From e2ab622a148c2698e589303e835a7660bfe5b42b Mon Sep 17 00:00:00 2001
From: Jonathan G Rennison <j.g.rennison@gmail.com>
Date: Sat, 11 Apr 2020 18:23:38 +0100
Subject: [PATCH] Avoid data race in modal progress mode

---
 src/gfx.cpp           |  7 ++++++-
 src/newgrf_config.cpp | 16 +++++++++++++---
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/gfx.cpp b/src/gfx.cpp
index e48de20a64..364fca20a2 100644
--- a/src/gfx.cpp
+++ b/src/gfx.cpp
@@ -1490,12 +1490,17 @@ void DrawDirtyBlocks()
 	if (HasModalProgress()) {
 		/* We are generating the world, so release our rights to the map and
 		 * painting while we are waiting a bit. */
+		bool is_first_modal_progress_loop = IsFirstModalProgressLoop();
 		_modal_progress_paint_mutex.unlock();
 		_modal_progress_work_mutex.unlock();
 
 		/* Wait a while and update _realtime_tick so we are given the rights */
-		if (!IsFirstModalProgressLoop()) CSleep(MODAL_PROGRESS_REDRAW_TIMEOUT);
+		if (!is_first_modal_progress_loop) CSleep(MODAL_PROGRESS_REDRAW_TIMEOUT);
+#if defined(__GNUC__) || defined(__clang__)
+		__atomic_add_fetch(&_realtime_tick, MODAL_PROGRESS_REDRAW_TIMEOUT, __ATOMIC_RELAXED);
+#else
 		_realtime_tick += MODAL_PROGRESS_REDRAW_TIMEOUT;
+#endif
 
 		/* Modal progress thread may need blitter access while we are waiting for it. */
 		VideoDriver::GetInstance()->ReleaseBlitterLock();
diff --git a/src/newgrf_config.cpp b/src/newgrf_config.cpp
index fce97bd289..b3b25506a8 100644
--- a/src/newgrf_config.cpp
+++ b/src/newgrf_config.cpp
@@ -721,8 +721,13 @@ class GRFFileScanner : FileScanner {
 	std::vector<GRFConfig *> grfs;
 
 public:
-	GRFFileScanner() : next_update(_realtime_tick), num_scanned(0)
+	GRFFileScanner() : num_scanned(0)
 	{
+#if defined(__GNUC__) || defined(__clang__)
+		this->next_update = __atomic_load_n(&_realtime_tick, __ATOMIC_RELAXED);
+#else
+		this->next_update = _realtime_tick;
+#endif
 	}
 
 	bool AddFile(const char *filename, size_t basepath_length, const char *tar_filename) override;
@@ -783,7 +788,12 @@ bool GRFFileScanner::AddFile(const char *filename, size_t basepath_length, const
 	}
 
 	this->num_scanned++;
-	if (this->next_update <= _realtime_tick) {
+#if defined(__GNUC__) || defined(__clang__)
+	const uint32 now = __atomic_load_n(&_realtime_tick, __ATOMIC_RELAXED);
+#else
+	const uint32 now = _realtime_tick;
+#endif
+	if (this->next_update <= now) {
 		_modal_progress_work_mutex.unlock();
 		_modal_progress_paint_mutex.lock();
 
@@ -795,7 +805,7 @@ bool GRFFileScanner::AddFile(const char *filename, size_t basepath_length, const
 		_modal_progress_work_mutex.lock();
 		_modal_progress_paint_mutex.unlock();
 
-		this->next_update = _realtime_tick + MODAL_PROGRESS_REDRAW_TIMEOUT;
+		this->next_update = now + MODAL_PROGRESS_REDRAW_TIMEOUT;
 	}
 
 	if (!added) {