bug symfony#59949 [Process] Use a pipe for stderr in pty mode to avoid mixed output between stdout and stderr (joelwurtz)

This PR was merged into the 6.4 branch.

Discussion
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[Process] Use a pipe for stderr in pty mode to avoid mixed output between stdout and stderr

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Issues        | Fix symfony#59927
| License       | MIT

This PR split the stderr in a pipe to correctly split stdout / stderr when using `->setPty`

When using `pty` for pipes, PHP will only open only one pty and share the "same" fd for all pipes (it use dup but the original fd is the same)

Which means there is a possibility than stdout goes to stderr (or vice versa) when using `pty` for both stdout and stderr.

I did not make a test as this behavior is erratic and happens under other conditions that are hard to find (see related issue to see reproducer, we have to do some iterations before it happens)

Commits
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ba755a8 fix(process): use a pipe for stderr in pty mode to avoid mixed output between stdout and stderr

Author: nicolas-grekas

src/Symfony/Component/Process/Pipes/UnixPipes.php

@@ -74,7 +74,7 @@ public function getDescriptors(): array
             return [
                 ['pty'],
                 ['pty'],
-                ['pty'],
+                ['pipe', 'w'], // stderr needs to be in a pipe to correctly split error and output, since PHP will use the same stream for both
             ];
         }
 

src/Symfony/Component/Process/Tests/ProcessTest.php

@@ -540,6 +540,20 @@ public function testExitCodeTextIsNullWhenExitCodeIsNull()
         $this->assertNull($process->getExitCodeText());
     }
 
+    public function testStderrNotMixedWithStdout()
+    {
+        if (!Process::isPtySupported()) {
+            $this->markTestSkipped('PTY is not supported on this operating system.');
+        }
+
+        $process = $this->getProcess('echo "foo" && echo "bar" >&2');
+        $process->setPty(true);
+        $process->run();
+
+        $this->assertSame("foo\r\n", $process->getOutput());
+        $this->assertSame("bar\n", $process->getErrorOutput());
+    }
+
     public function testPTYCommand()
     {
         if (!Process::isPtySupported()) {