repl: respect --print-build-logs & fix memory leak

243c0f18d[1] allowed the logger's progress bar to display during repl
builds, but startProgressBar() re-creates the entire logger from
scratch, discarding the value of printBuildLogs (and leaking the
previous logger). In this commit, we instead call logger->pause() and
logger->resume(), and in logger->pause(), we manually reset the
activities and stored values in the progress bar's state, so stuff from
multiple builds don't leak over into following ones.

[1]: 243c0f18da

Change-Id: Ie734d1f638d45759d232805d7e3c2005f7dea483

src/libcmd/repl.cc

@@ -300,7 +300,7 @@ ReplExitStatus NixRepl::mainLoop()
 
     /* Stop the progress bar because it interferes with the display of
        the repl. */
-    stopProgressBar();
+    logger->pause();
 
     std::string input;
 
@@ -684,9 +684,22 @@ ProcessLineResult NixRepl::processLine(std::string line)
             // TODO: this only shows a progress bar for explicitly initiated builds,
             // not eval-time fetching or builds performed for IFD.
             // But we can't just show it everywhere, since that would erase partial output from evaluation.
-            startProgressBar();
+            logger->resume();
             Finally stopLogger([&]() {
-                stopProgressBar();
+                logger->pause();
+                if (auto * progressBar = dynamic_cast<ProgressBar *>(logger)) {
+                    auto state(progressBar->state_.lock());
+                    bool const prevActive = state->active;
+                    bool const prevPaused = state->paused;
+                    bool const prevHaveUpdate = state->haveUpdate;
+                    // HACK: reset the state, except for those three booleans, so the
+                    // stats of multiple builds in the same repl session don't mingle.
+                    *state = ProgressBar::State{
+                        .active = prevActive,
+                        .paused = prevPaused,
+                        .haveUpdate = prevHaveUpdate,
+                    };
+                }
             });
 
             state->store->buildPaths({