authfs: replace tuple w/ anonymous struct in FileConfig

Also added some documentation.

Bug: 173507504
Test: atest
Change-Id: I34b3390531273a3b0d0ae1ef30adbed2164e57cf
diff --git a/authfs/src/main.rs b/authfs/src/main.rs
index a4b0d40..0db73e9 100644
--- a/authfs/src/main.rs
+++ b/authfs/src/main.rs
@@ -207,8 +207,8 @@
 
     let service = Arc::new(Mutex::new(service));
     let authenticator = FakeAuthenticator::always_succeed();
-    Ok(FileConfig::RemoteVerifiedReadonlyFile(
-        VerifiedFileReader::new(
+    Ok(FileConfig::RemoteVerifiedReadonlyFile {
+        reader: VerifiedFileReader::new(
             &authenticator,
             RemoteFileReader::new(Arc::clone(&service), remote_id),
             file_size,
@@ -216,13 +216,12 @@
             RemoteMerkleTreeReader::new(Arc::clone(&service), remote_id),
         )?,
         file_size,
-    ))
+    })
 }
 
 fn new_config_remote_unverified_file(remote_id: i32, file_size: u64) -> Result<FileConfig> {
-    let file_reader =
-        RemoteFileReader::new(Arc::new(Mutex::new(file::get_local_binder())), remote_id);
-    Ok(FileConfig::RemoteUnverifiedReadonlyFile(file_reader, file_size))
+    let reader = RemoteFileReader::new(Arc::new(Mutex::new(file::get_local_binder())), remote_id);
+    Ok(FileConfig::RemoteUnverifiedReadonlyFile { reader, file_size })
 }
 
 fn new_config_local_ro_file(
@@ -237,21 +236,21 @@
     let authenticator = FakeAuthenticator::always_succeed();
     let mut sig = Vec::new();
     let _ = File::open(signature)?.read_to_end(&mut sig)?;
-    let file_reader =
+    let reader =
         VerifiedFileReader::new(&authenticator, file_reader, file_size, sig, merkle_tree_reader)?;
-    Ok(FileConfig::LocalVerifiedReadonlyFile(file_reader, file_size))
+    Ok(FileConfig::LocalVerifiedReadonlyFile { reader, file_size })
 }
 
 fn new_config_local_ro_file_unverified(file_path: &PathBuf) -> Result<FileConfig> {
-    let file_reader = LocalFileReader::new(File::open(file_path)?)?;
-    let file_size = file_reader.len();
-    Ok(FileConfig::LocalUnverifiedReadonlyFile(file_reader, file_size))
+    let reader = LocalFileReader::new(File::open(file_path)?)?;
+    let file_size = reader.len();
+    Ok(FileConfig::LocalUnverifiedReadonlyFile { reader, file_size })
 }
 
 fn new_config_remote_new_verified_file(remote_id: i32) -> Result<FileConfig> {
     let remote_file =
         RemoteFileEditor::new(Arc::new(Mutex::new(file::get_local_binder())), remote_id);
-    Ok(FileConfig::RemoteVerifiedNewFile(VerifiedFileEditor::new(remote_file)))
+    Ok(FileConfig::RemoteVerifiedNewFile { editor: VerifiedFileEditor::new(remote_file) })
 }
 
 fn prepare_file_pool(args: &Args) -> Result<BTreeMap<Inode, FileConfig>> {