From 4eb9d9eca23c00a5186932c63e9b129e353cd0b1 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Sat, 17 Jan 2026 11:16:44 +0530
Subject: [PATCH] Explicitly release GIL while loading QImage in thumbnail
 renderer thread

---
 src/calibre/gui2/library/caches.py      | 3 ++-
 src/calibre/utils/imageops/imageops.cpp | 5 ++++-
 src/calibre/utils/imageops/imageops.h   | 1 +
 src/calibre/utils/imageops/imageops.sip | 9 +++------
 4 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/src/calibre/gui2/library/caches.py b/src/calibre/gui2/library/caches.py
index c01f33798a..e92837ffc5 100644
--- a/src/calibre/gui2/library/caches.py
+++ b/src/calibre/gui2/library/caches.py
@@ -21,6 +21,7 @@ from qt.core import QBuffer, QByteArray, QColor, QImage, QImageWriter, QIODevice
 from calibre.db.utils import ThumbnailCache as TC
 from calibre.utils import join_with_timeout
 from calibre.utils.img import resize_to_fit
+from calibre_extensions.imageops import load_from_data_without_gil
 
 
 class ThumbnailCache(TC):
@@ -129,7 +130,7 @@ class Thumbnailer:
         if not cover_as_bytes:
             return self.thumbnail_class(), b''
         cover: QImage = self.thumbnail_class()
-        if not cover.loadFromData(cover_as_bytes):
+        if not load_from_data_without_gil(cover, cover_as_bytes):
             return cover, b''
         cover = self.resize_to_fit(cover, width, height)
         serialized = self.serialize(cover)
diff --git a/src/calibre/utils/imageops/imageops.cpp b/src/calibre/utils/imageops/imageops.cpp
index 605180b3e6..622db1b559 100644
--- a/src/calibre/utils/imageops/imageops.cpp
+++ b/src/calibre/utils/imageops/imageops.cpp
@@ -639,7 +639,6 @@ void overlay(const QImage &image, QImage &canvas, unsigned int left, unsigned in
 } // }}}
 
 QColor dominant_color(const QImage &image) { // {{{
-    ScopedGILRelease PyGILRelease;
     if (image.isNull()) return QColor();
     QImage img;
     // Resize the image to a thumbnail for improved performance
@@ -1005,3 +1004,7 @@ QImage texture_image(const QImage &image, const QImage &texturei) {  // {{{
     }
     return canvas;
 } // }}}
+
+bool load_from_data_without_gil(QImage &image, const char *data, size_t len) {  // {{{
+    return image.loadFromData((const uchar*)data, len, NULL);
+} // }}}
diff --git a/src/calibre/utils/imageops/imageops.h b/src/calibre/utils/imageops/imageops.h
index 150be5b4ea..7a178d3fb2 100644
--- a/src/calibre/utils/imageops/imageops.h
+++ b/src/calibre/utils/imageops/imageops.h
@@ -25,6 +25,7 @@ QImage set_opacity(const QImage &image, double alpha);
 QImage texture_image(const QImage &image, const QImage &texturei);
 QImage ordered_dither(const QImage &image);
 QColor dominant_color(const QImage &image);
+bool load_from_data_without_gil(QImage &image, const char *data, size_t len);
 
 class ScopedGILRelease {
 public:
diff --git a/src/calibre/utils/imageops/imageops.sip b/src/calibre/utils/imageops/imageops.sip
index befdd5e6eb..b3833831fb 100644
--- a/src/calibre/utils/imageops/imageops.sip
+++ b/src/calibre/utils/imageops/imageops.sip
@@ -18,6 +18,8 @@
         } catch (...) { PyErr_SetString(PyExc_RuntimeError, "unknown error"); return NULL;}
 %End
 
+bool load_from_data_without_gil(QImage &image, const char *data /Array/, size_t len /ArraySize/) /ReleaseGIL/;
+
 QImage* remove_borders(const QImage &image, double fuzz);
 %MethodCode
         IMAGEOPS_PREFIX
@@ -74,12 +76,7 @@ QImage oil_paint(const QImage &image, const float radius=-1, const bool high_qua
         IMAGEOPS_SUFFIX
 %End
 
-QColor dominant_color(const QImage &image);
-%MethodCode
-        IMAGEOPS_PREFIX
-			sipRes = new QColor(dominant_color(*a0));
-        IMAGEOPS_SUFFIX
-%End
+QColor dominant_color(const QImage &image) /ReleaseGIL/;
 
 QImage quantize(const QImage &image, unsigned int maximum_colors, bool dither, const QList<QRgb> &palette);
 %MethodCode