Cygwin: posix_spawn: add Cygwin-specific code fixing process synchronisation

Newlib's posix_spawn has been taken from FreeBSD.  The code relies on
BSD-specific behaviour of vfork, namely the fact that vfork blocks
the parent until the child exits or calls execve as well as the fact
that the child shares parent memory in non-COW mode.

This behaviour can't be emulated by Cygwin.  Cygwin's vfork is
equivalent to fork.  This is POSIX-compliant, but it's lacking BSD's
vfork ingrained synchronization of the parent to wait for the child
calling execve, or the chance to just write a variable and the parent
will see the result.

So this requires a Cygwin-specific solution.  The core function of
posix_spawn, called do_posix_spawn is now implemented twice, once using
the BSD method, and once for Cygwin using Windows synchronization under
the hood waiting for the child to call execve and signalling errors
upstream.  The Windows specifics are hidden inside Cygwin, so newlib
only calls internal Cygwin functions.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

1 files changed, 64 insertions, 0 deletions

diff --git a/newlib/libc/posix/posix_spawn.c b/newlib/libc/posix/posix_spawn.c
index 19c5cd0fe..005471fde 100644
--- a/newlib/libc/posix/posix_spawn.c
+++ b/newlib/libc/posix/posix_spawn.c
@@ -254,6 +254,69 @@ process_file_actions(const posix_spawn_file_actions_t fa)
 	return (0);
 }
 
+#ifdef __CYGWIN__
+/* Cygwin's vfork does not follow BSD vfork semantics.  Rather it's equivalent
+   to fork.  While that's POSIX compliant, the below FreeBSD implementation
+   relying on BSD vfork semantics doesn't work as expected on Cygwin.  The
+   following Cygwin-specific code handles the synchronization FreeBSD gets
+   for free by using vfork. */
+
+extern int __posix_spawn_sem_create (void **semp);
+extern void __posix_spawn_sem_release (void *sem, int error);
+extern int __posix_spawn_sem_wait_and_close (void *sem, void *proc);
+extern int __posix_spawn_fork (void **proc);
+extern int __posix_spawn_execvpe (const char *path, char * const *argv,
+				  char *const *envp, void *sem,
+				  int use_env_path);
+
+
+static int
+do_posix_spawn(pid_t *pid, const char *path,
+	const posix_spawn_file_actions_t *fa,
+	const posix_spawnattr_t *sa,
+	char * const argv[], char * const envp[], int use_env_path)
+{
+	int error;
+	void *sem, *proc;
+	pid_t p;
+
+	error = __posix_spawn_sem_create(&sem);
+	if (error)
+		return error;
+
+	p = __posix_spawn_fork(&proc);
+	switch (p) {
+	case -1:
+		return (errno);
+	case 0:
+		if (sa != NULL) {
+			error = process_spawnattr(*sa);
+			if (error) {
+				__posix_spawn_sem_release(sem, error);
+				_exit(127);
+			}
+		}
+		if (fa != NULL) {
+			error = process_file_actions(*fa);
+			if (error) {
+				__posix_spawn_sem_release(sem, error);
+				_exit(127);
+			}
+		}
+		__posix_spawn_execvpe(path, argv,
+				      envp != NULL ? envp : *p_environ,
+				      sem, use_env_path);
+		_exit(127);
+	default:
+		error = __posix_spawn_sem_wait_and_close(sem, proc);
+		if (error != 0)
+			waitpid(p, NULL, WNOHANG);
+		else if (pid != NULL)
+			*pid = p;
+		return (error);
+	}
+}
+#else
 static int
 do_posix_spawn(pid_t *pid, const char *path,
 	const posix_spawn_file_actions_t *fa,
@@ -292,6 +355,7 @@ do_posix_spawn(pid_t *pid, const char *path,
 		return (error);
 	}
 }
+#endif
 
 int
 posix_spawn (pid_t *pid,