Add serial-buildorder argument to the build-tree

- Introduced a new command-line argument `--serial-buildorder` that
  outputs the tree-of package in a serial build order.
- The `--serial-buildorder` argument conflicts with the existing `--dot`
  argument for now. Showing the serial build order in the dot format
  would not bring any benefits.
- Ensured cyclic dependency detection and error handling during
  topological sorting.

Fixes #379

Signed-off-by: Nico Steinle <nico.steinle@eviden.com>

src/cli.rs

@@ -1024,6 +1024,24 @@ pub fn cli() -> Command {
                 .required(false)
                 .long("dot")
                 .help("Output the dependency DAG in the Graphviz DOT format")
+                .conflicts_with("serial-buildorder")
+            )
+            .arg(Arg::new("serial-buildorder")
+                .action(ArgAction::SetTrue)
+                .required(false)
+                .long("serial-buildorder")
+                .help("Output the dependencies in a serial build order (flattened tree)")
+                .long_help(indoc::indoc!(r#"
+                    A serial build order ensures that dependencies are built one at a time
+                    in a specific sequence. This approach follows a reversed topological ordering,
+                    where dependencies (or children) are listed and built first. Each dependency
+                    must be fully built before the next one begins, ensuring all components are
+                    available when needed.
+
+                    Keep in mind that the actual build order remains parallel, this serialized
+                    output is mainly useful for debugging purposes.
+                "#))
+                .conflicts_with("dot")
             )
         )
 

src/commands/tree_of.rs

@@ -60,6 +60,8 @@ pub async fn tree_of(matches: &ArgMatches, repo: Repository, config: &Configurat
 
     let dot = matches.get_flag("dot");
 
+    let serial_buildorder = matches.get_flag("serial-buildorder");
+
     repo.packages()
         .filter(|p| pname.as_ref().map(|n| p.name() == n).unwrap_or(true))
         .filter(|p| {
@@ -92,6 +94,19 @@ pub async fn tree_of(matches: &ArgMatches, repo: Repository, config: &Configurat
                 );
 
                 println!("{:?}", dot);
+                Ok(())
+            } else if serial_buildorder {
+                let petgraph: DiGraph<Package, DependencyType> = (*dag.dag()).clone().into();
+
+                let topo_sorted = petgraph::algo::toposort(&petgraph, None)
+                    .map_err(|_| Error::msg("Cyclic dependency found!"))?;
+
+                for node in topo_sorted.iter().rev() {
+                    let package = petgraph.node_weight(*node).unwrap();
+                    println!("{}", package.clone().display_name_version());
+                }
+                println!();
+
                 Ok(())
             } else {
                 let stdout = std::io::stdout();