tool: make file installation deterministic

Due to the use of hash maps, the order of file installation was not
deterministic. I've changed the code the use BTreeMaps instead, which
makes this deterministic. While I was here, I tried to simplify the
code a bit.

rust/tool/src/install.rs

@@ -1,4 +1,4 @@
-use std::collections::HashMap;
+use std::collections::BTreeMap;
 use std::fs;
 use std::os::unix::prelude::PermissionsExt;
 use std::path::{Path, PathBuf};
@@ -259,15 +259,17 @@ impl Installer {
             .write_secure_file(os_release.to_string().as_bytes())
             .context("Failed to write os-release file.")?;
 
-        let kernel_path = generation_artifacts
-            .unsigned_files
+        let kernel_path: &Path = generation_artifacts
+            .files
             .get(&esp_gen_paths.kernel)
-            .context("Failed to retrieve kernel path from GenerationArtifacts.")?;
+            .context("Failed to retrieve kernel path from GenerationArtifacts.")?
+            .into();
 
         let initrd_path = generation_artifacts
-            .unsigned_files
+            .files
             .get(&esp_gen_paths.initrd)
-            .context("Failed to retrieve initrd path from GenerationArtifacts.")?;
+            .context("Failed to retrieve initrd path from GenerationArtifacts.")?
+            .into();
 
         let lanzaboote_image = pe::lanzaboote_image(
             tempdir,
@@ -326,6 +328,22 @@ impl Installer {
     }
 }
 
+/// A location in the ESP together with information whether the file
+/// needs to be signed.
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum FileSource {
+    SignedFile(PathBuf),
+    UnsignedFile(PathBuf),
+}
+
+impl<'a> From<&'a FileSource> for &'a Path {
+    fn from(value: &'a FileSource) -> Self {
+        match value {
+            FileSource::SignedFile(p) | FileSource::UnsignedFile(p) => p,
+        }
+    }
+}
+
 /// Stores the source and destination of all artifacts needed to install all generations.
 ///
 /// The key feature of this data structure is that the mappings are automatically deduplicated
@@ -342,45 +360,56 @@ struct GenerationArtifacts {
     /// Temporary directory that stores all temporary files that are created when building the
     /// GenerationArtifacts.
     tempdir: TempDir,
-    /// Mapping of signed files from their destinations to their source.
-    signed_files: HashMap<PathBuf, PathBuf>,
-    /// Mapping of unsigned files from their destinations to their source.
-    unsigned_files: HashMap<PathBuf, PathBuf>,
+
+    /// A mapping from target location to source.
+    files: BTreeMap<PathBuf, FileSource>,
 }
 
 impl GenerationArtifacts {
     fn new() -> Result<Self> {
         Ok(Self {
             tempdir: TempDir::new().context("Failed to create temporary directory.")?,
-            signed_files: HashMap::new(),
-            unsigned_files: HashMap::new(),
+            files: Default::default(),
         })
     }
 
+    /// Add a file to be installed.
+    ///
+    /// Adding the same file multiple times with the same source is ok
+    /// and will drop the old source.
+    fn add_file(&mut self, from: FileSource, to: &Path) {
+        if let Some(_prev_from) = self.files.insert(to.to_path_buf(), from) {
+            // Should we log something here?
+        }
+    }
+
     /// Add source and destination of a PE file to be signed.
     ///
     /// Files are stored in the HashMap using their destination path as the key to ensure that the
     /// destination paths are unique.
     fn add_signed(&mut self, from: &Path, to: &Path) {
-        self.signed_files.insert(to.into(), from.into());
+        self.add_file(FileSource::SignedFile(from.to_path_buf()), to);
     }
 
     /// Add source and destination of an arbitrary file.
     fn add_unsigned(&mut self, from: &Path, to: &Path) {
-        self.unsigned_files.insert(to.into(), from.into());
+        self.add_file(FileSource::UnsignedFile(from.to_path_buf()), to);
     }
 
     /// Install all files to the ESP.
     fn install(&self, key_pair: &KeyPair) -> Result<()> {
-        for (to, from) in &self.signed_files {
-            install_signed(key_pair, from, to)
-                .with_context(|| format!("Failed to sign and install from {from:?} to {to:?}"))?;
+        for (to, from) in &self.files {
+            match from {
+                FileSource::SignedFile(from) => {
+                    install_signed(key_pair, from, to).with_context(|| {
+                        format!("Failed to sign and install from {from:?} to {to:?}")
+                    })?
+                }
+                FileSource::UnsignedFile(from) => install(from, to)
+                    .with_context(|| format!("Failed to install from {from:?} to {to:?}"))?,
+            }
         }
 
-        for (to, from) in &self.unsigned_files {
-            install(from, to)
-                .with_context(|| format!("Failed to install from {from:?} to {to:?}"))?;
-        }
         Ok(())
     }
 }