repl: show a progress bar while performing builds

In commit 946fc12e4e6d7e097c5b6ed4f6bc4d516b10b901, the progress bar in the
repl was disabled again because it was observed to erase incremental output
from attrset evaluations from the terminal. Let's try adding the progress bar
again, this time showing up only when a build is initiated, which does not have
incremental output that could be destroyed to begin with. While this does mean
that we won't have a progress bar for eval-time fetching or IFD, it's still
better than nothing.

Change-Id: If4eb1035cd0c876f5b4ff1e2434b9baf99f150ac

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index 9811ea30c..696bb3c12 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -605,6 +605,14 @@ ProcessLineResult NixRepl::processLine(std::string line)
         Path drvPathRaw = state->store->printStorePath(drvPath);
 
         if (command == ":b" || command == ":bl") {
+            // 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();
+            Finally stopLogger([&]() {
+                stopProgressBar();
+            });
+
             state->store->buildPaths({
                 DerivedPath::Built {
                     .drvPath = makeConstantStorePathRef(drvPath),