executor: properly separate stdout and stderr in log messages

... about processes producing both, stdout and stderr. By supporting
unique readability of the output, we facilitate the understanding
of the messages provided by actions.

2 files changed, 14 insertions, 5 deletions

diff --git a/src/buildtool/execution_engine/executor/TARGETS b/src/buildtool/execution_engine/executor/TARGETS
index 6855f87b..7a7718dd 100644
--- a/src/buildtool/execution_engine/executor/TARGETS
+++ b/src/buildtool/execution_engine/executor/TARGETS
@@ -17,6 +17,7 @@
     , ["src/buildtool/execution_api/remote", "bazel"]
     , ["src/buildtool/progress_reporting", "progress"]
     , ["src/utils/cpp", "hex_string"]
+    , ["src/utils/cpp", "prefix"]
     , ["@", "gsl", "", "gsl"]
     , ["src/buildtool/common", "common"]
     , ["src/buildtool/common/remote", "remote_common"]
diff --git a/src/buildtool/execution_engine/executor/executor.hpp b/src/buildtool/execution_engine/executor/executor.hpp
index ad933a93..d84f4e7e 100644
--- a/src/buildtool/execution_engine/executor/executor.hpp
+++ b/src/buildtool/execution_engine/executor/executor.hpp
@@ -45,6 +45,7 @@
 #include "src/buildtool/logging/logger.hpp"
 #include "src/buildtool/progress_reporting/progress.hpp"
 #include "src/utils/cpp/hex_string.hpp"
+#include "src/utils/cpp/prefix.hpp"
 
 /// \brief Implementations for executing actions and uploading artifacts.
 class ExecutorImpl {
@@ -594,20 +595,27 @@ class ExecutorImpl {
         auto build_message = [has_err, has_out, &logger, &action, &response]() {
             using namespace std::string_literals;
             auto message = ""s;
+            bool has_both = has_err and has_out;
             if (has_err or has_out) {
-                message += (has_err and has_out ? "Stdout and stderr"s
-                            : has_out           ? "Stdout"s
-                                                : "Stderr"s) +
+                message += (has_both  ? "Output"s
+                            : has_out ? "Stdout"s
+                                      : "Stderr"s) +
                            " of command ";
             }
             message += nlohmann::json(action->Command()).dump() +
                        " in environment " +
                        nlohmann::json(action->Env()).dump() + "\n";
             if (response->HasStdOut()) {
-                message += response->StdOut();
+                if (has_both) {
+                    message += "Stdout:\n";
+                }
+                message += PrefixLines(response->StdOut());
             }
             if (response->HasStdErr()) {
-                message += response->StdErr();
+                if (has_both) {
+                    message += "Stderr:\n";
+                }
+                message += PrefixLines(response->StdErr());
             }
             return message;
         };