Fix reading past the end of unterminated block strings

CHANGELOG.md

@@ -9,6 +9,10 @@ You can find and compare releases at the [GitHub release page](https://github.co
 
 ## Unreleased
 
+### Fixed
+
+- Fix reading past the end of unterminated block strings https://github.com/webonyx/graphql-php/pull/1989
+
 ## v15.37.3
 
 ### Fixed

src/Language/Lexer.php

@@ -531,6 +531,12 @@ private function readString(int $line, int $col, Token $prev): Token
         throw new SyntaxError($this->source, $this->position, 'Unterminated string.');
     }
 
+    /** Compares bytes, since decoding one byte at a time would land inside multibyte characters. */
+    private function isTripleQuoteAhead(): bool
+    {
+        return substr($this->source->body, $this->byteStreamPosition, 3) === '"""';
+    }
+
     /**
      * Reads a block string token from the source file.
      *
@@ -551,52 +557,31 @@ private function readBlockString(int $line, int $col, Token $prev): Token
 
         while ($code !== null) {
             // Closing Triple-Quote (""")
-            if ($code === 34) {
-                // Move 2 quotes
-                [, $nextCode] = $this->moveStringCursor(1, 1)->readChar();
-                [, $nextNextCode] = $this->moveStringCursor(1, 1)->readChar();
+            if ($code === 34 && $this->isTripleQuoteAhead()) {
+                $value .= $chunk;
 
-                if ($nextCode === 34 && $nextNextCode === 34) {
-                    $value .= $chunk;
-
-                    $this->moveStringCursor(1, 1);
-
-                    return new Token(
-                        Token::BLOCK_STRING,
-                        $start,
-                        $this->position,
-                        $line,
-                        $col,
-                        $prev,
-                        BlockString::dedentBlockStringLines($value)
-                    );
-                }
+                $this->moveStringCursor(3, 3);
 
-                // move cursor back to before the first quote
-                $this->moveStringCursor(-2, -2);
+                return new Token(
+                    Token::BLOCK_STRING,
+                    $start,
+                    $this->position,
+                    $line,
+                    $col,
+                    $prev,
+                    BlockString::dedentBlockStringLines($value)
+                );
             }
 
             $this->assertValidBlockStringCharacterCode($code, $this->position);
             $this->moveStringCursor(1, $bytes);
 
-            [, $nextCode] = $this->readChar();
-            [, $nextNextCode] = $this->moveStringCursor(1, 1)->readChar();
-            [, $nextNextNextCode] = $this->moveStringCursor(1, 1)->readChar();
-
             // Escape Triple-Quote (\""")
-            if (
-                $code === 92
-                && $nextCode === 34
-                && $nextNextCode === 34
-                && $nextNextNextCode === 34
-            ) {
-                $this->moveStringCursor(1, 1);
+            if ($code === 92 && $this->isTripleQuoteAhead()) {
+                $this->moveStringCursor(3, 3);
                 $value .= $chunk . '"""';
                 $chunk = '';
             } else {
-                // move cursor back to before the first quote
-                $this->moveStringCursor(-2, -2);
-
                 if ($code === 10) { // new line
                     ++$this->line;
                     $this->lineStart = $this->position;

tests/Language/LexerTest.php

@@ -50,7 +50,14 @@ private function lexOne(string $body): Token
     {
         $lexer = new Lexer(new Source($body));
 
-        return $lexer->advance();
+        set_error_handler(static function (int $severity, string $message): bool {
+            throw new \ErrorException($message, 0, $severity);
+        });
+        try {
+            return $lexer->advance();
+        } finally {
+            restore_error_handler();
+        }
     }
 
     private static function loc(int $line, int $column): SourceLocation
@@ -334,6 +341,14 @@ public function testLexesStringsWithUnescapedTabCharacter(): void
         );
     }
 
+    public function testLexesBlockStringsWithMultibyteCharactersAfterQuotes(): void
+    {
+        self::assertArraySubset(
+            ['kind' => Token::BLOCK_STRING, 'start' => 0, 'end' => 11, 'value' => 'a"é\\ü'],
+            (array) $this->lexOne('"""a"é\\ü"""')
+        );
+    }
+
     /** @see it('lexes block strings') */
     public function testLexesBlockString(): void
     {
@@ -479,6 +494,9 @@ public static function reportsUsefulBlockStringErrors(): iterable
     {
         yield ['"""', 'Unterminated string.', self::loc(1, 4)];
         yield ['"""no end quote', 'Unterminated string.', self::loc(1, 16)];
+        yield ['"""a"é', 'Unterminated string.', self::loc(1, 7)];
+        yield ['""""é', 'Unterminated string.', self::loc(1, 6)];
+        yield ['"""\\é', 'Unterminated string.', self::loc(1, 6)];
         yield [
             '"""contains unescaped ' . json_decode('"\u0007"') . ' control char"""',
             'Invalid character within String: "\\u0007"',