Check a font is flagged before reading the keys only a used font has

src/Writer/BackgroundWriter.php

@@ -163,18 +163,27 @@ public function writePatterns() // _putpatterns
 				$this->writer->write('/Font <<');
 
 				foreach ($this->mpdf->fonts as $font) {
-					if (!$font['used'] && $font['type'] === 'TTF') {
+					// The 'used' and 'type' keys are only reachable once a font is flagged, as a
+					// core font that has drawn nothing yet carries neither
+					if (!isset($font['fo']) || !$font['fo']) {
 						continue;
 					}
-					if (isset($font['fo']) && $font['fo']) {
-						if ($font['type'] === 'TTF' && ($font['sip'] || $font['smp'])) {
+
+					if (isset($font['type']) && $font['type'] === 'TTF') {
+						if (!$font['used']) {
+							continue;
+						}
+
+						if ($font['sip'] || $font['smp']) {
 							foreach ($font['n'] as $k => $fid) {
-								$this->writer->write('/F' . $font['subsetfontids'][$k] . ' ' . $font['n'][$k] . ' 0 R');
+								$this->writer->write('/F' . $font['subsetfontids'][$k] . ' ' . $fid . ' 0 R');
 							}
-						} else {
-							$this->writer->write('/F' . $font['i'] . ' ' . $font['n'] . ' 0 R');
+
+							continue;
 						}
 					}
+
+					$this->writer->write('/F' . $font['i'] . ' ' . $font['n'] . ' 0 R');
 				}
 				$this->writer->write('>>');
 			} else {

tests/Mpdf/BackgroundPatternResourcesTest.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace Mpdf;
+
+class BackgroundPatternResourcesTest extends \Yoast\PHPUnitPolyfills\TestCases\TestCase
+{
+
+	const SVG = __DIR__ . '/../data/img/pattern-with-text.svg';
+
+	/**
+	 * The page draws no text of its own, so its core font is registered but never marked used
+	 * and carries no 'used' key at all. Reading that key before checking 'fo' is a warning,
+	 * which PHPUnit turns into a test error.
+	 */
+	public function testPatternIsWrittenWithAnUnusedCoreFont()
+	{
+		$output = $this->render();
+
+		$this->assertStringContainsString(
+			'/PatternType 1',
+			$output,
+			'Expected the repeating SVG background to be written as a tiling pattern'
+		);
+	}
+
+	/**
+	 * The pattern's resource dictionary still has to name the font its Form XObject draws with.
+	 */
+	public function testPatternResourcesNameTheSvgFont()
+	{
+		$output = $this->render();
+
+		$this->assertSame(1, preg_match('#/PatternType 1.*?/Resources (\d+) 0 R#s', $output, $pattern));
+
+		$this->assertMatchesRegularExpression(
+			'#/Font\s*<<\s*/F\d+ [1-9]\d* 0 R#',
+			$this->object($output, $pattern[1])
+		);
+	}
+
+	/**
+	 * @param  string $pdf
+	 * @param  string $number
+	 * @return string
+	 */
+	private function object($pdf, $number)
+	{
+		preg_match('/(?:^|\s)' . $number . ' 0 obj\s*(.*?)endobj/s', $pdf, $matches);
+
+		return isset($matches[1]) ? $matches[1] : '';
+	}
+
+	/**
+	 * @return string
+	 */
+	private function render()
+	{
+		$mpdf = new Mpdf(['mode' => 'c']);
+		$mpdf->compress = false;
+
+		$mpdf->WriteHTML(
+			'<div style="background-image: url(\'' . self::SVG . '\');'
+			. ' background-repeat: repeat; height: 40mm"></div>'
+		);
+
+		$output = $mpdf->OutputBinaryData();
+		$mpdf->cleanup();
+
+		return $output;
+	}
+
+}