os: keep attr.Files alive when calling StartProcess

Updates #34810
Fixes #34858

Change-Id: Ie934861e51eeafe8a7fd6653c4223a5f5d45efe8
Reviewed-on: https://go-review.googlesource.com/c/go/+/201198Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/os/exec_posix.go

@@ -7,6 +7,7 @@
 package os
 
 import (
+	"runtime"
 	"syscall"
 )
 
@@ -49,9 +50,14 @@ func startProcess(name string, argv []string, attr *ProcAttr) (p *Process, err e
 	}
 
 	pid, h, e := syscall.StartProcess(name, argv, sysattr)
+
+	// Make sure we don't run the finalizers of attr.Files.
+	runtime.KeepAlive(attr)
+
 	if e != nil {
 		return nil, &PathError{"fork/exec", name, e}
 	}
+
 	return newProcess(pid, h), nil
 }