == 0);
+    }
+
+private:
+    // Theory of operation for State
+    //
+    // Initial state: num participants
+    //
+    // On strong: atomic.fetch_or(highBit)
+    //   Concurrent calls race to set the high bit.
+    //
+    // On weak: atomic.fetch_sub(1)
+    //   Decrements state by 1.  If this reduces state to 0, there was no strong arriver.
+    AtomicWord<uint64_t> _state;
+};
+
+}  // namespace mongo
diff --git a/src/mongo/util/strong_weak_finish_line_test.cpp b/src/mongo/util/strong_weak_finish_line_test.cpp
new file mode 100644
index 00000000000..62b1f8ae484
--- /dev/null
+++ b/src/mongo/util/strong_weak_finish_line_test.cpp
@@ -0,0 +1,87 @@
+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/platform/basic.h"
+
+#include "mongo/util/strong_weak_finish_line.h"
+
+#include "mongo/unittest/unittest.h"
+
+namespace mongo {
+namespace {
+
+TEST(StrongWeakFinishLineTest, WeakArrivalFollowedByStrong) {
+    StrongWeakFinishLine finishLine(100);
+
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveWeakly());
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveWeakly());
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT(finishLine.arriveStrongly());
+    ASSERT(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveStrongly());
+    ASSERT_FALSE(finishLine.arriveWeakly());
+}
+
+TEST(StrongWeakFinishLineTest, AllWeakArrival) {
+    StrongWeakFinishLine finishLine(3);
+
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveWeakly());
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveWeakly());
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT(finishLine.arriveWeakly());
+    ASSERT(finishLine.isReady());
+}
+
+TEST(StrongWeakFinishLineTest, LastWeakArrivalAfterStrongReturnsFalse) {
+    StrongWeakFinishLine finishLine(3);
+
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveWeakly());
+    ASSERT_FALSE(finishLine.isReady());
+
+    ASSERT(finishLine.arriveStrongly());
+    ASSERT(finishLine.isReady());
+
+    ASSERT_FALSE(finishLine.arriveWeakly());
+}
+
+}  // namespace
+}  // namespace mongo