[PyQt] QIconEngine ownership regression in PyQt5 5.12

[Ales Erjavec]

Hi,

There seems to be a regression in PyQt5 5.12 in QIconEngine ownership
transfer to QIcon.
I.e.

### code
icon1 = QIcon(IconEngine(...))
icon2 = QIcon(icon1)
del icon1 # the IconEngine is deleted along with the icon1
### /code

After this icon2 no longer renders properly. The QIconEngine's are
implicitly shared by QIcon
instances so the engine should not be deleted with icon1.

This works as expected in in PyQt5 5.11, 5.10, 5.9

A more complete example with an icon engine implementation:

### code

from PyQt5.QtCore import Qt, QRect
from PyQt5.QtGui import (
    QIcon, QIconEngine, QPainter, QColor, QPixmap, QStandardItem,
    QStandardItemModel
)
from PyQt5.QtWidgets import QApplication, QListView


class IconEngine(QIconEngine):
    """A simple QIconEngine drawing a single text character."""
    def __init__(self, char, color):
        # type: (str, QColor) -> None
        super().__init__()
        self.char = char
        self.color = QColor(color)

    def paint(self, painter, rect, mode, state):
        # type: (QPainter, QRect, QIcon.Mode, QIcon.State) -> None
        size = rect.size()
        if size.isNull():
            return
        dpr = painter.device().devicePixelRatioF()
        size = size * dpr
        painter.drawPixmap(rect, self.pixmap(size, mode, state))

    def pixmap(self, size, mode, state):
        # type: (QSize, QIcon.Mode, QIcon.State) -> QPixmap
        pm = QPixmap(size)
        pm.fill(Qt.transparent)
        painter = QPainter(pm)
        painter.setRenderHints(QPainter.TextAntialiasing)
        size = size.width()
        painter.setPen(self.color)
        painter.setBrush(self.color)
        margin = 1 + size // 16
        text_margin = size // 20
        rect = QRect(margin, margin, size - 2 * margin, size - 2 * margin)

        font = painter.font()
        font.setPixelSize(size - 2 * margin - 2 * text_margin)
        font.setBold(True)

        painter.setFont(font)
        painter.drawText(rect, Qt.AlignCenter, self.char)
        painter.end()
        return pm

    def clone(self):
        return IconEngine(self.char, self.color)

    def __del__(self):
        print("del")

def main(argv=[]):
    app = QApplication(argv)
    view = QListView()
    model = QStandardItemModel()
    item1 = QStandardItem("R")
    item1.setIcon(QIcon(IconEngine('R', QColor("red"))))
    item2 = QStandardItem("B")
    item2.setIcon(QIcon(IconEngine('B', QColor("blue"))))
    item3 = QStandardItem("G")
    item3.setIcon(QIcon(IconEngine('G', QColor("green"))))

    model.appendRow([item1])
    model.appendRow([item2])
    model.appendRow([item3])

    view.setModel(model)
    view.show()

    return app.exec_()


if __name__ == "__main__":
    import sys
    sys.exit(main())

### /code

In PyQt5 5.12 the __del__ method is called immediately after setting
the icon on the QStandardItem (and the icons do not render in the
view)
In PyQt5 5.11, 5.10, 5.9 it is called after main finishes (and icons
render correctly in the view).

Tested on macOS 10.14.5 with Python 3.6.6 and 3.7.0

Best regards
Aleš