From 279a434c1c8689f00b1ab8ed571f8732a803a7eb Mon Sep 17 00:00:00 2001
From: Eirik Aavitsland <eirik.aavitsland@qt.io>
Date: Tue, 17 Aug 2021 13:50:50 +0200
Subject: [PATCH] QDashStroker: cap the number of repetitions of the pattern
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Since the dashing is computed even outside the clipping and device
area, painting very long dashed lines could consume unexpected amounts
of time and resources. Fix by placing a limit on the dashing, and fall
back to solid line drawing if hit.

Fixes: QTBUG-95594
Pick-to: 6.2 6.1 5.15
Change-Id: Ida05ecd8fe6df402c9e669206fd5cec4a9f5386a
Reviewed-by: Robert Löhning <robert.loehning@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
---
 src/gui/painting/qpaintengine_raster.cpp |  5 +++
 src/gui/painting/qpaintengineex.cpp      |  2 +-
 src/gui/painting/qstroker.cpp            | 53 ++++++++++++++----------
 src/gui/painting/qstroker_p.h            |  1 +
 4 files changed, 38 insertions(+), 23 deletions(-)

diff --git a/src/gui/painting/qpaintengine_raster.cpp b/src/gui/painting/qpaintengine_raster.cpp
index 5ac1f748e5..70fce7d253 100644
--- a/src/gui/painting/qpaintengine_raster.cpp
+++ b/src/gui/painting/qpaintengine_raster.cpp
@@ -3265,6 +3265,11 @@ void QRasterPaintEnginePrivate::rasterizeLine_dashed(QLineF line,
 
     qreal length = line.length();
     Q_ASSERT(length > 0);
+    if (length / (patternLength * width) > QDashStroker::repetitionLimit()) {
+        rasterizer->rasterizeLine(line.p1(), line.p2(), width / length, squareCap);
+        return;
+    }
+
     while (length > 0) {
         const bool rasterize = *inDash;
         qreal dash = (pattern.at(*dashIndex) - *dashOffset) * width;
diff --git a/src/gui/painting/qpaintengineex.cpp b/src/gui/painting/qpaintengineex.cpp
index d85c2a951c..9ca3de5cf2 100644
--- a/src/gui/painting/qpaintengineex.cpp
+++ b/src/gui/painting/qpaintengineex.cpp
@@ -426,7 +426,7 @@ void QPaintEngineEx::stroke(const QVectorPath &path, const QPen &inPen)
         patternLength *= pw;
         if (qFuzzyIsNull(patternLength)) {
             pen.setStyle(Qt::NoPen);
-        } else if (extent / patternLength > 10000) {
+        } else if (extent / patternLength > QDashStroker::repetitionLimit()) {
             // approximate stream of tiny dashes with semi-transparent solid line
             pen.setStyle(Qt::SolidLine);
             QColor color(pen.color());
diff --git a/src/gui/painting/qstroker.cpp b/src/gui/painting/qstroker.cpp
index cd1a49150b..79194c7cf8 100644
--- a/src/gui/painting/qstroker.cpp
+++ b/src/gui/painting/qstroker.cpp
@@ -1179,32 +1179,41 @@ void QDashStroker::processCurrentSubpath()
 
         bool done = pos >= estop;
 
-        if (clipping) {
-            // Check if the entire line can be clipped away.
-            if (!lineIntersectsRect(prev, e, clip_tl, clip_br)) {
-                // Cut away full dash sequences.
-                elen -= qFloor(elen * invSumLength) * sumLength;
-                // Update dash offset.
-                while (!done) {
-                    qreal dpos = pos + dashes[idash] - doffset - estart;
+        // Check if the entire line should be clipped away or simplified
+        bool clipIt = clipping && !lineIntersectsRect(prev, e, clip_tl, clip_br);
+        bool skipDashing = elen * invSumLength > repetitionLimit();
+        if (skipDashing || clipIt) {
+            // Cut away full dash sequences.
+            elen -= std::floor(elen * invSumLength) * sumLength;
+            // Update dash offset.
+            while (!done) {
+                qreal dpos = pos + dashes[idash] - doffset - estart;
 
-                    Q_ASSERT(dpos >= 0);
+                Q_ASSERT(dpos >= 0);
 
-                    if (dpos > elen) { // dash extends this line
-                        doffset = dashes[idash] - (dpos - elen); // subtract the part already used
-                        pos = estop; // move pos to next path element
-                        done = true;
-                    } else { // Dash is on this line
-                        pos = dpos + estart;
-                        done = pos >= estop;
-                        if (++idash >= dashCount)
-                            idash = 0;
-                        doffset = 0; // full segment so no offset on next.
-                    }
+                if (dpos > elen) { // dash extends this line
+                    doffset = dashes[idash] - (dpos - elen); // subtract the part already used
+                    pos = estop; // move pos to next path element
+                    done = true;
+                } else { // Dash is on this line
+                    pos = dpos + estart;
+                    done = pos >= estop;
+                    if (++idash >= dashCount)
+                        idash = 0;
+                    doffset = 0; // full segment so no offset on next.
                 }
-                hasMoveTo = false;
-                move_to_pos = e;
             }
+            if (clipIt) {
+                hasMoveTo = false;
+            } else {
+                // skip costly dashing, just draw solid line
+                if (!hasMoveTo) {
+                    emitMoveTo(move_to_pos.x, move_to_pos.y);
+                    hasMoveTo = true;
+                }
+                emitLineTo(e.x, e.y);
+            }
+            move_to_pos = e;
         }
 
         // Dash away...
diff --git a/src/gui/painting/qstroker_p.h b/src/gui/painting/qstroker_p.h
index eb0e5d7ffa..0934ef5df7 100644
--- a/src/gui/painting/qstroker_p.h
+++ b/src/gui/painting/qstroker_p.h
@@ -268,6 +268,7 @@ public:
     QStroker *stroker() const { return m_stroker; }
 
     static QList<qfixed> patternForStyle(Qt::PenStyle style);
+    static int repetitionLimit() { return 10000; }
 
     void setDashPattern(const QList<qfixed> &dashPattern) { m_dashPattern = dashPattern; }
     QList<qfixed> dashPattern() const { return m_dashPattern; }