fdio: Expose wrappers for renameat2() EXCHANGE and NOREPLACE

I want the `RENAME_EXCHANGE` version for rpm-ostree, to atomically
swap `/usr/share/rpm` (a directory) with a new verison.  While
we're here we might as well expose `RENAME_NOREPLACE` in case
something else wants it.

These both have fallbacks to the non-atomic version.

Closes: https://github.com/GNOME/libglnx/pull/36

Makefile-libglnx.am

@@ -52,9 +52,14 @@ libglnx_la_CFLAGS = $(AM_CFLAGS) $(libglnx_cflags)
 libglnx_la_LDFLAGS = -avoid-version -Bsymbolic-functions -export-symbols-regex "^glnx_" -no-undefined -export-dynamic 
 libglnx_la_LIBADD = $(libglnx_libs)
 
-TESTS += test-libglnx-xattrs
+libglnx_tests = test-libglnx-xattrs test-libglnx-fdio
+TESTS += $(libglnx_tests)
 
-check_PROGRAMS += test-libglnx-xattrs
+check_PROGRAMS += $(libglnx_tests)
 test_libglnx_xattrs_SOURCES = $(libglnx_srcpath)/tests/test-libglnx-xattrs.c
 test_libglnx_xattrs_CFLAGS = $(AM_CFLAGS) $(libglnx_cflags)
 test_libglnx_xattrs_LDADD = $(libglnx_libs) libglnx.la
+
+test_libglnx_fdio_SOURCES = $(libglnx_srcpath)/tests/test-libglnx-fdio.c
+test_libglnx_fdio_CFLAGS = $(AM_CFLAGS) $(libglnx_cflags)
+test_libglnx_fdio_LDADD = $(libglnx_libs) libglnx.la

glnx-fdio.c

@@ -54,11 +54,13 @@
             sizeof(type) <= 4 ? 10 :                                    \
             sizeof(type) <= 8 ? 20 : sizeof(int[-2*(sizeof(type) > 8)])))
 
-static gboolean
-rename_file_noreplace_at (int olddirfd, const char *oldpath,
-                          int newdirfd, const char *newpath,
-                          gboolean ignore_eexist,
-                          GError **error)
+
+/* An implementation of renameat2(..., RENAME_NOREPLACE)
+ * with fallback to a non-atomic version.
+ */
+int
+glnx_renameat2_noreplace (int olddirfd, const char *oldpath,
+                          int newdirfd, const char *newpath)
 {
 #ifndef ENABLE_WRPSEUDO_COMPAT
   if (renameat2 (olddirfd, oldpath, newdirfd, newpath, RENAME_NOREPLACE) < 0)
@@ -66,9 +68,35 @@ rename_file_noreplace_at (int olddirfd, const char *oldpath,
       if (errno == EINVAL || errno == ENOSYS)
         {
           /* Fall through */
-          ;
         }
-      else if (errno == EEXIST && ignore_eexist)
+      else
+        {
+          return -1;
+        }
+    }
+  else
+    return TRUE;
+#endif
+
+  if (linkat (olddirfd, oldpath, newdirfd, newpath, 0) < 0)
+    return -1;
+
+  if (unlinkat (olddirfd, oldpath, 0) < 0)
+    return -1;
+
+  return 0;
+}
+
+static gboolean
+rename_file_noreplace_at (int olddirfd, const char *oldpath,
+                          int newdirfd, const char *newpath,
+                          gboolean ignore_eexist,
+                          GError **error)
+{
+  if (glnx_renameat2_noreplace (olddirfd, oldpath,
+                                newdirfd, newpath) < 0)
+    {
+      if (errno == EEXIST && ignore_eexist)
         {
           (void) unlinkat (olddirfd, oldpath, 0);
           return TRUE;
@@ -79,29 +107,46 @@ rename_file_noreplace_at (int olddirfd, const char *oldpath,
           return FALSE;
         }
     }
-  else
-    return TRUE;
-#endif
+  return TRUE;
+}
 
-  if (linkat (olddirfd, oldpath, newdirfd, newpath, 0) < 0)
+/* An implementation of renameat2(..., RENAME_EXCHANGE)
+ * with fallback to a non-atomic version.
+ */
+int
+glnx_renameat2_exchange (int olddirfd, const char *oldpath,
+                         int newdirfd, const char *newpath)
+{
+#ifndef ENABLE_WRPSEUDO_COMPAT
+  if (renameat2 (olddirfd, oldpath, newdirfd, newpath, RENAME_EXCHANGE) == 0)
+    return 0;
+  else
     {
-      if (errno == EEXIST && ignore_eexist)
-        /* Fall through */
-        ;
+      if (errno == ENOSYS || errno == EINVAL)
+        {
+          /* Fall through */
+        }
       else
         {
-          glnx_set_error_from_errno (error);
-          return FALSE;
+          return -1;
         }
     }
-  
-  if (unlinkat (olddirfd, oldpath, 0) < 0)
-    {
-      glnx_set_error_from_errno (error);
-      return FALSE;
-    }
+#endif
 
-  return TRUE;
+  /* Fallback */
+  { const char *old_tmp_name = glnx_strjoina (oldpath, ".XXXXXX");
+
+    /* Move old out of the way */
+    if (renameat (olddirfd, oldpath, olddirfd, old_tmp_name) < 0)
+      return -1;
+    /* Now move new into its place */
+    if (renameat (newdirfd, newpath, olddirfd, oldpath) < 0)
+      return -1;
+    /* And finally old(tmp) into new */
+    if (renameat (olddirfd, old_tmp_name, newdirfd, newpath) < 0)
+      return -1;
+  }
+  return 0;
 }
 
 gboolean

glnx-fdio.h

@@ -150,4 +150,10 @@ glnx_stream_fstat (GFileDescriptorBased *stream,
                    struct stat          *stbuf,
                    GError              **error);
 
+int glnx_renameat2_noreplace (int olddirfd, const char *oldpath,
+                              int newdirfd, const char *newpath);
+int glnx_renameat2_exchange (int olddirfd, const char *oldpath,
+                             int newdirfd, const char *newpath);
+
+
 G_END_DECLS

glnx-missing.h

@@ -48,5 +48,8 @@
 #ifndef RENAME_NOREPLACE
 #define RENAME_NOREPLACE (1 << 0)
 #endif
+#ifndef RENAME_EXCHANGE
+#define RENAME_EXCHANGE (1 << 1)
+#endif
 
 #include "glnx-missing-syscall.h"

tests/test-libglnx-fdio.c

@@ -0,0 +1,155 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2017 Red Hat, Inc.
+ *
+ * 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.
+ */
+
+#include "config.h"
+#include "libglnx.h"
+#include <glib.h>
+#include <stdlib.h>
+#include <gio/gio.h>
+#include <err.h>
+#include <string.h>
+
+static gboolean
+renameat_test_setup (int *out_srcfd, int *out_destfd,
+                     GError **error)
+{
+  glnx_fd_close int srcfd = -1;
+  glnx_fd_close int destfd = -1;
+
+  (void) glnx_shutil_rm_rf_at (AT_FDCWD, "srcdir", NULL, NULL);
+  if (mkdir ("srcdir", 0755) < 0)
+    err (1, "mkdir");
+  if (!glnx_opendirat (AT_FDCWD, "srcdir", TRUE, &srcfd, error))
+    return FALSE;
+  (void) glnx_shutil_rm_rf_at (AT_FDCWD, "destdir", NULL, NULL);
+  if (mkdir ("destdir", 0755) < 0)
+    err (1, "mkdir");
+  if (!glnx_opendirat (AT_FDCWD, "destdir", TRUE, &destfd, error))
+    return FALSE;
+
+  if (!glnx_file_replace_contents_at (srcfd, "foo", (guint8*)"foo contents", strlen ("foo contents"),
+                                      GLNX_FILE_REPLACE_NODATASYNC, NULL, error))
+    return FALSE;
+  if (!glnx_file_replace_contents_at (destfd, "bar", (guint8*)"bar contents", strlen ("bar contents"),
+                                      GLNX_FILE_REPLACE_NODATASYNC, NULL, error))
+    return FALSE;
+
+  *out_srcfd = srcfd; srcfd = -1;
+  *out_destfd = destfd; destfd = -1;
+  return TRUE;
+}
+
+static void
+test_renameat2_noreplace (void)
+{
+  g_autoptr(GError) local_error = NULL;
+  GError **error = &local_error;
+  glnx_fd_close int srcfd = -1;
+  glnx_fd_close int destfd = -1;
+  struct stat stbuf;
+
+  if (!renameat_test_setup (&srcfd, &destfd, error))
+    goto out;
+
+  if (glnx_renameat2_noreplace (srcfd, "foo", destfd, "bar") == 0)
+    g_assert_not_reached ();
+  else
+    {
+      g_assert_cmpint (errno, ==, EEXIST);
+    }
+
+  if (glnx_renameat2_noreplace (srcfd, "foo", destfd, "baz") < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+  if (fstatat (destfd, "bar", &stbuf, AT_SYMLINK_NOFOLLOW) < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+
+  if (fstatat (srcfd, "foo", &stbuf, AT_SYMLINK_NOFOLLOW) == 0)
+    g_assert_not_reached ();
+  else
+    g_assert_cmpint (errno, ==, ENOENT);
+
+ out:
+  g_assert_no_error (local_error);
+}
+
+static void
+test_renameat2_exchange (void)
+{
+  g_autoptr(GError) local_error = NULL;
+  GError **error = &local_error;
+  glnx_fd_close int srcfd = -1;
+  glnx_fd_close int destfd = -1;
+  struct stat stbuf;
+
+  if (!renameat_test_setup (&srcfd, &destfd, error))
+    goto out;
+
+  if (glnx_renameat2_exchange (AT_FDCWD, "srcdir", AT_FDCWD, "destdir") < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+
+  /* Ensure the dir fds are the same */
+  if (fstatat (srcfd, "foo", &stbuf, AT_SYMLINK_NOFOLLOW) < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+  if (fstatat (destfd, "bar", &stbuf, AT_SYMLINK_NOFOLLOW) < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+  /* But the dirs should be swapped */
+  if (fstatat (AT_FDCWD, "destdir/foo", &stbuf, AT_SYMLINK_NOFOLLOW) < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+  if (fstatat (AT_FDCWD, "srcdir/bar", &stbuf, AT_SYMLINK_NOFOLLOW) < 0)
+    {
+      glnx_set_error_from_errno (error);
+      goto out;
+    }
+
+ out:
+  g_assert_no_error (local_error);
+}
+
+int main (int argc, char **argv)
+{
+  int ret;
+
+  g_test_init (&argc, &argv, NULL);
+
+  g_test_add_func ("/renameat2-noreplace", test_renameat2_noreplace);
+  g_test_add_func ("/renameat2-exchange", test_renameat2_exchange);
+
+  ret = g_test_run();
+
+  return ret;
+}