[10.x] Fix decoding issue in MailLogTransport (#49727)

* Handle multipart messages and only decode content

* formatting

---------

Co-authored-by: Taylor Otwell <[email protected]>

src/Illuminate/Mail/Transport/LogTransport.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Mail\Transport;
 
+use Illuminate\Support\Str;
 use Psr\Log\LoggerInterface;
 use Symfony\Component\Mailer\Envelope;
 use Symfony\Component\Mailer\SentMessage;
@@ -33,17 +34,48 @@ public function __construct(LoggerInterface $logger)
      */
     public function send(RawMessage $message, Envelope $envelope = null): ?SentMessage
     {
-        $string = $message->toString();
+        $string = Str::of($message->toString());
 
-        if (str_contains($string, 'Content-Transfer-Encoding: quoted-printable')) {
-            $string = quoted_printable_decode($string);
+        if ($string->contains('Content-Type: multipart/')) {
+            $boundary = $string
+                ->after('boundary=')
+                ->before("\r\n")
+                ->prepend('--')
+                ->append("\r\n");
+
+            $string = $string
+                ->explode($boundary)
+                ->map($this->decodeQuotedPrintableContent(...))
+                ->implode($boundary);
+        } elseif ($string->contains('Content-Transfer-Encoding: quoted-printable')) {
+            $string = $this->decodeQuotedPrintableContent($string);
         }
 
-        $this->logger->debug($string);
+        $this->logger->debug((string) $string);
 
         return new SentMessage($message, $envelope ?? Envelope::create($message));
     }
 
+    /**
+     * Decode the given quoted printable content.
+     *
+     * @param  string  $part
+     * @return string
+     */
+    protected function decodeQuotedPrintableContent(string $part)
+    {
+        if (! str_contains($part, 'Content-Transfer-Encoding: quoted-printable')) {
+            return $part;
+        }
+
+        [$headers, $content] = explode("\r\n\r\n", $part, 2);
+
+        return implode("\r\n\r\n", [
+            $headers,
+            quoted_printable_decode($content),
+        ]);
+    }
+
     /**
      * Get the logger for the LogTransport instance.
      *

tests/Mail/MailLogTransportTest.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Tests\Mail;
 
+use Illuminate\Mail\Attachment;
 use Illuminate\Mail\Message;
 use Illuminate\Mail\Transport\LogTransport;
 use Monolog\Handler\StreamHandler;
@@ -59,6 +60,32 @@ public function testItDecodesTheMessageBeforeLogging()
         $this->assertStringContainsString('https://example.com/reset-password=5e113c71a4c210aff04b3fa66f1b1299', $actualLoggedValue);
     }
 
+    public function testItOnlyDecodesQuotedPrintablePartsOfTheMessageBeforeLogging()
+    {
+        $message = (new Message(new Email))
+            ->from('[email protected]', 'no-reply')
+            ->to('[email protected]', 'Taylor')
+            ->html(<<<'BODY'
+            Hi,
+
+            <a href="https://example.com/reset-password=5e113c71a4c210aff04b3fa66f1b1299">Click here to reset your password</a>.
+
+            All the best,
+
+            Burt & Irving
+            BODY)
+            ->text('A text part')
+            ->attach(Attachment::fromData(fn () => 'My attachment', 'attachment.txt'));
+
+        $actualLoggedValue = $this->getLoggedEmailMessage($message);
+
+        $this->assertStringContainsString('href=', $actualLoggedValue);
+        $this->assertStringContainsString('Burt & Irving', $actualLoggedValue);
+        $this->assertStringContainsString('https://example.com/reset-password=5e113c71a4c210aff04b3fa66f1b1299', $actualLoggedValue);
+        $this->assertStringContainsString('name=attachment.txt', $actualLoggedValue);
+        $this->assertStringContainsString('filename=attachment.txt', $actualLoggedValue);
+    }
+
     public function testGetLogTransportWithPsrLogger()
     {
         $this->app['config']->set('mail.driver', 'log');