libstore: turn Worker::updateStatistics into a promise

we'll now loop to update displayed statistics, and use this loop to
limit the update rate to 50 times per second. we could have updated
much more frequently before this (once per iteration of `runImpl`),
much faster than would ever be useful in practice. aggressive stats
updates can even impede progress due to terminal or network delays.

Change-Id: Ifba755a2569f73c919b1fbb06a142c0951395d6d

1 files changed, 4 insertions, 3 deletions

diff --git a/src/libstore/build/worker.hh b/src/libstore/build/worker.hh
index bb51a2114..d6cde8384 100644
--- a/src/libstore/build/worker.hh
+++ b/src/libstore/build/worker.hh
@@ -167,16 +167,17 @@ private:
     /**
       * Pass current stats counters to the logger for progress bar updates.
       */
-    void updateStatistics();
+    kj::Promise<Result<void>> updateStatistics();
 
-    bool statisticsOutdated = true;
+    AsyncSemaphore statisticsUpdateSignal{1};
+    std::optional<AsyncSemaphore::Token> statisticsUpdateInhibitor;
 
     /**
       * Mark statistics as outdated, such that `updateStatistics` will be called.
       */
     void updateStatisticsLater()
     {
-        statisticsOutdated = true;
+        statisticsUpdateInhibitor = {};
     }
 
     kj::Promise<Result<void>> runImpl();