Logger, ProgressBar: add a way to pause/resume

Add new virtual methods pause and resume to the Logger class, and
implement them in ProgressBar to allow to pause the bar refreshing.

src/libmain/progress-bar.cc

@@ -72,6 +72,7 @@ private:
         uint64_t corruptedPaths = 0, untrustedPaths = 0;
 
         bool active = true;
+        bool paused = false;
         bool haveUpdate = true;
     };
 
@@ -120,6 +121,17 @@ public:
         updateThread.join();
     }
 
+    void pause() override {
+        state_.lock()->paused = true;
+        writeToStderr("\r\e[K");
+    }
+
+    void resume() override {
+        state_.lock()->paused = false;
+        writeToStderr("\r\e[K");
+        updateCV.notify_one();
+    }
+
     bool isVerbose() override
     {
         return printBuildLogs;
@@ -338,6 +350,7 @@ public:
     {
         auto nextWakeup = std::chrono::milliseconds::max();
 
+        if (state.paused) return nextWakeup;
         state.haveUpdate = false;
         if (!state.active) return nextWakeup;
 

src/libutil/logging.hh

@@ -72,6 +72,9 @@ public:
 
     virtual void stop() { };
 
+    virtual void pause() { };
+    virtual void resume() { };
+
     // Whether the logger prints the whole build log
     virtual bool isVerbose() { return false; }