compose+rust: Parse includes via Rust too

This follows up to #1576
AKA commit 2e56784 - we now process
treefile inheritance in Rust code.  Previously for elements which
reference external files (`postprocess-script` and `add-files`)
we'd hardcoded things to only look in the first context dir.

Now we open file descriptors in the Rust side for these "externals"
as we're parsing, and load them C side.  Hence we'll correctly handle
a `postprocess-script` from an included config.

Other advantages are that the include handling was ugly un-typesafe C code
with no unit tests, now it's memory safe Rust with unit tests.

The downside here is I ended up spelling out the list of fields
again - there's probably a way to unify this via macros but
for now I think this is OK.

Closes: #1574
Approved by: jlebon

Makefile-lib.am

@@ -34,7 +34,7 @@ librpmostree_1_la_CFLAGS = $(AM_CFLAGS) -I$(srcdir)/libglnx -I$(srcdir)/src/libp
 	-fvisibility=hidden '-D_RPMOSTREE_EXTERN=__attribute((visibility("default"))) extern' \
 	$(PKGDEP_RPMOSTREE_CFLAGS)
 librpmostree_1_la_LDFLAGS = $(AM_LDFLAGS) -version-number 1:0:0 -Bsymbolic-functions
-librpmostree_1_la_LIBADD =  $(PKGDEP_RPMOSTREE_LIBS) librpmostreepriv.la
+librpmostree_1_la_LIBADD =  $(PKGDEP_RPMOSTREE_LIBS) librpmostreepriv.la $(librpmostree_rust_path)
 
 # bundled libdnf
 INTROSPECTION_SCANNER_ENV = env LD_LIBRARY_PATH=$(top_builddir)/libdnf-build/libdnf

rust/src/lib.rs

@@ -32,8 +32,9 @@ extern crate serde_json;
 extern crate serde_yaml;
 
 use std::ffi::{CStr, OsStr};
+use std::io::Seek;
 use std::os::unix::ffi::OsStrExt;
-use std::os::unix::io::{FromRawFd, IntoRawFd};
+use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd};
 use std::{io, ptr};
 
 mod glibutils;
@@ -94,6 +95,41 @@ pub extern "C" fn ror_treefile_new(
     )
 }
 
+#[no_mangle]
+pub extern "C" fn ror_treefile_get_dfd(tf: *mut Treefile) -> libc::c_int {
+    assert!(!tf.is_null());
+    let tf = unsafe { &mut *tf };
+    tf.primary_dfd.as_raw_fd()
+}
+
+#[no_mangle]
+pub extern "C" fn ror_treefile_get_postprocess_script_fd(tf: *mut Treefile) -> libc::c_int {
+    assert!(!tf.is_null());
+    let tf = unsafe { &mut *tf };
+    if let Some(ref mut fd) = tf.externals.postprocess_script.as_ref() {
+        // We always seek to the start
+        fd.seek(io::SeekFrom::Start(0)).unwrap();
+        fd.as_raw_fd()
+    } else {
+        -1
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn ror_treefile_get_add_file_fd(
+    tf: *mut Treefile,
+    filename: *const libc::c_char,
+) -> libc::c_int {
+    assert!(!tf.is_null());
+    let tf = unsafe { &mut *tf };
+    let filename = OsStr::from_bytes(bytes_from_nonnull(filename));
+    let filename = filename.to_string_lossy().into_owned();
+    let mut fd = tf.externals.add_files.get(&filename).expect("add-file");
+    // We always seek to the start
+    fd.seek(io::SeekFrom::Start(0)).unwrap();
+    fd.as_raw_fd()
+}
+
 #[no_mangle]
 pub extern "C" fn ror_treefile_to_json(
     tf: *mut Treefile,