ProgressBar: Delay before showing a new activity

Some activities are numerous but usually very short (e.g. copying a
source file to the store) which would cause a lot of flickering. So
only show activities that have been running for at least 10 ms.

src/libmain/progress-bar.cc

@@ -8,6 +8,7 @@
 #include <map>
 #include <thread>
 #include <iostream>
+#include <chrono>
 
 namespace nix {
 
@@ -48,6 +49,7 @@ private:
         bool visible = true;
         ActivityId parent;
         std::optional<std::string> name;
+        std::chrono::time_point<std::chrono::steady_clock> startTime;
     };
 
     struct ActivitiesByType
@@ -91,10 +93,11 @@ public:
         state_.lock()->active = isTTY;
         updateThread = std::thread([&]() {
             auto state(state_.lock());
+            auto nextWakeup = std::chrono::milliseconds::max();
             while (state->active) {
                 if (!state->haveUpdate)
-                    state.wait(updateCV);
-                draw(*state);
+                    state.wait_for(updateCV, nextWakeup);
+                nextWakeup = draw(*state);
                 state.wait_for(quitCV, std::chrono::milliseconds(50));
             }
         });
@@ -118,7 +121,8 @@ public:
         updateThread.join();
     }
 
-    bool isVerbose() override {
+    bool isVerbose() override
+    {
         return printBuildLogs;
     }
 
@@ -159,11 +163,13 @@ public:
         if (lvl <= verbosity && !s.empty() && type != actBuildWaiting)
             log(*state, lvl, s + "...");
 
-        state->activities.emplace_back(ActInfo());
+        state->activities.emplace_back(ActInfo {
+            .s = s,
+            .type = type,
+            .parent = parent,
+            .startTime = std::chrono::steady_clock::now()
+        });
         auto i = std::prev(state->activities.end());
-        i->s = s;
-        i->type = type;
-        i->parent = parent;
         state->its.emplace(act, i);
         state->activitiesByType[type].its.emplace(act, i);
 
@@ -327,10 +333,12 @@ public:
         updateCV.notify_one();
     }
 
-    void draw(State & state)
+    std::chrono::milliseconds draw(State & state)
     {
+        auto nextWakeup = std::chrono::milliseconds::max();
+
         state.haveUpdate = false;
-        if (!state.active) return;
+        if (!state.active) return nextWakeup;
 
         std::string line;
 
@@ -341,12 +349,25 @@ public:
             line += "]";
         }
 
+        auto now = std::chrono::steady_clock::now();
+
         if (!state.activities.empty()) {
             if (!status.empty()) line += " ";
             auto i = state.activities.rbegin();
 
-            while (i != state.activities.rend() && (!i->visible || (i->s.empty() && i->lastLine.empty())))
+            while (i != state.activities.rend()) {
+                if (i->visible && (!i->s.empty() || !i->lastLine.empty())) {
+                    /* Don't show activities until some time has
+                       passed, to avoid displaying very short
+                       activities. */
+                    auto delay = std::chrono::milliseconds(10);
+                    if (i->startTime + delay < now)
+                        break;
+                    else
+                        nextWakeup = std::min(nextWakeup, std::chrono::duration_cast<std::chrono::milliseconds>(delay - (now - i->startTime)));
+                }
                 ++i;
+            }
 
             if (i != state.activities.rend()) {
                 line += i->s;
@@ -366,6 +387,8 @@ public:
         if (width <= 0) width = std::numeric_limits<decltype(width)>::max();
 
         writeToStderr("\r" + filterANSIEscapes(line, false, width) + ANSI_NORMAL + "\e[K");
+
+        return nextWakeup;
     }
 
     std::string getStatus(State & state)