Prep patches for supporting container overrides

[cgwalters]

This is prep for work on #3364

---

treefile: Don't bail out if /etc/rpm-ostree doesn't exist

As it doesn't by default today, even in a container.

---

ex-rebuild: Print something in the case where we found no files

Now clearly we should have much more useful information here,
but this is a start.

---

treefile: Fix parsing of derived config fields

There may be a serde bug going on here. But I think this is
fallout from 72a6a77

Basically we have #[flatten] in 3 places; in both the base
and derive structs, and the base struct has the extra member.

It seems that order matters here. In order to parse the derive
config fields, it needs to be earlier in the struct.

I am at this moment a bit uncertain why this didn't break other
cases.

But this is prep for supporting override-remove in ex rebuild
too.

---

[jlebon]

Hmm, I don't follow what either is used for here. Is this just a sneaky way to avoid having to write out the actual chained iterator type?

[cgwalters]

A return value of impl T must have exactly one type determined at compile time. Now, one can return e.g. Box<&dyn T> which does dynamic dispatch.

More at https://internals.rust-lang.org/t/extending-impl-trait-to-allow-multiple-return-types/7921

either is just a convenient enum for returning at most two different types from a function - encapsulated as a single type.

It implements Iterator if both of its element types do, and delegates to them.

[cgwalters]

Trying it out:

$ git diff
diff --git a/rust/src/treefile.rs b/rust/src/treefile.rs
index 0828d595..dfe7cba3 100644
--- a/rust/src/treefile.rs
+++ b/rust/src/treefile.rs
@@ -2292,9 +2292,9 @@ fn iter_etc_treefiles() -> Result<impl Iterator<Item = Result<PathBuf>>> {
     // TODO use cap-std-ext's https://docs.rs/cap-std-ext/latest/cap_std_ext/dirext/trait.CapStdExtDirExt.html#tymethod.open_dir_optional
     let p = Path::new(CLIENT_TREEFILES_DIR);
     if !p.exists() {
-        return Ok(either::Left(std::iter::empty()));
+        return Ok(std::iter::empty());
     }
-    Ok(either::Right(read_dir(CLIENT_TREEFILES_DIR)?.filter_map(
+    Ok(read_dir(CLIENT_TREEFILES_DIR)?.filter_map(
         |res| match res {
             Ok(e) => {
                 let path = e.path();
@@ -2307,7 +2307,7 @@ fn iter_etc_treefiles() -> Result<impl Iterator<Item = Result<PathBuf>>> {
             }
             Err(err) => Some(Err(anyhow::Error::msg(err))),
         },
-    )))
+    ))
 }
 
 /// Create a new client treefile using the treefile dropins in /etc/rpm-ostree/origin.d/.
$ cargo b
cargo b
   Compiling rpmostree-rust v0.1.0 (/var/srv/walters/src/github/coreos/rpm-ostree)
error[E0308]: mismatched types
    --> rust/src/treefile.rs:2297:8
     |
2297 |       Ok(read_dir(CLIENT_TREEFILES_DIR)?.filter_map(
     |  ________^
2298 | |         |res| match res {
2299 | |             Ok(e) => {
2300 | |                 let path = e.path();
...    |
2309 | |         },
2310 | |     ))
     | |_____^ expected struct `std::iter::Empty`, found struct `std::iter::FilterMap`
     |
     = note: expected struct `std::iter::Empty<std::result::Result<PathBuf, anyhow::Error>>`
                found struct `std::iter::FilterMap<std::fs::ReadDir, [closure@rust/src/treefile.rs:2298:9: 2309:10]>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `rpmostree-rust` due to previous error
$