Preserve the previous session log across restarts (#1846)

re2_framework_log.txt was opened with truncate=true, so the previous
session's log was destroyed as soon as the framework was loaded. After a
crash this made it impossible to inspect the log of the session that
crashed: restarting the game, which is the first thing anyone does, had
already overwritten it.

Rename the existing log to re2_framework_log.prev.txt before creating the
logger, so the previous session survives a restart. All filesystem calls
use the std::error_code overloads, so a failure (e.g. a locked file)
quietly falls back to the old behaviour instead of blocking startup. At
most one previous session is kept: an existing .prev.txt is removed before
the rename.

Author: wdre9

src/REFramework.cpp

@@ -256,9 +256,30 @@ try {
 
 REFramework::REFramework(HMODULE reframework_module)
     : m_game_module{GetModuleHandle(0)}
-    , m_logger{spdlog::basic_logger_mt("REFramework", (get_persistent_dir("re2_framework_log.txt")).string(), true)}
     {
 
+    // Preserve the previous session's log instead of overwriting it, so it can still be
+    // inspected after a crash or a restart (re2_framework_log.txt -> re2_framework_log.prev.txt).
+    const auto log_path = get_persistent_dir("re2_framework_log.txt");
+    bool log_rotated = false;
+    std::string log_prev_path;
+
+    {
+        std::error_code ec;
+
+        if (std::filesystem::exists(log_path, ec)) {
+            auto prev = log_path.parent_path() / (log_path.stem().string() + ".prev" + log_path.extension().string());
+
+            std::filesystem::remove(prev, ec);
+            std::filesystem::rename(log_path, prev, ec);
+
+            log_rotated = !ec;
+            log_prev_path = prev.string();
+        }
+    }
+
+    m_logger = spdlog::basic_logger_mt("REFramework", log_path.string(), true);
+
     s_reframework_module = reframework_module;
     m_dist_sink = std::make_shared<spdlog::sinks::dist_sink_mt>();
 
@@ -270,6 +291,10 @@ REFramework::REFramework(HMODULE reframework_module)
     spdlog::set_default_logger(m_logger);
     spdlog::flush_on(spdlog::level::info);
 
+    if (log_rotated) {
+        spdlog::info("Previous log preserved as {}", log_prev_path);
+    }
+
     if (s_fallback_appdata) {
         spdlog::warn("Failed to write to current directory, falling back to appdata folder");
     }