glib-unix: New Unix-specific API

GLib historically has been designed to be "mostly" portable; there
are some functions only available on Unix like g_io_channel_unix_new(),
but these are typically paired with obvious counterparts for Win32.

However, as GLib is used not only by portable software, but components
targeting Unix (or even just Linux), there are a few cases where it
would be very convenient if GLib shipped built-in functionality.

This initial patch is a basic wrapper around pipe2(), including
fallbacks for older kernels.  This pairs well with the
existing g_spawn_*() API and its child_setup functionality.

However, in the future, I want to add a signal() wrapper here,
complete with proxying the signal to a mainloop.  I have initial code
for this, but doing it sanely (including factoring out gmain.c's
private worker thread), is a complex task, and I don't want to block
on that.

See also gwin32.h for Win32 specific functionality.

https://bugzilla.gnome.org/show_bug.cgi?id=644941

glib/glib-unix.c

@@ -0,0 +1,135 @@
+/* GLIB - Library of useful routines for C programming
+ * Copyright (C) 2011 Red Hat, Inc.
+ *
+ * glib-unix.c: Unix specific API wrappers and convenience functions 
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.	 See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Authors: Colin Walters <walters@verbum.org>
+ */
+
+#include "config.h"
+
+#include "glib-unix.h"
+
+#include <string.h>
+
+/**
+ * SECTION:gunix
+ * @short_description: Unix-specific utilities and integration
+ * @include: glib-unix.h
+ *
+ * Most of GLib is intended to be portable; in constrast, this set of
+ * functions is designed for programs which explicitly target UNIX, or
+ * are using it to build higher level abstractions which would be
+ * conditionally compiled if the platform matches G_OS_UNIX.
+ *
+ * To use these functions, you must explicitly include the
+ * "glib-unix.h" header.
+ */
+
+GQuark
+g_unix_error_quark (void)
+{
+  return g_quark_from_static_string ("g-unix-error-quark");
+}
+
+static gboolean
+g_unix_set_error_from_errno (GError **error)
+{
+  int saved_errno = errno;
+  g_set_error_literal (error,
+		       G_UNIX_ERROR,
+		       0,
+		       g_strerror (errno));
+  errno = saved_errno;
+  return FALSE;
+}
+
+static gboolean
+g_unix_set_error_from_errno_saved (GError **error,
+				    int      saved_errno)
+{
+  g_set_error_literal (error,
+		       G_UNIX_ERROR,
+		       0,
+		       g_strerror (saved_errno));
+  errno = saved_errno;
+  return FALSE;
+}
+
+/**
+ * g_unix_pipe_flags:
+ * @fds: Array of two integers
+ * @flags: Bitfield of file descriptor flags, see "man 2 fcntl"
+ * @error: a #GError
+ *
+ * Similar to the Unix pipe() call, but on modern systems like
+ * Linux uses the pipe2 system call, which atomically creates
+ * a pipe with the configured flags.  The only supported flag
+ * currently is FD_CLOEXEC.  If for example you want to configure
+ * O_NONBLOCK, that must still be done separately with fcntl().
+ *
+ * Note in particular this function does *not* take O_CLOEXEC, it
+ * takes FD_CLOEXEC as if for fcntl(); these are different on
+ * Linux/glibc.
+ *
+ * Returns: %TRUE on success, %FALSE if not (and errno will be set).
+ *
+ * Since: 2.30
+ */
+gboolean
+g_unix_pipe_flags (int     *fds,
+		   int      flags,
+		   GError **error)
+{
+  int ecode;
+
+  /* We only support FD_CLOEXEC */
+  g_return_val_if_fail ((flags & (FD_CLOEXEC)) == flags, FALSE);
+
+#ifdef HAVE_PIPE2
+  {
+    int pipe2_flags = 0;
+    if (flags & FD_CLOEXEC)
+      pipe2_flags |= O_CLOEXEC;
+    /* Atomic */
+    ecode = pipe2 (fds, pipe2_flags);
+    if (ecode == -1 && errno != ENOSYS)
+      return g_unix_set_error_from_errno (error);
+    /* Fall through on -ENOSYS, we must be running on an old kernel */
+  }
+#endif
+  ecode = pipe (fds);
+  if (ecode == -1)
+    return g_unix_set_error_from_errno (error);
+  ecode = fcntl (fds[0], flags);
+  if (ecode == -1)
+    {
+      int saved_errno = errno;
+      close (fds[0]);
+      return g_unix_set_error_from_errno_saved (error, saved_errno);
+    }
+  ecode = fcntl (fds[0], flags);
+  if (ecode == -1)
+    {
+      int saved_errno = errno;
+      close (fds[0]);
+      close (fds[1]);
+      return g_unix_set_error_from_errno_saved (error, saved_errno);
+    }
+  return TRUE;
+}