Fix data race reading the log buffer from the display thread

Copy entries under the buffer lock before rendering; other threads mutate the deque.

src/display/window_log.cc

@@ -1,6 +1,8 @@
 #include "config.h"
 
 #include <ctime>
+#include <mutex>
+#include <vector>
 #include <torrent/torrent.h>
 #include <torrent/system/callbacks.h>
 #include <torrent/system/thread.h>
@@ -41,13 +43,20 @@ WindowLog::redraw() {
 
   int pos = m_canvas->height();
 
-  for (iterator itr = m_log->end(), last = find_older(); itr != last && pos > 0; --pos) {
-    itr--;
+  std::vector<torrent::log_entry> entries;
 
+  {
+    std::lock_guard<torrent::log_buffer> guard(*m_log);
+
+    for (iterator itr = m_log->end(), last = find_older(); itr != last && entries.size() < static_cast<size_t>(pos); )
+      entries.push_back(*--itr);
+  }
+
+  for (const auto& entry : entries) {
     char buffer[16];
-    print_hhmmss_local(buffer, buffer + 16, static_cast<time_t>(itr->timestamp));
+    print_hhmmss_local(buffer, buffer + 16, static_cast<time_t>(entry.timestamp));
 
-    m_canvas->print(0, pos - 1, "(%s) %s", buffer, itr->message.c_str());
+    m_canvas->print(0, --pos, "(%s) %s", buffer, entry.message.c_str());
   }
 }
 
@@ -60,8 +69,13 @@ WindowLog::receive_update() {
   if (!is_active())
     return;
 
-  auto itr    = find_older();
-  auto height = std::min(std::distance(itr, (iterator)m_log->end()), (std::iterator_traits<iterator>::difference_type)10);
+  std::iterator_traits<iterator>::difference_type height;
+
+  {
+    std::lock_guard<torrent::log_buffer> guard(*m_log);
+
+    height = std::min(std::distance(find_older(), (iterator)m_log->end()), (std::iterator_traits<iterator>::difference_type)10);
+  }
 
   if (height != m_max_height) {
     m_min_height = height != 0 ? 1 : 0;

src/display/window_log_complete.cc

@@ -1,6 +1,8 @@
 #include "config.h"
 
 #include <ctime>
+#include <mutex>
+#include <vector>
 
 #include "canvas.h"
 #include "utils.h"
@@ -29,24 +31,34 @@ WindowLogComplete::redraw() {
 
   int pos = m_canvas->height();
 
-  for (iterator itr = m_log->end(), last = m_log->begin(); itr != last && pos > 0; ) {
-    itr--;
+  std::vector<torrent::log_entry> entries;
+
+  {
+    std::lock_guard<torrent::log_buffer> guard(*m_log);
+
+    for (iterator itr = m_log->end(), last = m_log->begin(); itr != last && entries.size() < static_cast<size_t>(pos); )
+      entries.push_back(*--itr);
+  }
+
+  for (const auto& entry : entries) {
+    if (pos <= 0)
+      break;
 
     char buffer[16];
 
     // Use an arbitrary min width of 60 for allowing multiple
     // lines. This should ensure we don't mess up the display when the
     // screen is shrunk too much.
-    unsigned int timeWidth = 3 + print_hhmmss_local(buffer, buffer + 16, static_cast<time_t>(itr->timestamp)) - buffer;
+    unsigned int timeWidth = 3 + print_hhmmss_local(buffer, buffer + 16, static_cast<time_t>(entry.timestamp)) - buffer;
 
     unsigned int logWidth  = m_canvas->width() > 60 ? (m_canvas->width() - timeWidth) : (60 - timeWidth);
-    unsigned int logHeight = (itr->message.size() + logWidth - 1) / logWidth;
+    unsigned int logHeight = (entry.message.size() + logWidth - 1) / logWidth;
 
     for (unsigned int j = logHeight; j > 0 && pos > 0; --j, --pos)
       if (j == 1)
-        m_canvas->print(0, pos - 1, "(%s) %s", buffer, itr->message.substr(0, m_canvas->width() - timeWidth).c_str());
+        m_canvas->print(0, pos - 1, "(%s) %s", buffer, entry.message.substr(0, m_canvas->width() - timeWidth).c_str());
       else
-        m_canvas->print(timeWidth, pos - 1, "%s", itr->message.substr(logWidth * (j - 1), m_canvas->width() - timeWidth).c_str());
+        m_canvas->print(timeWidth, pos - 1, "%s", entry.message.substr(logWidth * (j - 1), m_canvas->width() - timeWidth).c_str());
   }
 }