Fall back to /dev/stderr instead of std::cerr

This allows us to remap STDIO internally in the group process.

src/common/logging.cpp

@@ -72,7 +72,12 @@ Logger Logger::create_from_environment(std::string prefix) {
     if (log_file->is_open()) {
         return Logger(log_file, verbosity_level, prefix);
     } else {
-        return Logger(std::shared_ptr<std::ostream>(&std::cerr, [](auto) {}),
+        // For STDERR we sadly can't just use `std::cerr`. In the group process
+        // we need to capture all output generated by the process itself, and
+        // the only way to do this is by reopening the STDERR and STDOUT streams
+        // to a pipe. Luckily `/dev/stderr` stays unaffected, so we can still
+        // write there without causing infinite loops.
+        return Logger(std::make_shared<std::ofstream>("/dev/stderr"),
                       verbosity_level, prefix);
     }
 }