fix segfault when emitting log messages from fread

src/core/read/parallel_reader.cc

@@ -240,6 +240,7 @@ void ParallelReader::read_all()
   if (preframe.nrows_written() < g.max_nrows) {
     xassert(end_of_last_chunk == input_end);
   }
+  g.logger_.emit_pending_messages();
 }
 
 

src/core/utils/logger.cc

@@ -125,7 +125,7 @@ Message::Message(Logger* logger, bool warn)
 
 Message::~Message() {
   try {
-    logger_->emit(std::move(out_).str(), emit_as_warning_);
+    logger_->add(std::move(out_).str(), emit_as_warning_);
   }
   catch (...) {
     std::cerr << "unable to emit log message\n";
@@ -236,7 +236,7 @@ void Logger::use_pylogger(py::oobj logger) {
 
 Section Logger::section(std::string title) {
   if (enabled_) {
-    emit(std::move(title), false);
+    add(std::move(title), false);
     prefix_ += "  ";
   }
   return Section(this);
@@ -285,10 +285,32 @@ void Logger::end_section() noexcept {
 }
 
 
-void Logger::emit(std::string&& msg, bool warning) {
-  // std::lock_guard<std::mutex> pylock(dt::python_mutex());
+// This function must be called from the main thread
+void Logger::emit_pending_messages() {
+  xassert(dt::this_thread_index() == 0);
+  if (!pending_messages_.empty()) {
+    for (size_t i = 0; i < pending_messages_.size(); i++) {
+      auto msg = std::move(pending_messages_[i]);
+      emit_(std::move(msg.first), msg.second);
+    }
+    pending_messages_.clear();
+  }
+}
+
+
+void Logger::add(std::string&& msg, bool is_warning) {
   PythonLock pylock;
   CallLoggerLock loglock;
+  if (dt::this_thread_index() == 0) {
+    emit_pending_messages();
+    emit_(std::move(msg), is_warning);
+  } else {
+    pending_messages_.push_back(std::make_pair(std::move(msg), is_warning));
+  }
+}
+
+
+void Logger::emit_(std::string&& msg, bool warning) {
   // Use user-defined logger object
   if (pylogger_) {
     HidePythonError hpe;

src/core/utils/logger.h

@@ -23,6 +23,9 @@
 #define dt_UTILS_LOGGER_h
 #include <iomanip>
 #include <sstream>    // std::ostringstream
+#include <string>
+#include <utility>
+#include <vector>
 #include "_dt.h"
 namespace py { class DefaultLogger; }
 namespace dt {
@@ -72,6 +75,7 @@ class Logger {
   private:
     py::oobj pylogger_;
     std::string prefix_;
+    std::vector<std::pair<std::string, bool>> pending_messages_;
     bool enabled_;
     bool use_colors_;
     size_t : 48;
@@ -98,9 +102,12 @@ class Logger {
     // "python" -> returns the stored python logger object
     py::oobj get_pylogger(bool fallback_to_default = true) const;
 
+    void emit_pending_messages();
+
   private:
     void end_section() noexcept;
-    void emit(std::string&& msg, bool as_warning);
+    void add(std::string&& msg, bool as_warning);
+    void emit_(std::string&& msg, bool as_warning);
 
     friend class Section;
     friend class Message;