Demonstrate Rust code from sysroot

This is a demo/prototype for starting to add new Rust
code only for `OstreeSysroot`.

https://github.com/ostreedev/ostree/issues/2427#issuecomment-1728210940

Cargo.toml

@@ -31,7 +31,7 @@ name = "integration"
 path = "rust-bindings/tests/tests.rs"
 
 [workspace]
-members = [".", "rust-bindings/sys"]
+members = [".", "src/sysroot-rs", "rust-bindings/sys"]
 
 [dependencies]
 base64 = "0.20.0"

Makefile-sysroot-rs.am

@@ -0,0 +1,23 @@
+# Makefile for Rust lib
+#
+# SPDX-License-Identifier: LGPL-2.0+
+#
+# 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, see <https://www.gnu.org/licenses/>.
+
+
+target/release/libostreesysrs.so: src/sysroot-rs/src/lib.rs
+	cargo build --release --workspace
+
+libostreesysrs_DATA = target/release/libostreesysrs.so
+libostreesysrsdir = $(pkglibexecdir)

Makefile.am

@@ -134,6 +134,7 @@ include Makefile-switchroot.am
 if BUILDOPT_FUSE
 include src/rofiles-fuse/Makefile-inc.am
 endif
+include Makefile-sysroot-rs.am
 include Makefile-tests.am
 include Makefile-boot.am
 include Makefile-man.am

src/libostree/ostree-sysroot.c

@@ -21,6 +21,7 @@
 #include "config.h"
 
 #include "otutil.h"
+#include <dlfcn.h>
 #include <err.h>
 #include <sys/file.h>
 #include <sys/mount.h>
@@ -76,6 +77,29 @@ enum
 
 G_DEFINE_TYPE (OstreeSysroot, ostree_sysroot, G_TYPE_OBJECT)
 
+static void *
+load_sysroot_rs (void)
+{
+  static gssize initialized = 0;
+  static void *sysrootrs = NULL;
+  if (g_once_init_enter (&initialized))
+    {
+      const char *uninstalled = g_getenv ("OSTREE_UNINSTALLED");
+      g_autofree char *path = NULL;
+      if (uninstalled)
+        path = g_strdup_printf ("%s/target/release/libostreesysrs.so", uninstalled);
+      else
+        path = g_strdup_printf ("%s/libostreesysrs.so", PKGLIBEXECDIR);
+      sysrootrs = dlopen (path, RTLD_NOW);
+      if (sysrootrs == NULL)
+        {
+          errx (EXIT_FAILURE, "failed to initialize libostreesysrs.so: %s", dlerror ());
+        }
+      g_once_init_leave (&initialized, 1);
+    }
+  return sysrootrs;
+}
+
 static void
 ostree_sysroot_finalize (GObject *object)
 {
@@ -447,11 +471,16 @@ ostree_sysroot_is_booted (OstreeSysroot *self)
 gboolean
 _ostree_sysroot_bump_mtime (OstreeSysroot *self, GError **error)
 {
+  void *lib = load_sysroot_rs ();
+  int (*f) (int fd) = dlsym (lib, "_ostreesys_bump_mtime");
+  g_assert (f);
+
   /* Allow other systems to monitor for changes */
-  if (utimensat (self->sysroot_fd, "ostree/deploy", NULL, 0) < 0)
+  int r = f (self->sysroot_fd);
+  if (r < 0)
     {
-      glnx_set_prefix_error_from_errno (error, "%s", "futimens");
-      return FALSE;
+      errno = r;
+      return glnx_throw_errno_prefix (error, "futimens");
     }
   return TRUE;
 }

src/sysroot-rs/Cargo.toml

@@ -0,0 +1,20 @@
+[package]
+description = "Internal Rust sysroot code"
+edition = "2021"
+keywords = ["ostree", "libostree"]
+license = "MIT"
+name = "ostreesysrs"
+readme = "rust-bindings/README.md"
+repository = "https://github.com/ostreedev/ostree"
+rust-version = "1.70.0"
+version = "0.0.0"
+publish = false
+
+[lib]
+name = "ostreesysrs"
+crate-type = ["cdylib"]
+
+[dependencies]
+ostree = { package = "ostree", path = "../..", version = "0.19", features = ["v2022_6"] }
+libc = "0.2"
+rustix = { version = "0.38", features = ["fs"] }

src/sysroot-rs/src/lib.rs

@@ -0,0 +1,17 @@
+#[no_mangle]
+pub extern "C" fn _ostreesys_bump_mtime(fd: libc::c_int) -> libc::c_int {
+    let fd = unsafe { std::os::fd::BorrowedFd::borrow_raw(fd) };
+    let now = rustix::fs::Timespec {
+        tv_sec: 0,
+        tv_nsec: rustix::fs::UTIME_NOW,
+    };
+    let ts = rustix::fs::Timestamps {
+        last_access: now.clone(),
+        last_modification: now.clone(),
+    };
+    if let Err(e) = rustix::fs::utimensat(fd, "ostree/deploy", &ts, rustix::fs::AtFlags::empty()) {
+        e.raw_os_error().into()
+    } else {
+        0
+    }
+}