Project metadata wrongly encoded in PDF exported from print layout on Windows

[agiudiceandrea]

Describe the bug

On Windows, if the project metadata (title, author, subject, ...) contain characters not supported or not well mapped by the system 8 bit codepage, then the metadata of the PDF file exported from the print layout (using the "Export RDF metadata" option) are wrongly encoded.

Project metadata with the string Test Eλληνικά Русский ÀÇÑ ŞÖÜ ŁĞÇŞ

Metadata of the exported PDF with the wrongly encoded string Test E??????? ??????? ÀÇÑ SÖÜ LGÇS

Test project
Test_project.zip

QGIS and OS versions

QGIS 3.10.x, 3.16.x, 3.17-master
Windows 7 / 10, windows-1252 system codepage

Additional context

The issue doesn't occur if the print layout is exported as GeoPDF instead of PDF: this previously unreported issue was fixed for GeoPDF with #40156

I think the issue is due to the use of QString::toLocal8Bit() instead of QString::toUtf8() in

QGIS/src/core/qgsmaprenderertask.cpp

Lines 348 to 364 in 3b4e360

	GDALSetMetadataItem( outputDS.get(), "CREATION_DATE", creationDateString.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "AUTHOR", mGeoPdfExportDetails.author.toLocal8Bit().constData(), nullptr );
	const QString creator = QStringLiteral( "QGIS %1" ).arg( Qgis::version() );
	GDALSetMetadataItem( outputDS.get(), "CREATOR", creator.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "PRODUCER", creator.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "SUBJECT", mGeoPdfExportDetails.subject.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "TITLE", mGeoPdfExportDetails.title.toLocal8Bit().constData(), nullptr );
	const QgsAbstractMetadataBase::KeywordMap keywords = mGeoPdfExportDetails.keywords;
	QStringList allKeywords;
	for ( auto it = keywords.constBegin(); it != keywords.constEnd(); ++it )
	{
	allKeywords.append( QStringLiteral( "%1: %2" ).arg( it.key(), it.value().join( ',' ) ) );
	}
	const QString keywordString = allKeywords.join( ';' );
	GDALSetMetadataItem( outputDS.get(), "KEYWORDS", keywordString.toLocal8Bit().constData(), nullptr );

and/or in

QGIS/src/core/layout/qgslayoutexporter.cpp

Lines 1596 to 1612 in 3b4e360

	GDALSetMetadataItem( outputDS.get(), "CREATION_DATE", creationDateString.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "AUTHOR", mLayout->project()->metadata().author().toLocal8Bit().constData(), nullptr );
	const QString creator = QStringLiteral( "QGIS %1" ).arg( Qgis::version() );
	GDALSetMetadataItem( outputDS.get(), "CREATOR", creator.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "PRODUCER", creator.toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "SUBJECT", mLayout->project()->metadata().abstract().toLocal8Bit().constData(), nullptr );
	GDALSetMetadataItem( outputDS.get(), "TITLE", mLayout->project()->metadata().title().toLocal8Bit().constData(), nullptr );
	const QgsAbstractMetadataBase::KeywordMap keywords = mLayout->project()->metadata().keywords();
	QStringList allKeywords;
	for ( auto it = keywords.constBegin(); it != keywords.constEnd(); ++it )
	{
	allKeywords.append( QStringLiteral( "%1: %2" ).arg( it.key(), it.value().join( ',' ) ) );
	}
	const QString keywordString = allKeywords.join( ';' );
	GDALSetMetadataItem( outputDS.get(), "KEYWORDS", keywordString.toLocal8Bit().constData(), nullptr );

[agiudiceandrea]

Issue fixed by #40574