PHPUnit process is blocked when there's a lot of output and a test with separate process

src/Util/PHP/DefaultPhpProcess.php

@@ -12,12 +12,15 @@
 use function array_merge;
 use function fclose;
 use function file_put_contents;
+use function fread;
 use function fwrite;
 use function is_array;
 use function is_resource;
 use function proc_close;
 use function proc_open;
-use function stream_get_contents;
+use function proc_terminate;
+use function sprintf;
+use function stream_select;
 use function sys_get_temp_dir;
 use function tempnam;
 use function unlink;
@@ -112,16 +115,64 @@ protected function runProcess(string $job, array $settings): array
 
         $stderr = $stdout = '';
 
-        if (isset($pipes[1])) {
-            $stdout = stream_get_contents($pipes[1]);
+        unset($pipes[0]);
+        $timeout = 5;
 
-            fclose($pipes[1]);
-        }
+        while (true) {
+            $r = $pipes;
+            $w = null;
+            $e = null;
+
+            $n = @stream_select($r, $w, $e, $timeout);
+
+            if ($n === false) {
+                break;
+            }
+
+            if ($n === 0) {
+                proc_terminate($process, 9);
+
+                throw new PhpProcessException(
+                    sprintf(
+                        'Job execution aborted after %d seconds',
+                        $timeout,
+                    ),
+                );
+            }
 
-        if (isset($pipes[2])) {
-            $stderr = stream_get_contents($pipes[2]);
+            if ($n > 0) {
+                foreach ($r as $pipe) {
+                    $pipeOffset = 0;
 
-            fclose($pipes[2]);
+                    foreach ($pipes as $i => $origPipe) {
+                        if ($pipe === $origPipe) {
+                            $pipeOffset = $i;
+
+                            break;
+                        }
+                    }
+
+                    if (!$pipeOffset) {
+                        break;
+                    }
+
+                    $line = fread($pipe, 8192);
+
+                    if ($line === '' || $line === false) {
+                        fclose($pipes[$pipeOffset]);
+
+                        unset($pipes[$pipeOffset]);
+                    } elseif ($pipeOffset === 1) {
+                        $stdout .= $line;
+                    } else {
+                        $stderr .= $line;
+                    }
+                }
+
+                if (empty($pipes)) {
+                    break;
+                }
+            }
         }
 
         proc_close($process);

tests/end-to-end/regression/5993.phpt

@@ -0,0 +1,14 @@
+--TEST--
+https://github.com/sebastianbergmann/phpunit/issues/5993
+--FILE--
+<?php declare(strict_types=1);
+$_SERVER['argv'][] = '--do-not-cache-result';
+$_SERVER['argv'][] = '--no-configuration';
+$_SERVER['argv'][] = '--debug';
+$_SERVER['argv'][] = __DIR__ . '/5993/Issue5993Test.php';
+
+require_once __DIR__ . '/../../bootstrap.php';
+
+(new PHPUnit\TextUI\Application)->run($_SERVER['argv']);
+--EXPECTF--
+%A

tests/end-to-end/regression/5993/Issue5993Test.php

@@ -0,0 +1,29 @@
+--TEST--
+PHPUnit process is blocked when there's a lot of output and a test with separate process
+--INI--
+error_reporting=-1
+display_errors=1
+display_startup_errors=1
+memory_limit=-1
+zend.assertions=1
+assert.exception=1
+--SKIPIF--
+<?php
+for ($i = 0; $i < 390; $i++) {
+    \trigger_error("error {$i}");
+}
+?>
+--FILE--
+<?php
+
+use PHPUnit\Framework\Attributes\RunInSeparateProcess;
+use PHPUnit\Framework\TestCase;
+
+final class Issue5993Test extends TestCase
+{
+    #[RunInSeparateProcess]
+    public function testOne(): void
+    {
+        $this->assertTrue(true);
+    }
+}