libstore: flush log lines only from handleBuilderOutput

also change the tests to ensure that we handle all lines correctly.

Change-Id: I13de286bda8eee57acd53af1ab5c081d3048b500

lix/libstore/build/derivation-goal.cc

@@ -1250,6 +1250,11 @@ try {
         lastChildActivity = AIO().provider.getTimer().now();
 
         if (data.empty()) {
+            if (!currentLogLine.empty()) {
+                if (flushLine() == Logger::BufferState::NeedsFlush) {
+                    TRY_AWAIT(act->getLogger().flush());
+                }
+            }
             co_return std::nullopt;
         }
 
@@ -1359,11 +1364,7 @@ try {
         );
     }
 
-    const auto r = TRY_AWAIT(handlers);
-    if (!currentLogLine.empty() && flushLine() == Logger::BufferState::NeedsFlush) {
-        TRY_AWAIT(act->getLogger().flush());
-    }
-    co_return r;
+    co_return TRY_AWAIT(handlers);
 } catch (...) {
     co_return result::current_exception();
 }

tests/functional/build-hook-ca-fixed.nix

@@ -36,8 +36,9 @@ let
   input3 = mkDerivation {
     shell = busybox;
     name = "build-remote-input-3";
+    # `echo -n` tests handling of logs without trailing newlines
     buildCommand = ''
-      echo hi-input3
+      echo -n hi-input3
       read x < ${input2}
       echo $x BAZ > $out
     '';

tests/functional/build-hook.nix

@@ -39,8 +39,9 @@ let
   input3 = mkDerivation {
     shell = busybox;
     name = "build-remote-input-3";
+    # `echo -n` tests handling of logs without trailing newlines
     buildCommand = ''
-      echo hi-input3
+      echo -n hi-input3
       read x < ${input2}
       echo $x BAZ > $out
     '';