From 82baeb5e925bf838ac0fa4e1c613125d3854dafa Mon Sep 17 00:00:00 2001
From: NotPppp1116 <264624490+NotPppp1116@users.noreply.github.com>
Date: Thu, 16 Jul 2026 19:12:32 +0100
Subject: [PATCH] render: handle explicit sync fallback safely (#14986)

* render: handle explicit sync fallback safely

* render: avoid blocking explicit sync fallback

* ci: retry flaky Hyprland VM tests

* render: avoid blocking explicit sync fallback

---------

Co-authored-by: Pppp1116 <pcaiadoguerreiro@gmail.com>
Co-authored-by: Pppp1116 <ACCOUNT1_NOREPLY>
---
 src/output/Monitor.cpp               |  6 ++++--
 src/output/MonitorFrameScheduler.cpp |  7 +++++++
 src/render/GLRenderer.cpp            | 12 ++++++++++--
 3 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/src/output/Monitor.cpp b/src/output/Monitor.cpp
index f3f271b4b..2ce8885ee 100644
--- a/src/output/Monitor.cpp
+++ b/src/output/Monitor.cpp
@@ -2188,11 +2188,13 @@ bool CMonitor::attemptDirectScanout() {
     if (g_pHyprRenderer->explicitSyncSupported() && isMultiGPU()) {
         auto sync = g_pHyprRenderer->createSyncFDManager();
 
-        if (sync->fd().isValid()) {
+        if (sync && sync->isValid()) {
             m_inFence = sync->takeFd();
             m_output->state->setExplicitInFence(m_inFence.get());
-        } else
+        } else {
+            m_inFence.reset();
             m_output->state->resetExplicitFences(); // good luck.
+        }
     } else
         m_output->state->resetExplicitFences();
 
diff --git a/src/output/MonitorFrameScheduler.cpp b/src/output/MonitorFrameScheduler.cpp
index 18b09074a..9299bfbbb 100644
--- a/src/output/MonitorFrameScheduler.cpp
+++ b/src/output/MonitorFrameScheduler.cpp
@@ -129,6 +129,13 @@ void CMonitorFrameScheduler::onFrame() {
 
 void CMonitorFrameScheduler::onFinishRender() {
     m_sync = g_pHyprRenderer->createSyncFDManager(); // this destroys the old sync
+    if (!m_sync || !m_sync->isValid()) {
+        Log::logger->log(Log::ERR, "CMonitorFrameScheduler: explicit sync failed, falling back to frame events");
+        m_sync.reset();
+        m_renderAtFrame = true;
+        return;
+    }
+
     g_pEventLoopManager->doOnReadable(m_sync->fd().duplicate(), [this, self = m_self] {
         if (!self) // might've gotten destroyed
             return;
diff --git a/src/render/GLRenderer.cpp b/src/render/GLRenderer.cpp
index 255e5f84a..1dab5a0b7 100644
--- a/src/render/GLRenderer.cpp
+++ b/src/render/GLRenderer.cpp
@@ -148,9 +148,17 @@ void CHyprGLRenderer::endRender(const std::function<void()>& renderingDoneCallba
             PMONITOR->m_output->state->setExplicitInFence(PMONITOR->m_inFence.get());
         }
     } else {
-        Log::logger->log(Log::ERR, "renderer: Explicit sync failed, releasing resources");
+        Log::logger->log(Log::ERR, "renderer: Explicit sync failed, falling back to implicit sync");
 
-        m_usedAsyncBuffers.clear(); // release all buffer refs and hope implicit sync works
+        // Establish an implicit synchronization point without blocking the render loop.
+        glFlush();
+
+        if (m_renderMode == RENDER_MODE_NORMAL && PMONITOR) {
+            PMONITOR->m_inFence.reset();
+            PMONITOR->m_output->state->resetExplicitFences();
+        }
+
+        m_usedAsyncBuffers.clear();
         if (renderingDoneCallback)
             renderingDoneCallback();
     }