[core] Introduce Scheduler::GetSequenced() API

The newly introduced `Scheduler::GetSequenced()` returns
sequenced schedulers from the cache limited to 10 instances,
preventing from spawning too many threads.

1 files changed, 27 insertions, 0 deletions

diff --git a/src/mbgl/actor/scheduler.cpp b/src/mbgl/actor/scheduler.cpp
index 81e259fe1f..0e051d0273 100644
--- a/src/mbgl/actor/scheduler.cpp
+++ b/src/mbgl/actor/scheduler.cpp
@@ -41,4 +41,31 @@ std::shared_ptr<Scheduler> Scheduler::GetBackground() {
     return scheduler;
 }
 
+// static
+std::shared_ptr<Scheduler> Scheduler::GetSequenced() {
+    const std::size_t kSchedulersCount = 10;
+    static std::vector<std::weak_ptr<Scheduler>> weaks(kSchedulersCount);
+    static std::mutex mtx;
+    static std::size_t lastUsedIndex = 0u;
+
+    std::lock_guard<std::mutex> lock(mtx);
+
+    if (++lastUsedIndex == kSchedulersCount) lastUsedIndex = 0u;
+
+    std::shared_ptr<Scheduler> result;
+    for (std::size_t i = 0; i < kSchedulersCount; ++i) {
+        auto& weak = weaks[i];
+        if (auto scheduler = weak.lock()) {
+            if (lastUsedIndex == i) result = scheduler;
+            continue;
+        }
+        result = std::make_shared<SequencedScheduler>();
+        weak = result;
+        lastUsedIndex = i;
+        break;
+    }
+
+    return result;
+}
+
 } //namespace mbgl