Enable recursive directory traversal in casc

This enables recursively discovering all policy files in the specified
path(s), which is expected to be the most common use case.

Additional ergonomics, like defaulting to CWD if no path is specified is
future work.

Cargo.toml

@@ -19,3 +19,4 @@ lalrpop-util = "0.19"
 regex = "1"
 sexp = "1.1"
 thiserror = "1.0"
+walkdir = "2"

README.md

@@ -27,7 +27,9 @@ on the secilc package.
 
 ## casc
 The Cascade compiler is named casc, and will be located at target/debug/casc after a
-successful build.  It takes one argument, the name of a policy file to be built:
+successful build.  Input files are supplied as arguments.  Directory arguments
+are searched recursively for policy files.  If no valid policy files are found,
+casc will exit with an error.
 
 ```
 $ casc my_policy.cas

src/bin/casc.rs

@@ -6,19 +6,37 @@ use selinux_cascade::error::ErrorItem;
 use clap::Parser;
 use std::fs::File;
 use std::io::{Error, ErrorKind, Write};
+use walkdir::WalkDir;
 
 #[derive(Parser, Debug)]
-#[clap(author, version, name = "casc")]
+#[clap(author, version, name = "casc", about)]
 struct Args {
+    /// List of input files to process.  Directories are searched recursively.
     #[clap(required(true))]
     input_file: Vec<String>,
 }
 
 fn main() -> std::io::Result<()> {
     let args = Args::parse();
-    let policies: Vec<&str> = args.input_file.iter().map(|s| s as &str).collect();
+    let policies: Vec<String> = match get_policy_files(args.input_file) {
+        Ok(mut s) => {
+            s.sort_unstable();
+            s
+        }
+        Err(e) => {
+            eprintln!("{}", e);
+            return Err(e);
+        }
+    };
+    if policies.is_empty() {
+        // Files supplied on command line, but no .cas files found
+        return Err(Error::new(
+            ErrorKind::InvalidData,
+            "No policy source files found",
+        ));
+    }
     let mut out_file = File::create("out.cil")?;
-    let res = compile_system_policy(policies);
+    let res = compile_system_policy(policies.iter().map(|s| s as &str).collect());
     match res {
         Err(error_list) => {
             for e in error_list {
@@ -35,3 +53,19 @@ fn main() -> std::io::Result<()> {
         Ok(s) => out_file.write_all(s.as_bytes()),
     }
 }
+
+// Create a list of policy files
+fn get_policy_files(filenames: Vec<String>) -> Result<Vec<String>, Error> {
+    let mut policy_files = Vec::new();
+    for file in filenames {
+        for entry in WalkDir::new(file) {
+            let entry = entry?;
+            if entry.file_type().is_file() && entry.path().extension().unwrap_or_default() == "cas"
+            {
+                let filename = entry.path().display().to_string();
+                policy_files.push(filename);
+            }
+        }
+    }
+    Ok(policy_files)
+}