fs: Move file reading helper from composefs-boot

Some bits in bootc will end up using this for the same
reason composefs-boot internals are.

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

crates/composefs-boot/src/bootloader.rs

@@ -1,9 +1,7 @@
 use core::ops::Range;
-use std::{
-    collections::HashMap, ffi::OsStr, fs::File, io::Read, os::unix::ffi::OsStrExt, str::from_utf8,
-};
+use std::{collections::HashMap, ffi::OsStr, os::unix::ffi::OsStrExt, str::from_utf8};
 
-use anyhow::{bail, ensure, Result};
+use anyhow::{bail, Result};
 
 use composefs::{
     fsverity::FsVerityHashValue,
@@ -112,24 +110,6 @@ impl BootLoaderEntryFile {
     }
 }
 
-pub fn read_file<ObjectID: FsVerityHashValue>(
-    file: &RegularFile<ObjectID>,
-    repo: &Repository<ObjectID>,
-) -> Result<Box<[u8]>> {
-    match file {
-        RegularFile::Inline(data) => Ok(data.clone()),
-        RegularFile::External(id, size) => {
-            let mut data = vec![];
-            File::from(repo.open_object(id)?).read_to_end(&mut data)?;
-            ensure!(
-                *size == data.len() as u64,
-                "File content doesn't have the expected length"
-            );
-            Ok(data.into_boxed_slice())
-        }
-    }
-}
-
 #[derive(Debug)]
 pub struct Type1Entry<ObjectID: FsVerityHashValue> {
     /// This is the basename of the bootloader entry .conf file
@@ -181,7 +161,7 @@ impl<ObjectID: FsVerityHashValue> Type1Entry<ObjectID> {
         root: &Directory<ObjectID>,
         repo: &Repository<ObjectID>,
     ) -> Result<Self> {
-        let entry = BootLoaderEntryFile::new(from_utf8(&read_file(file, repo)?)?);
+        let entry = BootLoaderEntryFile::new(from_utf8(&composefs::fs::read_file(file, repo)?)?);
 
         let mut files = HashMap::new();
         for key in ["linux", "initrd", "efi"] {

crates/composefs-boot/src/write_boot.rs

@@ -8,7 +8,7 @@ use anyhow::{ensure, Result};
 use composefs::{fsverity::FsVerityHashValue, repository::Repository};
 
 use crate::{
-    bootloader::{read_file, BootEntry, Type1Entry, Type2Entry},
+    bootloader::{BootEntry, Type1Entry, Type2Entry},
     cmdline::get_cmdline_composefs,
     uki,
 };
@@ -38,7 +38,7 @@ pub fn write_t1_simple<ObjectID: FsVerityHashValue>(
         let file_path = bootdir.join(pathname.strip_prefix(Path::new("/"))?);
         // SAFETY: what safety? :)
         create_dir_all(file_path.parent().unwrap())?;
-        write(file_path, read_file(file, repo)?)?;
+        write(file_path, composefs::fs::read_file(file, repo)?)?;
     }
 
     // And now the loader entry itself
@@ -59,7 +59,7 @@ pub fn write_t2_simple<ObjectID: FsVerityHashValue>(
     let efi_linux = bootdir.join("EFI/Linux");
     create_dir_all(&efi_linux)?;
     let filename = efi_linux.join(t2.filename.as_ref());
-    let content = read_file(&t2.file, repo)?;
+    let content = composefs::fs::read_file(&t2.file, repo)?;
     let (composefs, _) = get_cmdline_composefs::<ObjectID>(uki::get_cmdline(&content)?)?;
 
     ensure!(

crates/composefs/src/fs.rs

@@ -3,7 +3,7 @@ use std::{
     collections::{BTreeMap, HashMap},
     ffi::{CStr, OsStr},
     fs::File,
-    io::Write,
+    io::{Read, Write},
     mem::MaybeUninit,
     os::unix::ffi::OsStrExt,
     path::Path,
@@ -313,6 +313,9 @@ impl<ObjectID: FsVerityHashValue> FilesystemReader<'_, ObjectID> {
     }
 }
 
+/// Load a filesystem tree from the given path. A repository may
+/// be provided; if it is, then all files found in the filesystem
+/// are copied in.
 pub fn read_filesystem<ObjectID: FsVerityHashValue>(
     dirfd: impl AsFd,
     path: &Path,
@@ -332,6 +335,25 @@ pub fn read_filesystem<ObjectID: FsVerityHashValue>(
     })
 }
 
+/// Read the contents of a file.
+pub fn read_file<ObjectID: FsVerityHashValue>(
+    file: &RegularFile<ObjectID>,
+    repo: &Repository<ObjectID>,
+) -> Result<Box<[u8]>> {
+    match file {
+        RegularFile::Inline(data) => Ok(data.clone()),
+        RegularFile::External(id, size) => {
+            let mut data = Vec::with_capacity(*size as usize);
+            std::fs::File::from(repo.open_object(id)?).read_to_end(&mut data)?;
+            ensure!(
+                *size == data.len() as u64,
+                "File content doesn't have the expected length"
+            );
+            Ok(data.into_boxed_slice())
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;