Merge branch '6.3' into 6.4

* 6.3:
  [Process] Fix silencing `wait` when using a sigchild-enabled binary
  Add before/after examples to 6.3 UPGRADE guide

Author: nicolas-grekas

Process.php

@@ -331,7 +331,7 @@ public function start(callable $callback = null, array $env = [])
 
             // See https://unix.stackexchange.com/questions/71205/background-process-pipe-input
             $commandline = '{ ('.$commandline.') <&3 3<&- 3>/dev/null & } 3<&0;';
-            $commandline .= 'pid=$!; echo $pid >&3; wait $pid; code=$?; echo $code >&3; exit $code';
+            $commandline .= 'pid=$!; echo $pid >&3; wait $pid 2>/dev/null; code=$?; echo $code >&3; exit $code';
 
             // Workaround for the bug, when PTS functionality is enabled.
             // @see : https://bugs.php.net/69442

Tests/ProcessTest.php

@@ -1522,7 +1522,7 @@ public function testWaitStoppedDeadProcess()
         $process->wait();
         $this->assertFalse($process->isRunning());
 
-        if ('\\' !== \DIRECTORY_SEPARATOR) {
+        if ('\\' !== \DIRECTORY_SEPARATOR && !\Closure::bind(fn () => $this->isSigchildEnabled(), $process, $process)()) {
             $this->assertSame(0, $process->getExitCode());
         }
     }